Priority queue
A priority queue is somewhat similar to a queue, with an important distinction: each item is added to a priority queue with a priority level, and will be later removed from the queue with the highest priority element first. That is, the items are (conceptually) stored in the queue in priority order instead of in insertion order.
You are encouraged to solve this task according to the task description, using any language you may know.
- Task
Create a priority queue. The queue must support at least two operations:
- Insertion. An element is added to the queue with a priority (a numeric value).
- Top item removal. Deletes the element or one of the elements with the current top priority and return it.
Optionally, other operations may be defined, such as peeking (find what current top priority/top element is), merging (combining two priority queues into one), etc.
To test your implementation, insert a number of elements into the queue, each with some random priority.
Then dequeue them sequentially; now the elements should be sorted by priority.
You can use the following task/priority items as input data:
Priority Task
══════════ ════════════════
3 Clear drains
4 Feed cat
5 Make tea
1 Solve RC tasks
2 Tax return
The implementation should try to be efficient. A typical implementation has O(log n) insertion and extraction time, where n is the number of items in the queue.
You may choose to impose certain limits such as small range of allowed priority levels, limited capacity, etc. If so, discuss the reasons behind it.
11l
<lang 11l>V items = [(3, ‘Clear drains’), (4, ‘Feed cat’), (5, ‘Make tea’), (1, ‘Solve RC tasks’), (2, ‘Tax return’)] minheap:heapify(&items) L !items.empty
print(minheap:pop(&items))</lang>
- Output:
(1, Solve RC tasks) (2, Tax return) (3, Clear drains) (4, Feed cat) (5, Make tea)
AArch64 Assembly
<lang AArch64 Assembly> /* ARM assembly AARCH64 Raspberry PI 3B */ /* program priorQueue64.s */
/*******************************************/ /* Constantes file */ /*******************************************/ /* for this file see task include a file in language AArch64 assembly*/ .include "../includeConstantesARM64.inc"
.equ NBMAXIELEMENTS, 100
/*******************************************/ /* Structures */ /********************************************/ /* example structure item */
.struct 0
item_priority: // priority
.struct item_priority + 8
item_address: // string address
.struct item_address + 8
item_fin: /* example structure heap */
.struct 0
heap_size: // heap size
.struct heap_size + 8
heap_items: // structure of items
.struct heap_items + (item_fin * NBMAXIELEMENTS)
heap_fin:
/*********************************/
/* Initialized data */
/*********************************/
.data
szMessEmpty: .asciz "Empty queue. \n"
szMessNotEmpty: .asciz "Not empty queue. \n"
szMessError: .asciz "Error detected !!!!. \n"
szMessResult: .asciz "Priority : @ : @ \n" // message result
szString1: .asciz "Clear drains" szString2: .asciz "Feed cat" szString3: .asciz "Make tea" szString4: .asciz "Solve RC tasks" szString5: .asciz "Tax return" szCarriageReturn: .asciz "\n" /*********************************/ /* UnInitialized data */ /*********************************/ .bss .align 4 sZoneConv: .skip 24 Queue1: .skip heap_fin // queue memory place /*********************************/ /* code section */ /*********************************/ .text .global main main: // entry of program
ldr x0,qAdrQueue1 // queue structure address bl isEmpty cbz x0,1f ldr x0,qAdrszMessEmpty bl affichageMess // display message empty b 2f
1:
ldr x0,qAdrszMessNotEmpty bl affichageMess // display message not empty
2:
// init item 1 ldr x0,qAdrQueue1 // queue structure address mov x1,#3 // priority ldr x2,qAdrszString1 bl pushQueue // add item in queue cmp x0,#-1 // error ? beq 99f ldr x0,qAdrQueue1 // queue structure address bl isEmpty cbz x0,3f // not empty ldr x0,qAdrszMessEmpty bl affichageMess // display message empty b 4f
3:
ldr x0,qAdrszMessNotEmpty bl affichageMess // display message not empty
4:
// init item 2 ldr x0,qAdrQueue1 // queue structure address mov x1,#4 // priority ldr x2,qAdrszString2 bl pushQueue // add item in queue cmp x0,#-1 // error ? beq 99f // init item 3 ldr x0,qAdrQueue1 // queue structure address mov x1,#5 // priority ldr x2,qAdrszString3 bl pushQueue // add item in queue cmp x0,#-1 // error ? beq 99f // init item 4 ldr x0,qAdrQueue1 // queue structure address mov x1,#1 // priority ldr x2,qAdrszString4 bl pushQueue // add item in queue cmp x0,#-1 // error ? beq 99f // init item 5 ldr x0,qAdrQueue1 // queue structure address mov x1,#2 // priority ldr x2,qAdrszString5 bl pushQueue // add item in queue cmp x0,#-1 // error ? beq 99f
5:
ldr x0,qAdrQueue1 // queue structure address bl popQueue // return item cmp x0,#-1 // end ? beq 100f mov x2,x1 // save string address ldr x1,qAdrsZoneConv // conversion priority bl conversion10 // decimal conversion ldr x0,qAdrszMessResult ldr x1,qAdrsZoneConv bl strInsertAtCharInc mov x1,x2 // string address bl strInsertAtCharInc bl affichageMess // display message b 5b // loop
99: // error
ldr x0,qAdrszMessError bl affichageMess
100: // standard end of the program
mov x0, #0 // return code mov x8, #EXIT // request to exit program svc #0 // perform the system call
qAdrQueue1: .quad Queue1 qAdrszString1: .quad szString1 qAdrszString2: .quad szString2 qAdrszString3: .quad szString3 qAdrszString4: .quad szString4 qAdrszString5: .quad szString5 qAdrszMessError: .quad szMessError qAdrszMessEmpty: .quad szMessEmpty qAdrszMessNotEmpty: .quad szMessNotEmpty qAdrszMessResult: .quad szMessResult qAdrszCarriageReturn: .quad szCarriageReturn //qAdrsMessPriority: .quad sMessPriority qAdrsZoneConv: .quad sZoneConv /******************************************************************/ /* test if queue empty */ /******************************************************************/ /* x0 contains the address of queue structure */ isEmpty:
stp x1,lr,[sp,-16]! // save registres ldr x1,[x0,#heap_size] // heap size cmp x1,#0 cset x0,eq ldp x1,lr,[sp],16 // restaur des 2 registres ret
/******************************************************************/ /* add item in queue */ /******************************************************************/ /* x0 contains the address of queue structure */ /* x1 contains the priority of item */ /* x2 contains the string address */ pushQueue:
stp x1,lr,[sp,-16]! // save registres stp x2,x3,[sp,-16]! // save registres stp x4,x5,[sp,-16]! // save registres stp x6,x7,[sp,-16]! // save registres stp x8,x9,[sp,-16]! // save registres ldr x3,[x0,#heap_size] // heap size cbnz x3,1f // heap empty ? add x4,x0,#heap_items // address of item structure str x1,[x4,#item_priority] // store in first item str x2,[x4,#item_address] mov x3,#1 // heap size str x3,[x0,#heap_size] // new heap size b 100f
1:
mov x4,x3 // maxi index lsr x5,x4,#1 // current index = maxi / 2 mov x8,x1 // save priority mov x9,x2 // save string address
2: // insertion loop
cmp x4,#0 // end loop ? ble 3f mov x6,#item_fin // item size madd x6,x5,x6,x0 // item shift add x6,x6,#heap_items // compute address item ldr x7,[x6,#item_priority] // load priority cmp x7,x8 // compare priority ble 3f // <= end loop mov x1,x4 // last index mov x2,x5 // current index bl exchange mov x4,x5 // last index = current index lsr x5,x5,#1 // current index / 2 b 2b
3: // store item at last index find
mov x6,#item_fin // item size madd x6,x4,x6,x0 // item shift add x6,x6,#heap_items // item address str x8,[x6,#item_priority] str x9,[x6,#item_address] add x3,x3,#1 // increment heap size cmp x3,#NBMAXIELEMENTS // maxi ? bge 99f // yes -> error str x3,[x0,#heap_size] // store new size b 100f
99:
mov x0,#-1 // error
100:
ldp x8,x9,[sp],16 // restaur des 2 registres ldp x6,x7,[sp],16 // restaur des 2 registres ldp x4,x5,[sp],16 // restaur des 2 registres ldp x2,x3,[sp],16 // restaur des 2 registres ldp x1,lr,[sp],16 // restaur des 2 registres ret
/******************************************************************/ /* swap two elements of table */ /******************************************************************/ /* x0 contains the address of table */ /* x1 contains the first index */ /* x2 contains the second index */ exchange:
stp x1,lr,[sp,-16]! // save registres stp x2,x3,[sp,-16]! // save registres stp x4,x5,[sp,-16]! // save registres stp x6,x7,[sp,-16]! // save registres add x5,x0,#heap_items // address items begin mov x3,#item_fin // item size madd x4,x1,x3,x5 // compute item 1 address madd x6,x2,x3,x5 // compute item 2 address ldr x5,[x4,#item_priority] // exchange ldr x3,[x6,#item_priority] str x3,[x4,#item_priority] str x5,[x6,#item_priority] ldr x5,[x4,#item_address] ldr x3,[x6,#item_address] str x5,[x6,#item_address] str x3,[x4,#item_address]
100:
ldp x6,x7,[sp],16 // restaur des 2 registres ldp x4,x5,[sp],16 // restaur des 2 registres ldp x2,x3,[sp],16 // restaur des 2 registres ldp x1,lr,[sp],16 // restaur des 2 registres ret
/******************************************************************/ /* move one element of table */ /******************************************************************/ /* x0 contains the address of table */ /* x1 contains the origin index */ /* x2 contains the destination index */ moveItem:
stp x1,lr,[sp,-16]! // save registres stp x2,x3,[sp,-16]! // save registres stp x4,x5,[sp,-16]! // save registres stp x6,x7,[sp,-16]! // save registres add x5,x0,#heap_items // address items begin mov x3,#item_fin // item size madd x4,x1,x3,x5 // compute item 1 address madd x6,x2,x3,x5 // compute item 2 address ldr x5,[x4,#item_priority] // exchange str x5,[x6,#item_priority] ldr x5,[x4,#item_address] str x5,[x6,#item_address]
100:
ldp x6,x7,[sp],16 // restaur des 2 registres ldp x4,x5,[sp],16 // restaur des 2 registres ldp x2,x3,[sp],16 // restaur des 2 registres ldp x1,lr,[sp],16 // restaur des 2 registres ret
/******************************************************************/ /* pop queue */ /******************************************************************/ /* x0 contains the address of queue structure */ /* x0 return priority */ /* x1 return string address */ popQueue:
stp x10,lr,[sp,-16]! // save registres stp x2,x3,[sp,-16]! // save registres stp x4,x5,[sp,-16]! // save registres stp x6,x7,[sp,-16]! // save registres stp x8,x9,[sp,-16]! // save registres mov x1,x0 // save address queue bl isEmpty // control if empty queue cmp x0,#1 // yes -> error beq 99f
mov x0,x1 // restaur address queue add x4,x0,#heap_items // address of item structure ldr x8,[x4,#item_priority] // save priority first item ldr x9,[x4,#item_address] // save address string first item ldr x3,[x0,#heap_size] // heap size sub x7,x3,#1 // last item mov x1,x7 mov x2,#0 // first item bl moveItem // move last item in first item cmp x7,#1 // one only item ? beq 10f // yes -> end mov x4,#0 // first index
1:
cmp x4,x7 // = last index
bge 10f // yes -> end
mov x5,x7 // last index
cmp x4,#0 // init current index
mov x6,#1 // = 1
lsl x1,x4,#1 // else = first index * 2
csel x6,x6,x1,eq
cmp x6,x7 // current index > last index
bgt 2f // yes
// no compar priority current item last item
mov x1,#item_fin
madd x1,x6,x1,x0
add x1,x1,#heap_items // address of current item structure
ldr x1,[x1,#item_priority]
mov x10,#item_fin
madd x10,x5,x10,x0
add x10,x10,#heap_items // address of last item structure
ldr x10,[x10,#item_priority]
cmp x1,x10
csel x5,x6,x5,lt
2:
add x10,x6,#1 // increment current index cmp x10,x7 // end ? bgt 3f // yes mov x1,#item_fin // no compare priority madd x1,x10,x1,x0 add x1,x1,#heap_items // address of item structure ldr x1,[x1,#item_priority] mov x2,#item_fin madd x2,x5,x2,x0 add x2,x2,#heap_items // address of item structure ldr x2,[x2,#item_priority] cmp x1,x2 csel x5,x10,x5,lt
3:
mov x1,x5 // move item mov x2,x4 bl moveItem mov x4,x5 b 1b // and loop
10:
sub x3,x3,#1 str x3,[x0,#heap_size] // new heap size mov x0,x8 // return priority mov x1,x9 // return string address b 100f
99:
mov x0,#-1 // error
100:
ldp x8,x9,[sp],16 // restaur des 2 registres ldp x6,x7,[sp],16 // restaur des 2 registres ldp x4,x5,[sp],16 // restaur des 2 registres ldp x2,x3,[sp],16 // restaur des 2 registres ldp x10,lr,[sp],16 // restaur des 2 registres ret
/********************************************************/ /* File Include fonctions */ /********************************************************/ /* for this file see task include a file in language AArch64 assembly */ .include "../includeARM64.inc" </lang>
- Output:
Empty queue. Not empty queue. Priority : 1 : Solve RC tasks Priority : 2 : Tax return Priority : 3 : Clear drains Priority : 4 : Feed cat Priority : 5 : Make tea
Action!
The user must type in the monitor the following command after compilation and before running the program!
SET EndProg=*
<lang Action!>CARD EndProg ;required for ALLOCATE.ACT
INCLUDE "D2:ALLOCATE.ACT" ;from the Action! Tool Kit. You must type 'SET EndProg=*' from the monitor after compiling, but before running this program!
DEFINE PTR="CARD" DEFINE NODE_SIZE="5" TYPE QueueNode=[
BYTE priority PTR data ;CHAR ARRAY PTR nxt]
QueueNode POINTER queueFront,queueRear
BYTE FUNC IsEmpty()
IF queueFront=0 THEN RETURN (1) FI
RETURN (0)
PROC Push(BYTE p CHAR ARRAY d)
QueueNode POINTER node,curr,prev
node=Alloc(NODE_SIZE) node.priority=p node.data=d node.nxt=0
IF IsEmpty() THEN queueFront=node queueRear=node RETURN FI
curr=queueFront prev=0 WHILE curr#0 AND curr.priority<=p DO prev=curr curr=curr.nxt OD
IF prev=0 THEN queueFront=node ELSEIF curr=0 THEN queueRear.nxt=node queueRear=node ELSE prev.nxt=node FI node.nxt=curr
RETURN
PTR FUNC Pop()
QueueNode POINTER node
IF IsEmpty() THEN
PrintE("Error: queue is empty!")
Break()
FI
node=queueFront queueFront=node.nxt
RETURN (node)
PROC TestIsEmpty()
IF IsEmpty() THEN
PrintE("Queue is empty")
ELSE
PrintE("Queue is not empty")
FI
RETURN
PROC TestPush(BYTE p CHAR ARRAY d)
PrintF("Push priority=%B task=%S%E",p,d)
Push(p,d)
RETURN
PROC TestPop()
QueueNode POINTER node
node=Pop()
PrintF("Pop priority=%B task=%S%E",node.priority,node.data)
Free(node,NODE_SIZE)
RETURN
PROC Main()
AllocInit(0) queueFront=0 queueRear=0
Put(125) PutE() ;clear screen
TestIsEmpty() TestPush(3,"Clear drains") TestPush(4,"Feed cat") TestPush(5,"Make tea") TestPush(1,"Solve RC tasks") TestPush(2,"Tax return") TestIsEmpty() TestPop() TestPop() TestPop() TestPop() TestPop() TestIsEmpty()
RETURN</lang>
- Output:
Screenshot from Atari 8-bit computer
Queue is empty Push priority=3 task=Clear drains Push priority=4 task=Feed cat Push priority=5 task=Make tea Push priority=1 task=Solve RC tasks Push priority=2 task=Tax return Queue is not empty Pop priority=1 task=Solve RC tasks Pop priority=2 task=Tax return Pop priority=3 task=Clear drains Pop priority=4 task=Feed cat Pop priority=5 task=Make tea Queue is empty
Ada
Ada 2012 includes container classes for priority queues.
<lang Ada>with Ada.Containers.Synchronized_Queue_Interfaces; with Ada.Containers.Unbounded_Priority_Queues; with Ada.Strings.Unbounded;
procedure Priority_Queues is
use Ada.Containers;
use Ada.Strings.Unbounded;
type Queue_Element is record
Priority : Natural;
Content : Unbounded_String;
end record;
function Get_Priority (Element : Queue_Element) return Natural is
begin
return Element.Priority;
end Get_Priority;
function Before (Left, Right : Natural) return Boolean is
begin
return Left > Right;
end Before;
package String_Queues is new Synchronized_Queue_Interfaces
(Element_Type => Queue_Element);
package String_Priority_Queues is new Unbounded_Priority_Queues
(Queue_Interfaces => String_Queues,
Queue_Priority => Natural);
My_Queue : String_Priority_Queues.Queue;
begin
My_Queue.Enqueue (New_Item => (Priority => 3, Content => To_Unbounded_String ("Clear drains")));
My_Queue.Enqueue (New_Item => (Priority => 4, Content => To_Unbounded_String ("Feed cat")));
My_Queue.Enqueue (New_Item => (Priority => 5, Content => To_Unbounded_String ("Make tea")));
My_Queue.Enqueue (New_Item => (Priority => 1, Content => To_Unbounded_String ("Solve RC tasks")));
My_Queue.Enqueue (New_Item => (Priority => 2, Content => To_Unbounded_String ("Tax return")));
declare
Element : Queue_Element;
begin
while My_Queue.Current_Use > 0 loop
My_Queue.Dequeue (Element => Element);
Ada.Text_IO.Put_Line (Natural'Image (Element.Priority) & " => " & To_String (Element.Content));
end loop;
end;
end Priority_Queues;</lang>
- Output:
ARM Assembly
<lang ARM Assembly> /* ARM assembly Raspberry PI */ /* program priorqueue.s */
/* Constantes */ .equ STDOUT, 1 @ Linux output console .equ EXIT, 1 @ Linux syscall .equ WRITE, 4 @ Linux syscall
.equ NBMAXIELEMENTS, 100
/*******************************************/ /* Structures */ /********************************************/ /* example structure item */
.struct 0
item_priority: @ priority
.struct item_priority + 4
item_address: @ string address
.struct item_address + 4
item_fin: /* example structure heap */
.struct 0
heap_size: @ heap size
.struct heap_size + 4
heap_items: @ structure of items
.struct heap_items + (item_fin * NBMAXIELEMENTS)
heap_fin:
/*********************************/
/* Initialized data */
/*********************************/
.data
szMessEmpty: .asciz "Empty queue. \n"
szMessNotEmpty: .asciz "Not empty queue. \n"
szMessError: .asciz "Error detected !!!!. \n"
szMessResult: .ascii "Priority : " @ message result
sMessPriority: .fill 11, 1, ' '
.asciz " : "
szString1: .asciz "Clear drains" szString2: .asciz "Feed cat" szString3: .asciz "Make tea" szString4: .asciz "Solve RC tasks" szString5: .asciz "Tax return" szCarriageReturn: .asciz "\n" /*********************************/ /* UnInitialized data */ /*********************************/ .bss .align 4 Queue1: .skip heap_fin @ queue memory place /*********************************/ /* code section */ /*********************************/ .text .global main main: @ entry of program
ldr r0,iAdrQueue1 @ queue structure address bl isEmpty cmp r0,#0 beq 1f ldr r0,iAdrszMessEmpty bl affichageMess @ display message empty b 2f
1:
ldr r0,iAdrszMessNotEmpty bl affichageMess @ display message not empty
2:
@ init item 1 ldr r0,iAdrQueue1 @ queue structure address mov r1,#3 @ priority ldr r2,iAdrszString1 bl pushQueue @ add item in queue cmp r0,#-1 @ error ? beq 99f
ldr r0,iAdrQueue1 @ queue structure address bl isEmpty cmp r0,#0 @ not empty beq 3f ldr r0,iAdrszMessEmpty bl affichageMess @ display message empty b 4f
3:
ldr r0,iAdrszMessNotEmpty bl affichageMess @ display message not empty
4:
@ init item 2 ldr r0,iAdrQueue1 @ queue structure address mov r1,#4 @ priority ldr r2,iAdrszString2 bl pushQueue @ add item in queue cmp r0,#-1 @ error ? beq 99f @ init item 3 ldr r0,iAdrQueue1 @ queue structure address mov r1,#5 @ priority ldr r2,iAdrszString3 bl pushQueue @ add item in queue cmp r0,#-1 @ error ? beq 99f @ init item 4 ldr r0,iAdrQueue1 @ queue structure address mov r1,#1 @ priority ldr r2,iAdrszString4 bl pushQueue @ add item in queue cmp r0,#-1 @ error ? beq 99f @ init item 5 ldr r0,iAdrQueue1 @ queue structure address mov r1,#2 @ priority ldr r2,iAdrszString5 bl pushQueue @ add item in queue cmp r0,#-1 @ error ? beq 99f
5:
ldr r0,iAdrQueue1 @ queue structure address bl popQueue @ return item cmp r0,#-1 @ end ? beq 100f mov r2,r1 @ save string address ldr r1,iAdrsMessPriority @ conversion priority bl conversion10 @ decimal conversion ldr r0,iAdrszMessResult bl affichageMess @ display message mov r0,r2 @ string address bl affichageMess @ display message ldr r0,iAdrszCarriageReturn bl affichageMess
b 5b @ loop
99:
@ error ldr r0,iAdrszMessError bl affichageMess
100: @ standard end of the program
mov r0, #0 @ return code mov r7, #EXIT @ request to exit program svc #0 @ perform the system call
iAdrQueue1: .int Queue1 iAdrszString1: .int szString1 iAdrszString2: .int szString2 iAdrszString3: .int szString3 iAdrszString4: .int szString4 iAdrszString5: .int szString5 iAdrszMessError: .int szMessError iAdrszMessEmpty: .int szMessEmpty iAdrszMessNotEmpty: .int szMessNotEmpty iAdrszMessResult: .int szMessResult iAdrszCarriageReturn: .int szCarriageReturn iAdrsMessPriority: .int sMessPriority
/******************************************************************/ /* test if queue empty */ /******************************************************************/ /* r0 contains the address of queue structure */ isEmpty:
push {r1,lr} @ save registres
ldr r1,[r0,#heap_size] @ heap size
cmp r1,#0
moveq r0,#1 @ empty queue
movne r0,#0 @ not empty
pop {r1,lr} @ restaur registers
bx lr @ return
/******************************************************************/ /* add item in queue */ /******************************************************************/ /* r0 contains the address of queue structure */ /* r1 contains the priority of item */ /* r2 contains the string address */ pushQueue:
push {r1-r9,lr} @ save registres
ldr r3,[r0,#heap_size] @ heap size
cmp r3,#0 @ heap empty ?
bne 1f
add r4,r0,#heap_items @ address of item structure
str r1,[r4,#item_priority] @ store in first item
str r2,[r4,#item_address]
mov r3,#1 @ heap size
str r3,[r0,#heap_size] @ new heap size
b 100f
1:
mov r4,r3 @ maxi index lsr r5,r4,#1 @ current index = maxi / 2 mov r8,r1 @ save priority mov r9,r2 @ save string address
2: @ insertion loop
cmp r4,#0 @ end loop ? ble 3f mov r6,#item_fin @ item size mul r6,r5,r6 @ item shift add r6,r0 add r6,#heap_items @ compute address item ldr r7,[r6,#item_priority] @ load priority cmp r7,r8 @ compare priority ble 3f @ <= end loop mov r1,r4 @ last index mov r2,r5 @ current index bl exchange mov r4,r5 @ last index = current index lsr r5,#1 @ current index / 2 b 2b
3: @ store item at last index find
mov r6,#item_fin @ item size mul r6,r4,r6 @ item shift add r6,r0 add r6,#heap_items @ item address str r8,[r6,#item_priority] str r9,[r6,#item_address] add r3,#1 @ increment heap size cmp r3,#NBMAXIELEMENTS @ maxi ? movge r0,#-1 @ yes -> error bge 100f str r3,[r0,#heap_size] @ store new size
100:
pop {r1-r9,lr} @ restaur registers
bx lr @ return
/******************************************************************/ /* swap two elements of table */ /******************************************************************/ /* r0 contains the address of table */ /* r1 contains the first index */ /* r2 contains the second index */ exchange:
push {r3-r6,lr} @ save registers
add r5,r0,#heap_items @ address items begin
mov r3,#item_fin @ item size
mul r4,r1,r3 @ compute item 1 shift
add r4,r5 @ compute item 1 address
mul r6,r2,r3 @ compute item 2 shift
add r6,r5 @ compute item 2 address
ldr r5,[r4,#item_priority] @ exchange
ldr r3,[r6,#item_priority]
str r3,[r4,#item_priority]
str r5,[r6,#item_priority]
ldr r5,[r4,#item_address]
ldr r3,[r6,#item_address]
str r5,[r6,#item_address]
str r3,[r4,#item_address]
100:
pop {r3-r6,lr}
bx lr @ return
/******************************************************************/ /* move one element of table */ /******************************************************************/ /* r0 contains the address of table */ /* r1 contains the origin index */ /* r2 contains the destination index */ moveItem:
push {r3-r6,lr} @ save registers
add r5,r0,#heap_items @ address items begin
mov r3,#item_fin @ item size
mul r4,r1,r3 @ compute item 1 shift
add r4,r5 @ compute item 1 address
mul r6,r2,r3 @ compute item 2 shift
add r6,r5 @ compute item 2 address
ldr r5,[r4,#item_priority] @ exchange
str r5,[r6,#item_priority]
ldr r5,[r4,#item_address]
str r5,[r6,#item_address]
100:
pop {r3-r6,lr}
bx lr @ return
/******************************************************************/
/* pop queue */
/******************************************************************/
/* r0 contains the address of queue structure */
/* r0 return priority */
/* r1 return string address */
popQueue:
push {r2-r10,lr} @ save registres
mov r1,r0 @ save address queue
bl isEmpty @ control if empty queue
cmp r0,#1 @ yes -> error
moveq r0,#-1
beq 100f
@ save données à retourner
mov r0,r1 @ restaur address queue
add r4,r0,#heap_items @ address of item structure
ldr r8,[r4,#item_priority] @ save priority first item
ldr r9,[r4,#item_address] @ save address string first item
ldr r3,[r0,#heap_size] @ heap size
sub r7,r3,#1 @ last item
mov r1,r7
mov r2,#0 @ first item
bl moveItem @ move last item in first item
cmp r7,#1 @ one only item ? beq 10f @ yes -> end mov r4,#0 @ first index
1:
cmp r4,r7 @ = last index
bge 10f @ yes -> end
mov r5,r7 @ last index
cmp r4,#0 @ init current index
moveq r6,#1 @ = 1
lslne r6,r4,#1 @ else = first index * 2
cmp r6,r7 @ current index > last index
bgt 2f @ yes
@ no compar priority current item last item
mov r1,#item_fin
mul r1,r6,r1
add r1,r0
add r1,#heap_items @ address of current item structure
ldr r1,[r1,#item_priority]
mov r10,#item_fin
mul r10,r5,r10
add r10,r0
add r10,#heap_items @ address of last item structure
ldr r10,[r10,#item_priority]
cmp r1,r10
movlt r5,r6
2:
add r10,r6,#1 @ increment current index cmp r10,r7 @ end ? bgt 3f @ yes mov r1,#item_fin @ no compare priority mul r1,r10,r1 add r1,r0 add r1,#heap_items @ address of item structure ldr r1,[r1,#item_priority] mov r2,#item_fin mul r2,r5,r2 add r2,r0 add r2,#heap_items @ address of item structure ldr r2,[r2,#item_priority] cmp r1,r2 movlt r5,r10
3:
mov r1,r5 @ move item mov r2,r4 bl moveItem mov r4,r5 b 1b @ and loop
10:
sub r3,#1 str r3,[r0,#heap_size] @ new heap size mov r0,r8 @ return priority mov r1,r9 @ return string address
100:
pop {r2-r10,lr} @ restaur registers
bx lr @ return
/******************************************************************/ /* display text with size calculation */ /******************************************************************/ /* r0 contains the address of the message */ affichageMess:
push {r0,r1,r2,r7,lr} @ save registres
mov r2,#0 @ counter length
1: @ loop length calculation
ldrb r1,[r0,r2] @ read octet start position + index
cmp r1,#0 @ if 0 its over
addne r2,r2,#1 @ else add 1 in the length
bne 1b @ and loop
@ so here r2 contains the length of the message
mov r1,r0 @ address message in r1
mov r0,#STDOUT @ code to write to the standard output Linux
mov r7, #WRITE @ code call system "write"
svc #0 @ call systeme
pop {r0,r1,r2,r7,lr} @ restaur registers */
bx lr @ return
/******************************************************************/ /* Converting a register to a decimal */ /******************************************************************/ /* r0 contains value and r1 address area */ .equ LGZONECAL, 10 conversion10:
push {r1-r4,lr} @ save registers
mov r3,r1
mov r2,#LGZONECAL
1: @ start loop
bl divisionpar10 @ r0 <- dividende. quotient ->r0 reste -> r1
add r1,#48 @ digit
strb r1,[r3,r2] @ store digit on area
cmp r0,#0 @ stop if quotient = 0
subne r2,#1 @ previous position
bne 1b @ else loop
@ end replaces digit in front of area
mov r4,#0
2:
ldrb r1,[r3,r2] strb r1,[r3,r4] @ store in area begin add r4,#1 add r2,#1 @ previous position cmp r2,#LGZONECAL @ end ble 2b @ loop mov r1,#' '
3:
strb r1,[r3,r4] add r4,#1 cmp r4,#LGZONECAL @ end ble 3b
100:
pop {r1-r4,lr} @ restaur registres
bx lr @return
/***************************************************/ /* division par 10 signé */ /* Thanks to http://thinkingeek.com/arm-assembler-raspberry-pi/* /* and http://www.hackersdelight.org/ */ /***************************************************/ /* r0 dividende */ /* r0 quotient */ /* r1 remainder */ divisionpar10:
/* r0 contains the argument to be divided by 10 */
push {r2-r4} @ save registers */
mov r4,r0
mov r3,#0x6667 @ r3 <- magic_number lower
movt r3,#0x6666 @ r3 <- magic_number upper
smull r1, r2, r3, r0 @ r1 <- Lower32Bits(r1*r0). r2 <- Upper32Bits(r1*r0)
mov r2, r2, ASR #2 @ r2 <- r2 >> 2
mov r1, r0, LSR #31 @ r1 <- r0 >> 31
add r0, r2, r1 @ r0 <- r2 + r1
add r2,r0,r0, lsl #2 @ r2 <- r0 * 5
sub r1,r4,r2, lsl #1 @ r1 <- r4 - (r2 * 2) = r4 - (r0 * 10)
pop {r2-r4}
bx lr @ return
</lang>
- Output:
Empty queue. Not empty queue. Priority : 1 : Solve RC tasks Priority : 2 : Tax return Priority : 3 : Clear drains Priority : 4 : Feed cat Priority : 5 : Make tea
AutoHotkey
<lang AutoHotkey>;----------------------------------- PQ_TopItem(Queue,Task:=""){ ; remove and return top priority item TopPriority := PQ_TopPriority(Queue) for T, P in Queue if (P = TopPriority) && ((T=Task)||!Task) return T , Queue.Remove(T) return 0 }
- -----------------------------------
PQ_AddTask(Queue,Task,Priority){ ; insert and return new task for T, P in Queue if (T=Task) || !(Priority && Task) return 0 return Task, Queue[Task] := Priority }
- -----------------------------------
PQ_DelTask(Queue, Task){ ; delete and return task for T, P in Queue if (T = Task) return Task, Queue.Remove(Task) }
- -----------------------------------
PQ_Peek(Queue){ ; peek and return top priority task(s) TopPriority := PQ_TopPriority(Queue) for T, P in Queue if (P = TopPriority) PeekList .= (PeekList?"`n":"") "`t" T return PeekList }
- -----------------------------------
PQ_Check(Queue,Task){ ; check task and return its priority for T, P in Queue if (T = Task) return P return 0 }
- -----------------------------------
PQ_Edit(Queue,Task,Priority){ ; Update task priority and return its new priority for T, P in Queue if (T = Task) return Priority, Queue[T]:=Priority return 0 }
- -----------------------------------
PQ_View(Queue){ ; view current Queue for T, P in Queue Res .= P " : " T "`n" Sort, Res, FMySort return "Priority Queue=`n" Res } MySort(a,b){ RegExMatch(a,"(\d+) : (.*)", x), RegExMatch(b,"(\d+) : (.*)", y) return x1>y1?1:x1<y1?-1: x2>y2?1:x2<y2?-1: 0 }
- -----------------------------------
PQ_TopPriority(Queue){ ; return queue's top priority for T, P in Queue TopPriority := TopPriority?TopPriority:P , TopPriority := TopPriority<P?TopPriority:P return, TopPriority }</lang> Examples:<lang AutoHotkey>data = ( 3 Clear drains 1 test 4 Feed cat 5 Make tea 1 Solve RC tasks 2 Tax return ) PQ:=[] ; Create Priority Queue PQ[Task, Priority] loop, parse, data, `n, `r F:= StrSplit(A_LoopField, "`t") , PQ[F[2]] := F[1] PQ_View(PQ) MsgBox, 262208,, % "Top Priority item(s)=`n" PQ_Peek(PQ) "`n`n" PQ_View(PQ) MsgBox, 262208,, % "Add : " PQ_AddTask(PQ, "AutoHotkey", 2) "`n`n" PQ_View(PQ) MsgBox, 262208,, % "Remove Top Item : " PQ_TopItem(PQ) "`n`n" PQ_View(PQ) MsgBox, 262208,, % "Remove specific top item : " PQ_TopItem(PQ,"test") "`n`n" PQ_View(PQ) MsgBox, 262208,, % "Delete Item : " PQ_DelTask(PQ, "Clear drains") "`n`n" PQ_View(PQ) MsgBox, 262208,, % (Task:="Tax return") " new priority = " PQ_Edit(PQ,task, 7) "`n`n" PQ_View(PQ) MsgBox, 262208,, % (Task:="Feed cat") " priority = " PQ_Check(PQ,task)"`n`n" PQ_View(PQ) ^Esc:: ExitApp</lang>
Axiom
Axiom already has a heap domain for ordered sets. We define a domain for ordered key-entry pairs and then define a priority queue using the heap domain over the pairs: <lang Axiom>)abbrev Domain ORDKE OrderedKeyEntry OrderedKeyEntry(Key:OrderedSet,Entry:SetCategory): Exports == Implementation where
Exports == OrderedSet with
construct: (Key,Entry) -> %
elt: (%,"key") -> Key
elt: (%,"entry") -> Entry
Implementation == add
Rep := Record(k:Key,e:Entry)
x,y: %
construct(a,b) == construct(a,b)$Rep @ %
elt(x,"key"):Key == (x@Rep).k
elt(x,"entry"):Entry == (x@Rep).e
x < y == x.key < y.key
x = y == x.key = y.key
hash x == hash(x.key)
if Entry has CoercibleTo OutputForm then
coerce(x):OutputForm == bracket [(x.key)::OutputForm,(x.entry)::OutputForm]
)abbrev Domain PRIORITY PriorityQueue S ==> OrderedKeyEntry(Key,Entry) PriorityQueue(Key:OrderedSet,Entry:SetCategory): Exports == Implementation where
Exports == PriorityQueueAggregate S with
heap : List S -> %
setelt: (%,Key,Entry) -> Entry
Implementation == Heap(S) add
setelt(x:%,key:Key,entry:Entry) ==
insert!(construct(key,entry)$S,x)
entry</lang>For an example:<lang Axiom>pq := empty()$PriorityQueue(Integer,String)
pq(3):="Clear drains"; pq(4):="Feed cat"; pq(5):="Make tea"; pq(1):="Solve RC tasks"; pq(2):="Tax return"; [extract!(pq) for i in 1..#pq]</lang>
- Output:
[[5,"Make tea"], [4,"Feed cat"], [3,"Clear drains"], [2,"Tax return"],
[1,"Solve RC tasks"]]
Type: List(OrderedKeyEntry(Integer,String))
Batch File
Batch has only a data structure, the environment that incidentally sorts itself automatically by key. The environment has a limit of 64K <lang Batch File> @echo off setlocal enabledelayedexpansion
call :push 10 "item ten" call :push 2 "item two" call :push 100 "item one hundred" call :push 5 "item five"
call :pop & echo !order! !item! call :pop & echo !order! !item! call :pop & echo !order! !item! call :pop & echo !order! !item! call :pop & echo !order! !item!
goto:eof
- push
set temp=000%1 set queu%temp:~-3%=%2 goto:eof
- pop
set queu >nul 2>nul if %errorlevel% equ 1 (set order=-1&set item=no more items & goto:eof) for /f "tokens=1,2 delims==" %%a in ('set queu') do set %%a=& set order=%%a& set item=%%~b& goto:next
- next
set order= %order:~-3% goto:eof</lang>
- Output:
002 item two 005 item five 010 item ten 100 item one hundred -1 no more items
C
Using a dynamic array as a binary heap. Stores integer priority and a character pointer. Supports push and pop. <lang c>#include <stdio.h>
- include <stdlib.h>
typedef struct {
int priority; char *data;
} node_t;
typedef struct {
node_t *nodes; int len; int size;
} heap_t;
void push (heap_t *h, int priority, char *data) {
if (h->len + 1 >= h->size) {
h->size = h->size ? h->size * 2 : 4;
h->nodes = (node_t *)realloc(h->nodes, h->size * sizeof (node_t));
}
int i = h->len + 1;
int j = i / 2;
while (i > 1 && h->nodes[j].priority > priority) {
h->nodes[i] = h->nodes[j];
i = j;
j = j / 2;
}
h->nodes[i].priority = priority;
h->nodes[i].data = data;
h->len++;
}
char *pop (heap_t *h) {
int i, j, k;
if (!h->len) {
return NULL;
}
char *data = h->nodes[1].data;
h->nodes[1] = h->nodes[h->len];
h->len--;
i = 1;
while (i!=h->len+1) {
k = h->len+1;
j = 2 * i;
if (j <= h->len && h->nodes[j].priority < h->nodes[k].priority) {
k = j;
}
if (j + 1 <= h->len && h->nodes[j + 1].priority < h->nodes[k].priority) {
k = j + 1;
}
h->nodes[i] = h->nodes[k];
i = k;
}
return data;
}
int main () {
heap_t *h = (heap_t *)calloc(1, sizeof (heap_t));
push(h, 3, "Clear drains");
push(h, 4, "Feed cat");
push(h, 5, "Make tea");
push(h, 1, "Solve RC tasks");
push(h, 2, "Tax return");
int i;
for (i = 0; i < 5; i++) {
printf("%s\n", pop(h));
}
return 0;
} </lang>
- Output:
Solve RC tasks Tax return Clear drains Feed cat Make tea
Pairing heap w/ generic data types
header file: <lang C> typedef struct _pq_node_t {
long int key; struct _pq_node_t *next, *down;
} pq_node_t, *heap_t;
extern heap_t heap_merge(heap_t, heap_t); extern heap_t heap_pop(heap_t);
- define NEW_PQ_ELE(p, k) \
do { \
(p) = (typeof(p)) malloc(sizeof(*p)); \ ((pq_node_t *) (p))->next = ((pq_node_t *) (p))->down = NULL; \ ((pq_node_t *) (p))->key = (k); \
} while (0)
- define HEAP_PUSH(p, k, h) \
NEW_PQ_ELE(p, k); \ *(h) = heap_merge(((pq_node_t *) (p)), *(h))
</lang> implementation: <lang C>
- include <stdlib.h>
- include "pairheap.h"
static heap_t add_child(heap_t h, heap_t g) {
if (h->down != NULL)
g->next = h->down;
h->down = g;
}
heap_t heap_merge(heap_t a, heap_t b) {
if (a == NULL) return b;
if (b == NULL) return a;
if (a->key < b->key) {
add_child(a, b); return a;
} else {
add_child(b, a); return b;
}
}
/* NOTE: caller should have pointer to top of heap, since otherwise it won't
* be reclaimed. (we do not free the top.) */
heap_t two_pass_merge(heap_t h) {
if (h == NULL || h->next == NULL)
return h;
else {
pq_node_t *a = h, *b = h->next, *rest = b->next; a->next = b->next = NULL; return heap_merge(heap_merge(a, b), two_pass_merge(rest));
}
}
heap_t heap_pop(heap_t h) {
return two_pass_merge(h->down);
} </lang> usage: <lang C>
- include <stdio.h>
- include <string.h>
- include <stdlib.h>
- include "pairheap.h"
struct task {
pq_node_t hd; char task[40];
};
void main() {
heap_t heap = NULL; struct task *new;
HEAP_PUSH(new, 3, &heap); strcpy(new->task, "Clear drains.");
HEAP_PUSH(new, 4, &heap); strcpy(new->task, "Feed cat.");
HEAP_PUSH(new, 5, &heap); strcpy(new->task, "Make tea.");
HEAP_PUSH(new, 1, &heap); strcpy(new->task, "Solve RC tasks.");
HEAP_PUSH(new, 2, &heap); strcpy(new->task, "Tax return.");
while (heap != NULL) {
struct task *top = (struct task *) heap; printf("%s\n", top->task); heap = heap_pop(heap); free(top);
}
} </lang>
- Output:
Solve RC tasks. Tax return. Clear drains. Feed cat. Make tea.
C#
.NET 6 solution
<lang csharp>using System; using System.Collections.Generic;
namespace PriorityQueueExample { class Program { static void Main(string[] args) { // Starting with .NET 6.0 preview 2 (released March 11th, 2021), there's a built-in priority queue var p = new PriorityQueue<string, int>(); p.Enqueue("Clear drains", 3); p.Enqueue("Feed cat", 4); p.Enqueue("Make tea", 5); p.Enqueue("Solve RC tasks", 1); p.Enqueue("Tax return", 2); while (p.TryDequeue(out string task, out int priority)) { Console.WriteLine($"{priority}\t{task}"); } } } }
/* Output:
1 Solve RC tasks 2 Tax return 3 Clear drains 4 Feed cat 5 Make tea
*/</lang>
Pre-.NET 6 solution
<lang csharp>using System;
namespace PriorityQueue {
class Program
{
static void Main(string[] args)
{
PriorityQueue PQ = new PriorityQueue();
PQ.push(3, "Clear drains");
PQ.push(4, "Feed cat");
PQ.push(5, "Make tea");
PQ.push(1, "Solve RC tasks");
PQ.push(2, "Tax return");
while (!PQ.Empty)
{
var Val = PQ.pop();
Console.WriteLine(Val[0] + " : " + Val[1]);
}
Console.ReadKey();
}
}
class PriorityQueue
{
private System.Collections.SortedList PseudoQueue;
public bool Empty
{
get
{
return PseudoQueue.Count == 0;
}
}
public PriorityQueue()
{
PseudoQueue = new System.Collections.SortedList();
}
public void push(object Priority, object Value)
{
PseudoQueue.Add(Priority, Value);
}
public object[] pop()
{
object[] ReturnValue = { null, null };
if (PseudoQueue.Count > 0)
{
ReturnValue[0] = PseudoQueue.GetKey(0);
ReturnValue[1] = PseudoQueue.GetByIndex(0);
PseudoQueue.RemoveAt(0);
}
return ReturnValue;
}
}
}</lang>
Min Heap Priority Queue
The above code is not really a true Priority Queue as it does not allow duplicate keys; also, the SortedList on which it is based does not have O(log n) insertions and removals for random data as a true Priority Queue does. The below code implements a true Min Heap Priority Queue: <lang csharp>namespace PriorityQ {
using KeyT = UInt32;
using System;
using System.Collections.Generic;
using System.Linq;
class Tuple<K, V> { // for DotNet 3.5 without Tuple's
public K Item1; public V Item2;
public Tuple(K k, V v) { Item1 = k; Item2 = v; }
public override string ToString() {
return "(" + Item1.ToString() + ", " + Item2.ToString() + ")";
}
}
class MinHeapPQ<V> {
private struct HeapEntry {
public KeyT k; public V v;
public HeapEntry(KeyT k, V v) { this.k = k; this.v = v; }
}
private List<HeapEntry> pq;
private MinHeapPQ() { this.pq = new List<HeapEntry>(); }
private bool mt { get { return pq.Count == 0; } }
private int sz {
get {
var cnt = pq.Count;
return (cnt == 0) ? 0 : cnt - 1;
}
}
private Tuple<KeyT, V> pkmn {
get {
if (pq.Count == 0) return null;
else {
var mn = pq[0];
return new Tuple<KeyT, V>(mn.k, mn.v);
}
}
}
private void psh(KeyT k, V v) { // add extra very high item if none
if (pq.Count == 0) pq.Add(new HeapEntry(UInt32.MaxValue, v));
var i = pq.Count; pq.Add(pq[i - 1]); // copy bottom item...
for (var ni = i >> 1; ni > 0; i >>= 1, ni >>= 1) {
var t = pq[ni - 1];
if (t.k > k) pq[i - 1] = t; else break;
}
pq[i - 1] = new HeapEntry(k, v);
}
private void siftdown(KeyT k, V v, int ndx) {
var cnt = pq.Count - 1; var i = ndx;
for (var ni = i + i + 1; ni < cnt; ni = ni + ni + 1) {
var oi = i; var lk = pq[ni].k; var rk = pq[ni + 1].k;
var nk = k;
if (k > lk) { i = ni; nk = lk; }
if (nk > rk) { ni += 1; i = ni; }
if (i != oi) pq[oi] = pq[i]; else break;
}
pq[i] = new HeapEntry(k, v);
}
private void rplcmin(KeyT k, V v) {
if (pq.Count > 1) siftdown(k, v, 0);
}
private void dltmin() {
var lsti = pq.Count - 2;
if (lsti <= 0) pq.Clear();
else {
var lkv = pq[lsti];
pq.RemoveAt(lsti); siftdown(lkv.k, lkv.v, 0);
}
}
private void reheap(int i) {
var lfti = i + i + 1;
if (lfti < sz) {
var rghti = lfti + 1; reheap(lfti); reheap(rghti);
var ckv = pq[i]; siftdown(ckv.k, ckv.v, i);
}
}
private void bld(IEnumerable<Tuple<KeyT, V>> sq) {
var sqm = from e in sq
select new HeapEntry(e.Item1, e.Item2);
pq = sqm.ToList<HeapEntry>();
var sz = pq.Count;
if (sz > 0) {
var lkv = pq[sz - 1];
pq.Add(new HeapEntry(KeyT.MaxValue, lkv.v));
reheap(0);
}
}
private IEnumerable<Tuple<KeyT, V>> sq() {
return from e in pq
where e.k != KeyT.MaxValue
select new Tuple<KeyT, V>(e.k, e.v); }
private void adj(Func<KeyT, V, Tuple<KeyT, V>> f) {
var cnt = pq.Count - 1;
for (var i = 0; i < cnt; ++i) {
var e = pq[i];
var r = f(e.k, e.v);
pq[i] = new HeapEntry(r.Item1, r.Item2);
}
reheap(0);
}
public static MinHeapPQ<V> empty { get { return new MinHeapPQ<V>(); } }
public static bool isEmpty(MinHeapPQ<V> pq) { return pq.mt; }
public static int size(MinHeapPQ<V> pq) { return pq.sz; }
public static Tuple<KeyT, V> peekMin(MinHeapPQ<V> pq) { return pq.pkmn; }
public static MinHeapPQ<V> push(KeyT k, V v, MinHeapPQ<V> pq) {
pq.psh(k, v); return pq; }
public static MinHeapPQ<V> replaceMin(KeyT k, V v, MinHeapPQ<V> pq) {
pq.rplcmin(k, v); return pq; }
public static MinHeapPQ<V> deleteMin(MinHeapPQ<V> pq) { pq.dltmin(); return pq; }
public static MinHeapPQ<V> merge(MinHeapPQ<V> pq1, MinHeapPQ<V> pq2) {
return fromSeq(pq1.sq().Concat(pq2.sq())); }
public static MinHeapPQ<V> adjust(Func<KeyT, V, Tuple<KeyT, V>> f, MinHeapPQ<V> pq) {
pq.adj(f); return pq; }
public static MinHeapPQ<V> fromSeq(IEnumerable<Tuple<KeyT, V>> sq) {
var pq = new MinHeapPQ<V>(); pq.bld(sq); return pq; }
public static Tuple<Tuple<KeyT, V>, MinHeapPQ<V>> popMin(MinHeapPQ<V> pq) {
var rslt = pq.pkmn; if (rslt == null) return null;
pq.dltmin(); return new Tuple<Tuple<KeyT, V>, MinHeapPQ<V>>(rslt, pq); }
public static IEnumerable<Tuple<KeyT, V>> toSeq(MinHeapPQ<V> pq) {
for (; !pq.mt; pq.dltmin()) yield return pq.pkmn; }
public static IEnumerable<Tuple<KeyT, V>> sort(IEnumerable<Tuple<KeyT, V>> sq) {
return toSeq(fromSeq(sq)); }
}
}</lang>
The above class code offers a full set of static methods and properties:
1. "empty" to create a new empty queue, 2. "isEmpty" to test if a queue is empty, 3. "size" to get the number of elements in the queue, 4. "peekMin" to retrieve the lowest priority key/value pair entry as a Tuple (possibly null for empty queues), 5. "push" to insert an entry, 6. "deleteMin" to remove the lowest priority entry, 7. "replaceMin" to replace the lowest priority and adjust the queue according to the value (faster than a "deleteMin" followed by a "push"), 8. "adjust" to apply a function to every key/value entry pair and reheapify the result, 9. "merge" to merge two queues into a single reheapified result, 10. "fromSeq" to build a queue from a sequence of key/value pair tuples, 11. "popMin" which is a convenience function combining a "peekMin" with a "deleteMin", returning null if the queue is empty and a tuple of the result otherwise, 12. "toSeq" to output an ordered sequence of the queue contents as Tuple's of the key/value pairs, and 13. "sort" which is a convenience function combining "fromSeq" followed by "toSeq".
The first four are all O(1) and the remainder O(log n) except "adjust" and "fromSeq" are O(n), "merge" is O(m + n) where m and n are the sizes of the two queues, and "toSeq" and "sort" are O(n log n); "replaceMin" is still O(log n) but faster than a "deleteMin" followed by a "push" by a constant factor.
Note that the Key type "KeyT" is not generic in order to give better comparison efficiency than using generic comparison using the IComparible interface but can be changed to different numeric types using the "using KeyT = ???" type alias.
The above code can be tested as per the page specification by the following code: <lang csharp> static void Main(string[] args) {
Tuple<uint, string>[] ins = { new Tuple<uint,string>(3u, "Clear drains"),
new Tuple<uint,string>(4u, "Feed cat"),
new Tuple<uint,string>(5u, "Make tea"),
new Tuple<uint,string>(1u, "Solve RC tasks"),
new Tuple<uint,string>(2u, "Tax return") };
var spq = ins.Aggregate(MinHeapPQ<string>.empty, (pq, t) => MinHeapPQ<string>.push(t.Item1, t.Item2, pq));
foreach (var e in MinHeapPQ<string>.toSeq(spq)) Console.WriteLine(e); Console.WriteLine();
foreach (var e in MinHeapPQ<string>.sort(ins)) Console.WriteLine(e); Console.WriteLine();
var npq = MinHeapPQ<string>.fromSeq(ins);
foreach (var e in MinHeapPQ<string>.toSeq(MinHeapPQ<string>.merge(npq, npq)))
Console.WriteLine(e); Console.WriteLine();
var npq = MinHeapPQ<string>.fromSeq(ins);
foreach (var e in MinHeapPQ<string>.toSeq(MinHeapPQ<string>.merge(npq, npq)))
Console.WriteLine(e);
foreach (var e in MinHeapPQ<string>.toSeq(MinHeapPQ<string>.adjust((k, v) => new Tuple<uint,string>(6u - k, v), npq)))
Console.WriteLine(e); Console.WriteLine();
}</lang>
It tests building the queue the slow way using repeated "push"'s - O(n log n), the faster "fromSeq" (included in the "sort") - O(n), and also tests the "merge" and "adjust" methods.
The output of the above test is as follows:
- Output:
(1, Solve RC tasks) (2, Tax return) (3, Clear drains) (4, Feed cat) (5, Make tea) (1, Solve RC tasks) (2, Tax return) (3, Clear drains) (4, Feed cat) (5, Make tea) (1, Solve RC tasks) (1, Solve RC tasks) (2, Tax return) (2, Tax return) (3, Clear drains) (3, Clear drains) (4, Feed cat) (4, Feed cat) (5, Make tea) (5, Make tea) (1, Make tea) (2, Feed cat) (3, Clear drains) (4, Tax return) (5, Solve RC tasks)
C++
The C++ standard library contains the std::priority_queue opaque data structure. It implements a max-heap.
<lang cpp>#include <iostream>
- include <string>
- include <queue>
- include <utility>
int main() {
std::priority_queue<std::pair<int, std::string> > pq; pq.push(std::make_pair(3, "Clear drains")); pq.push(std::make_pair(4, "Feed cat")); pq.push(std::make_pair(5, "Make tea")); pq.push(std::make_pair(1, "Solve RC tasks")); pq.push(std::make_pair(2, "Tax return"));
while (!pq.empty()) {
std::cout << pq.top().first << ", " << pq.top().second << std::endl;
pq.pop();
}
return 0;
}</lang>
- Output:
5, Make tea 4, Feed cat 3, Clear drains 2, Tax return 1, Solve RC tasks
Alternately, you can use a pre-existing container of yours and use the heap operations to manipulate it:
<lang cpp>#include <iostream>
- include <string>
- include <vector>
- include <algorithm>
- include <utility>
int main() {
std::vector<std::pair<int, std::string> > pq; pq.push_back(std::make_pair(3, "Clear drains")); pq.push_back(std::make_pair(4, "Feed cat")); pq.push_back(std::make_pair(5, "Make tea")); pq.push_back(std::make_pair(1, "Solve RC tasks"));
// heapify std::make_heap(pq.begin(), pq.end());
// enqueue pq.push_back(std::make_pair(2, "Tax return")); std::push_heap(pq.begin(), pq.end());
while (!pq.empty()) {
// peek
std::cout << pq[0].first << ", " << pq[0].second << std::endl;
// dequeue
std::pop_heap(pq.begin(), pq.end());
pq.pop_back();
}
return 0;
}</lang>
- Output:
5, Make tea 4, Feed cat 3, Clear drains 2, Tax return 1, Solve RC tasks
Clojure
<lang clojure>user=> (use 'clojure.data.priority-map)
- priority-map can be used as a priority queue
user=> (def p (priority-map "Clear drains" 3, "Feed cat" 4, "Make tea" 5, "Solve RC tasks" 1))
- 'user/p
user=> p {"Solve RC tasks" 1, "Clear drains" 3, "Feed cat" 4, "Make tea" 5}
- You can use assoc or conj to add items
user=> (assoc p "Tax return" 2) {"Solve RC tasks" 1, "Tax return" 2, "Clear drains" 3, "Feed cat" 4, "Make tea" 5}
- peek to get first item, pop to give you back the priority-map with the first item removed
user=> (peek p) ["Solve RC tasks" 1]
- Merge priority-maps together
user=> (into p [["Wax Car" 4]["Paint Fence" 1]["Sand Floor" 3]]) {"Solve RC tasks" 1, "Paint Fence" 1, "Clear drains" 3, "Sand Floor" 3, "Wax Car" 4, "Feed cat" 4, "Make tea" 5}</lang>
CLU
This is a priority queue based on a binary heap. It uses CLU's dynamic array to store the data.
There are no intrinsic limits on what kind of data can be used for the priority or the values themselves, except that the priority datatype must support the less-than operator.
<lang clu>prio_queue = cluster [P, T: type] is new, empty, push, pop
where P has lt: proctype (P,P) returns (bool)
item = struct[prio: P, val: T]
rep = array[item]
new = proc () returns (cvt)
return (rep$create(0))
end new
empty = proc (pq: cvt) returns (bool)
return (rep$empty(pq))
end empty
parent = proc (k: int) returns (int)
return ((k-1)/2)
end parent
left = proc (k: int) returns (int)
return (2*k + 1)
end left
right = proc (k: int) returns (int)
return (2*k + 2)
end right
swap = proc (pq: rep, a: int, b: int)
temp: item := pq[a]
pq[a] := pq[b]
pq[b] := temp
end swap
min_heapify = proc (pq: rep, k: int)
l: int := left(k)
r: int := right(k)
smallest: int := k
if l < rep$size(pq) cand pq[l].prio < pq[smallest].prio then
smallest := l
end
if r < rep$size(pq) cand pq[r].prio < pq[smallest].prio then
smallest := r
end
if smallest ~= k then
swap(pq, k, smallest)
min_heapify(pq, smallest)
end
end min_heapify
push = proc (pq: cvt, prio: P, val: T)
rep$addh(pq, item${prio: prio, val: val})
i: int := rep$high(pq)
while i ~= 0 cand pq[i].prio < pq[parent(i)].prio do
swap(pq, i, parent(i))
i := parent(i)
end
end push
pop = proc (pq: cvt) returns (P, T) signals (empty)
if empty(up(pq)) then signal empty end
if rep$size(pq) = 1 then
i: item := rep$remh(pq)
return (i.prio, i.val)
end
root: item := pq[0]
pq[0] := rep$remh(pq)
min_heapify(pq, 0)
return (root.prio, root.val)
end pop
end prio_queue
start_up = proc ()
% use ints for priority and strings for data
prioq = prio_queue[int,string]
% make the priority queue
pq: prioq := prioq$new()
% add some tasks
prioq$push(pq, 3, "Clear drains")
prioq$push(pq, 4, "Feed cat")
prioq$push(pq, 5, "Make tea")
prioq$push(pq, 1, "Solve RC tasks")
prioq$push(pq, 2, "Tax return")
% print them all out in order
po: stream := stream$primary_output()
while ~prioq$empty(pq) do
prio: int task: string
prio, task := prioq$pop(pq)
stream$putl(po, int$unparse(prio) || ": " || task)
end
end start_up</lang>
- Output:
1: Solve RC tasks 2: Tax return 3: Clear drains 4: Feed cat 5: Make tea
CoffeeScript
<lang coffeescript> PriorityQueue = ->
# Use closure style for object creation (so no "new" required).
# Private variables are toward top.
h = []
better = (a, b) ->
h[a].priority < h[b].priority
swap = (a, b) ->
[h[a], h[b]] = [h[b], h[a]]
sift_down = ->
max = h.length
n = 0
while n < max
c1 = 2*n + 1
c2 = c1 + 1
best = n
best = c1 if c1 < max and better(c1, best)
best = c2 if c2 < max and better(c2, best)
return if best == n
swap n, best
n = best
sift_up = ->
n = h.length - 1
while n > 0
parent = Math.floor((n-1) / 2)
return if better parent, n
swap n, parent
n = parent
# now return the public interface, which is an object that only
# has functions on it
self =
size: ->
h.length
push: (priority, value) ->
elem =
priority: priority
value: value
h.push elem
sift_up()
pop: ->
throw Error("cannot pop from empty queue") if h.length == 0
value = h[0].value
last = h.pop()
if h.length > 0
h[0] = last
sift_down()
value
- test
do ->
pq = PriorityQueue() pq.push 3, "Clear drains" pq.push 4, "Feed cat" pq.push 5, "Make tea" pq.push 1, "Solve RC tasks" pq.push 2, "Tax return"
while pq.size() > 0
console.log pq.pop()
# test high performance
for n in [1..100000]
priority = Math.random()
pq.push priority, priority
v = pq.pop()
console.log "First random element was #{v}"
while pq.size() > 0
new_v = pq.pop()
throw Error "Queue broken" if new_v < v
v = new_v
console.log "Final random element was #{v}"
</lang>
output
<lang> > coffee priority_queue.coffee Solve RC tasks Tax return Clear drains Feed cat Make tea First random element was 0.00002744467929005623 Final random element was 0.9999718656763434 </lang>
Common Lisp
In this task were implemented to versions of the functions, the non-destructive ones that will not change the state of the priority queue and the destructive ones that will change. The destructive ones work very similarly to the 'pop' and 'push' functions. <lang lisp>
- priority-queue's are implemented with association lists
(defun make-pq (alist)
(sort (copy-alist alist) (lambda (a b) (< (car a) (car b)))))
- Will change the state of pq
(define-modify-macro insert-pq (pair)
(lambda (pq pair) (sort-alist (cons pair pq))))
(define-modify-macro remove-pq-aux () cdr)
(defmacro remove-pq (pq)
`(let ((aux (copy-alist ,pq)))
(REMOVE-PQ-AUX ,pq)
(car aux)))
- Will not change the state of pq
(defun insert-pq-non-destructive (pair pq)
(sort-alist (cons pair pq)))
(defun remove-pq-non-destructive (pq)
(cdr pq))
- testing
(defparameter a (make-pq '((1 . "Solve RC tasks") (3 . "Clear drains") (2 . "Tax return") (5 . "Make tea")))) (format t "~a~&" a) (insert-pq a '(4 . "Feed cat")) (format t "~a~&" a) (format t "~a~&" (remove-pq a)) (format t "~a~&" a) (format t "~a~&" (remove-pq a)) (format t "~a~&" a) </lang>
- Output:
((1 . Solve RC tasks) (2 . Tax return) (3 . Clear drains) (5 . Make tea)) ((1 . Solve RC tasks) (2 . Tax return) (3 . Clear drains) (4 . Feed cat) (5 . Make tea)) (1 . Solve RC tasks) ((2 . Tax return) (3 . Clear drains) (4 . Feed cat) (5 . Make tea)) (2 . Tax return) ((3 . Clear drains) (4 . Feed cat) (5 . Make tea))
Component Pascal
BlackBox Component Builder <lang oberon2> MODULE PQueues; IMPORT StdLog,Boxes;
TYPE
Rank* = POINTER TO RECORD
p-: LONGINT; (* Priority *)
value-: Boxes.Object
END;
PQueue* = POINTER TO RECORD
a: POINTER TO ARRAY OF Rank;
size-: LONGINT;
END;
PROCEDURE NewRank*(p: LONGINT; v: Boxes.Object): Rank;
VAR
r: Rank;
BEGIN
NEW(r);r.p := p;r.value := v;
RETURN r
END NewRank;
PROCEDURE NewPQueue*(cap: LONGINT): PQueue;
VAR
pq: PQueue;
BEGIN
NEW(pq);pq.size := 0;
NEW(pq.a,cap);pq.a[0] := NewRank(MIN(INTEGER),NIL);
RETURN pq
END NewPQueue;
PROCEDURE (pq: PQueue) Push*(r:Rank), NEW;
VAR
i: LONGINT;
BEGIN
INC(pq.size);
i := pq.size;
WHILE r.p < pq.a[i DIV 2].p DO
pq.a[i] := pq.a[i DIV 2];i := i DIV 2
END;
pq.a[i] := r
END Push;
PROCEDURE (pq: PQueue) Pop*(): Rank,NEW;
VAR
r,y: Rank;
i,j: LONGINT;
ok: BOOLEAN;
BEGIN
r := pq.a[1]; (* Priority object *)
y := pq.a[pq.size]; DEC(pq.size); i := 1; ok := FALSE;
WHILE (i <= pq.size DIV 2) & ~ok DO
j := i + 1;
IF (j < pq.size) & (pq.a[i].p > pq.a[j + 1].p) THEN INC(j) END;
IF y.p > pq.a[j].p THEN pq.a[i] := pq.a[j]; i := j ELSE ok := TRUE END
END;
pq.a[i] := y;
RETURN r
END Pop;
PROCEDURE (pq: PQueue) IsEmpty*(): BOOLEAN,NEW;
BEGIN
RETURN pq.size = 0
END IsEmpty;
PROCEDURE Test*;
VAR
pq: PQueue;
r: Rank;
PROCEDURE ShowRank(r:Rank);
BEGIN
StdLog.Int(r.p);StdLog.String(":> ");StdLog.String(r.value.AsString());StdLog.Ln;
END ShowRank;
BEGIN
pq := NewPQueue(128);
pq.Push(NewRank(3,Boxes.NewString("Clear drains")));
pq.Push(NewRank(4,Boxes.NewString("Feed cat")));
pq.Push(NewRank(5,Boxes.NewString("Make tea")));
pq.Push(NewRank(1,Boxes.NewString("Solve RC tasks")));
pq.Push(NewRank(2,Boxes.NewString("Tax return")));
ShowRank(pq.Pop());
ShowRank(pq.Pop());
ShowRank(pq.Pop());
ShowRank(pq.Pop());
ShowRank(pq.Pop());
END Test;
END PQueues. </lang> Interface extracted from the implementation <lang oberon2> DEFINITION PQueues;
IMPORT Boxes;
TYPE
PQueue = POINTER TO RECORD
size-: LONGINT;
(pq: PQueue) IsEmpty (): BOOLEAN, NEW;
(pq: PQueue) Pop (): Rank, NEW;
(pq: PQueue) Push (r: Rank), NEW
END;
Rank = POINTER TO RECORD
p-: LONGINT;
value-: Boxes.Object
END;
PROCEDURE NewPQueue (cap: LONGINT): PQueue; PROCEDURE NewRank (p: LONGINT; v: Boxes.Object): Rank; PROCEDURE Test;
END PQueues.
</lang>
Execute: ^Q PQueues.Test
Output:
1:> Solve RC tasks 2:> Tax return 3:> Clear drains 4:> Feed cat 5:> Make tea
D
<lang d>import std.stdio, std.container, std.array, std.typecons;
void main() {
alias tuple T;
auto heap = heapify([T(3, "Clear drains"),
T(4, "Feed cat"),
T(5, "Make tea"),
T(1, "Solve RC tasks"),
T(2, "Tax return")]);
while (!heap.empty) {
writeln(heap.front);
heap.removeFront();
}
}</lang>
- Output:
Tuple!(int,string)(5, "Make tea") Tuple!(int,string)(4, "Feed cat") Tuple!(int,string)(3, "Clear drains") Tuple!(int,string)(2, "Tax return") Tuple!(int,string)(1, "Solve RC tasks")
Delphi
Boost.Generics.Collection is part of DelphiBoostLib <lang Delphi>program Priority_queue;
{$APPTYPE CONSOLE}
uses
System.SysUtils, Boost.Generics.Collection;
var
Queue: TPriorityQueue<String>;
begin
Queue := TPriorityQueue<String>.Create(['Clear drains', 'Feed cat', 'Make tea', 'Solve RC tasks', 'Tax return'], [3, 4, 5, 1, 2]);
while not Queue.IsEmpty do
with Queue.DequeueEx do
Writeln(Priority, ', ', value);
end.</lang>
- Output:
1, Solve RC tasks 2, Tax return 3, Clear drains 4, Feed cat 5, Make tea
EchoLisp
We use the built-in binary tree library. Each tree node has a datum (key . value). The functions (bin-tree-pop-first tree) and (bin-tree-pop-last tree) allow to extract the node with highest or lowest priority. <lang lisp> (lib 'tree) (define tasks (make-bin-tree 3 "Clear drains")) (bin-tree-insert tasks 2 "Tax return") (bin-tree-insert tasks 5 "Make tea") (bin-tree-insert tasks 1 "Solve RC tasks") (bin-tree-insert tasks 4 "Feed 🐡")
(bin-tree-pop-first tasks) → (1 . "Solve RC tasks") (bin-tree-pop-first tasks) → (2 . "Tax return") (bin-tree-pop-first tasks) → (3 . "Clear drains") (bin-tree-pop-first tasks) → (4 . "Feed 🐡") (bin-tree-pop-first tasks) → (5 . "Make tea") (bin-tree-pop-first tasks) → null
- similarly
(bin-tree-pop-last tasks) → (5 . "Make tea") (bin-tree-pop-last tasks) → (4 . "Feed 🐡")
- etc.
</lang>
Elixir
<lang elixir>defmodule Priority do
def create, do: :gb_trees.empty
def insert( element, priority, queue ), do: :gb_trees.enter( priority, element, queue )
def peek( queue ) do
{_priority, element, _new_queue} = :gb_trees.take_smallest( queue )
element
end
def task do
items = [{3, "Clear drains"}, {4, "Feed cat"}, {5, "Make tea"}, {1, "Solve RC tasks"}, {2, "Tax return"}]
queue = Enum.reduce(items, create, fn({priority, element}, acc) -> insert( element, priority, acc ) end)
IO.puts "peek priority: #{peek( queue )}"
Enum.reduce(1..length(items), queue, fn(_n, q) -> write_top( q ) end)
end
def top( queue ) do
{_priority, element, new_queue} = :gb_trees.take_smallest( queue )
{element, new_queue}
end
defp write_top( q ) do
{element, new_queue} = top( q )
IO.puts "top priority: #{element}"
new_queue
end
end
Priority.task</lang>
- Output:
peek priority: Solve RC tasks top priority: Solve RC tasks top priority: Tax return top priority: Clear drains top priority: Feed cat top priority: Make tea
Erlang
Using built in gb_trees module, with the suggested interface for this task. <lang Erlang> -module( priority_queue ).
-export( [create/0, insert/3, peek/1, task/0, top/1] ).
create() -> gb_trees:empty().
insert( Element, Priority, Queue ) -> gb_trees:enter( Priority, Element, Queue ).
peek( Queue ) ->
{_Priority, Element, _New_queue} = gb_trees:take_smallest( Queue ),
Element.
task() ->
Items = [{3, "Clear drains"}, {4, "Feed cat"}, {5, "Make tea"}, {1, "Solve RC tasks"}, {2, "Tax return"}],
Queue = lists:foldl( fun({Priority, Element}, Acc) -> insert( Element, Priority, Acc ) end, create(), Items ),
io:fwrite( "peek priority: ~p~n", [peek( Queue )] ),
lists:foldl( fun(_N, Q) -> write_top( Q ) end, Queue, lists:seq(1, erlang:length(Items)) ).
top( Queue ) ->
{_Priority, Element, New_queue} = gb_trees:take_smallest( Queue ),
{Element, New_queue}.
write_top( Q ) ->
{Element, New_queue} = top( Q ),
io:fwrite( "top priority: ~p~n", [Element] ),
New_queue.
</lang>
- Output:
12> priority_queue:task(). peek priority: "Solve RC tasks" top priority: "Solve RC tasks" top priority: "Tax return" top priority: "Clear drains" top priority: "Feed cat" top priority: "Make tea"
F#
The below codes all provide the standard priority queue functions of "peekMin", "push", and "deleteMin"; as well, "replaceMin" which can be much more efficient that a "deleteMin" followed by a "push" for some types of queues), "popMin" (generally a convenience function for "peekMin" followed by "deleteMin"), "adjust" for applying a function to all queue entries and reheapifying, "fromSeq" for building a queue from a sequence, "toSeq" for outputting a sorted sequence of the queue contents, and "sort" which is a convenience function combining the latter two functions are provided. Finally, the queue's all provide a "merge" function to combine two queues into one, and an "adjust" function which applies a function to every heap element and reheapifies.
Functional
Binomial Heap Priority Queue
The following Binomial Heap Priority Queue code has been adapted from a version by "DeeJay" updated for changes in F# over the intervening years, and implementing the O(1) "peekMin" mentioned in that post; in addition to the above standard priority queue functions, it also implements the "merge" function for which the Binomial Heap is particularly suited, taking O(log n) time rather than the usual O(n) (or worse) time: <lang fsharp>[<RequireQualifiedAccess>] module PriorityQ =
// type 'a treeElement = Element of uint32 * 'a
type 'a treeElement = struct val k:uint32 val v:'a new(k,v) = { k=k;v=v } end
type 'a tree = Node of uint32 * 'a treeElement * 'a tree list
type 'a heap = 'a tree list
[<CompilationRepresentation(CompilationRepresentationFlags.UseNullAsTrueValue)>] [<NoEquality; NoComparison>] type 'a outerheap = | HeapEmpty | HeapNotEmpty of 'a treeElement * 'a heap
let empty = HeapEmpty
let isEmpty = function | HeapEmpty -> true | _ -> false
let inline private rank (Node(r,_,_)) = r
let inline private root (Node(_,x,_)) = x
exception Empty_Heap
let peekMin = function | HeapEmpty -> None
| HeapNotEmpty(min, _) -> Some (min.k, min.v)
let rec private findMin heap =
match heap with | [] -> raise Empty_Heap //guarded so should never happen
| [node] -> root node,[]
| topnode::heap' ->
let min,subheap = findMin heap' in let rtn = root topnode
match subheap with
| [] -> if rtn.k > min.k then min,[] else rtn,[]
| minnode::heap ->
let rmn = root minnode
if rtn.k <= rmn.k then rtn,heap
else rmn,minnode::topnode::heap
let private mergeTree (Node(r,kv1,ts1) as tree1) (Node (_,kv2,ts2) as tree2) = if kv1.k > kv2.k then Node(r+1u,kv2,tree1::ts2) else Node(r+1u,kv1,tree2::ts1)
let rec private insTree (newnode: 'a tree) heap =
match heap with
| [] -> [newnode]
| topnode::heap' -> if (rank newnode) < (rank topnode) then newnode::heap
else insTree (mergeTree newnode topnode) heap'
let push k v = let kv = treeElement(k,v) in let nn = Node(0u,kv,[])
function | HeapEmpty -> HeapNotEmpty(kv,[nn])
| HeapNotEmpty(min,heap) -> let nmin = if k > min.k then min else kv
HeapNotEmpty(nmin,insTree nn heap)
let rec private merge' heap1 heap2 = //doesn't guaranty minimum tree node as head!!!
match heap1,heap2 with
| _,[] -> heap1
| [],_ -> heap2
| topheap1::heap1',topheap2::heap2' ->
match compare (rank topheap1) (rank topheap2) with
| -1 -> topheap1::merge' heap1' heap2
| 1 -> topheap2::merge' heap1 heap2'
| _ -> insTree (mergeTree topheap1 topheap2) (merge' heap1' heap2')
let merge oheap1 oheap2 = match oheap1,oheap2 with
| _,HeapEmpty -> oheap1
| HeapEmpty,_ -> oheap2
| HeapNotEmpty(min1,heap1),HeapNotEmpty(min2,heap2) ->
let min = if min1.k > min2.k then min2 else min1
HeapNotEmpty(min,merge' heap1 heap2)
let rec private removeMinTree = function
| [] -> raise Empty_Heap // will never happen as already guarded
| [node] -> node,[]
| t::ts -> let t',ts' = removeMinTree ts
if (root t).k <= (root t').k then t,ts else t',t::ts'
let deleteMin =
function | HeapEmpty -> HeapEmpty
| HeapNotEmpty(_,heap) ->
match heap with
| [] -> HeapEmpty // should never occur: non empty heap with no elements
| [Node(_,_,heap')] -> match heap' with
| [] -> HeapEmpty
| _ -> let min,_ = findMin heap'
HeapNotEmpty(min,heap')
| _::_ -> let Node(_,_,ts1),ts2 = removeMinTree heap
let nheap = merge' (List.rev ts1) ts2 in let min,_ = findMin nheap
HeapNotEmpty(min,nheap)
let replaceMin k v pq = push k v (deleteMin pq)
let fromSeq sq = Seq.fold (fun pq (k, v) -> push k v pq) empty sq
let popMin pq = match peekMin pq with
| None -> None
| Some(kv) -> Some(kv, deleteMin pq)
let toSeq pq = Seq.unfold popMin pq
let sort sq = sq |> fromSeq |> toSeq
let adjust f pq = pq |> toSeq |> Seq.map (fun (k, v) -> f k v) |> fromSeq</lang>
"isEmpty", "empty", and "peekMin" all have O(1) performance, "push" is O(1) amortized performance with O(log n) worst case, and the rest are O(log n) except for "fromSeq" (and thus "sort" and "adjust") which have O(n log n) performance since they use repeated "deleteMin" with one per entry.
No "size" function is provided, but it would be implemented by summing the total size of all the nested tree lists, which each have a "Count" property and thus would be quite fast.
Note that the current "adjust" function is horribly inefficient as it outputs the original queue as a sorted sequence (O(n log n) time complexity), maps the adjusting function to each element, and rebuilds the queue be repeated "push" operations of the resulting sequence. This could be improved by re-writing to output the sequence in unsorted order (using an internal function that doesn't use repeated "deleteMin" operations) and then rebuilding from the adjusted sequence; doing this would make the "adjust" operation take O(n) amortized time.
The "sort" function also uses a similar technique of building a queue from a sequence by repeated "push" operations (however, those only take O(n) amortized time for the Binomial Heap), then outputting a sorted sequence by repeated "popMin" operations for a combined O(n log n) time complexity.
Min Heap Priority Queue
The following code implementing a Min Heap Priority Queue is adapted from the ML PRIORITY_QUEUE code by Lawrence C. Paulson including separating the key/value pairs as separate entries in the data structure for better comparison efficiency; it implements an efficient "fromSeq" function using reheapify for which the Min Heap is particularly suited as it has only O(n) instead of O(n log n) computational time complexity, which method is also used for the "adjust" and "merge" functions: <lang fsharp>[<RequireQualifiedAccess>] module PriorityQ =
type HeapEntry<'V> = struct val k:uint32 val v:'V new(k,v) = {k=k;v=v} end
[<CompilationRepresentation(CompilationRepresentationFlags.UseNullAsTrueValue)>]
[<NoEquality; NoComparison>]
type PQ<'V> =
| Mt
| Br of HeapEntry<'V> * PQ<'V> * PQ<'V>
let empty = Mt
let isEmpty = function | Mt -> true
| _ -> false
// Return number of elements in the priority queue.
// /O(log(n)^2)/
let rec size = function
| Mt -> 0
| Br(_, ll, rr) ->
let n = size rr
// rest n p q, where n = size ll, and size ll - size rr = 0 or 1
// returns 1 + size ll - size rr.
let rec rest n pl pr =
match pl with
| Mt -> 1
| Br(_, pll, plr) ->
match pr with
| Mt -> 2
| Br(_, prl, prr) ->
let nm1 = n - 1 in let d = nm1 >>> 1
if (nm1 &&& 1) = 0
then rest d pll prl // subtree sizes: (d or d+1), d; d, d
else rest d plr prr // subtree sizes: d+1, (d or d+1); d+1, d
2 * n + rest n ll rr
let peekMin = function | Br(kv, _, _) -> Some(kv.k, kv.v)
| _ -> None
let rec push wk wv =
function | Mt -> Br(HeapEntry(wk, wv), Mt, Mt)
| Br(vkv, ll, rr) ->
if wk <= vkv.k then
Br(HeapEntry(wk, wv), push vkv.k vkv.v rr, ll)
else Br(vkv, push wk wv rr, ll)
let inline private siftdown wk wv pql pqr =
let rec sift pl pr =
match pl with
| Mt -> Br(HeapEntry(wk, wv), Mt, Mt)
| Br(vkvl, pll, plr) ->
match pr with
| Mt -> if wk <= vkvl.k then Br(HeapEntry(wk, wv), pl, Mt)
else Br(vkvl, Br(HeapEntry(wk, wv), Mt, Mt), Mt)
| Br(vkvr, prl, prr) ->
if wk <= vkvl.k && wk <= vkvr.k then Br(HeapEntry(wk, wv), pl, pr)
elif vkvl.k <= vkvr.k then Br(vkvl, sift pll plr, pr)
else Br(vkvr, pl, sift prl prr)
sift pql pqr
let replaceMin wk wv = function | Mt -> Mt
| Br(_, ll, rr) -> siftdown wk wv ll rr
let deleteMin = function
| Mt -> Mt
| Br(_, ll, Mt) -> ll
| Br(vkv, ll, rr) ->
let rec leftrem = function | Mt -> vkv, Mt // should never happen
| Br(kvd, Mt, _) -> kvd, Mt
| Br(vkv, Br(kvd, _, _), Mt) ->
kvd, Br(vkv, Mt, Mt)
| Br(vkv, pl, pr) -> let kvd, pqd = leftrem pl
kvd, Br(vkv, pr, pqd)
let (kvd, pqd) = leftrem ll
siftdown kvd.k kvd.v rr pqd;
let adjust f pq =
let rec adj = function
| Mt -> Mt
| Br(vkv, ll, rr) -> let nk, nv = f vkv.k vkv.v
siftdown nk nv (adj ll) (adj rr)
adj pq
let fromSeq sq =
if Seq.isEmpty sq then Mt
else let nmrtr = sq.GetEnumerator()
let rec build lvl = if lvl = 0 || not (nmrtr.MoveNext()) then Mt
else let ck, cv = nmrtr.Current
let lft = lvl >>> 1
let rght = (lvl - 1) >>> 1
siftdown ck cv (build lft) (build rght)
build (sq |> Seq.length)
let merge (pq1:PQ<_>) (pq2:PQ<_>) = // merges without using a sequence
match pq1 with
| Mt -> pq2
| _ ->
match pq2 with
| Mt -> pq1
| _ ->
let rec zipper lvl pq rest =
if lvl = 0 then Mt, pq, rest else
let lft = lvl >>> 1 in let rght = (lvl - 1) >>> 1
match pq with
| Mt ->
match rest with
| [] | Mt :: _ -> Mt, pq, [] // Mt in list never happens
| Br(kv, ll, Mt) :: tl ->
let pl, pql, rstl = zipper lft ll tl
let pr, pqr, rstr = zipper rght pql rstl
siftdown kv.k kv.v pl pr, pqr, rstr
| Br(kv, ll, rr) :: tl ->
let pl, pql, rstl = zipper lft ll (rr :: tl)
let pr, pqr, rstr = zipper rght pql rstl
siftdown kv.k kv.v pl pr, pqr, rstr
| Br(kv, ll, Mt) ->
let pl, pql, rstl = zipper lft ll rest
let pr, pqr, rstr = zipper rght pql rstl
siftdown kv.k kv.v pl pr, pqr, rstr
| Br(kv, ll, rr) ->
let pl, pql, rstl = zipper lft ll (rr :: rest)
let pr, pqr, rstr = zipper rght pql rstl
siftdown kv.k kv.v pl pr, pqr, rstr
let sz = size pq1 + size pq2
let pq, _, _ = zipper sz pq1 [pq2] in pq
let popMin pq = match peekMin pq with
| None -> None
| Some(kv) -> Some(kv, deleteMin pq)
let toSeq pq = Seq.unfold popMin pq
let sort sq = sq |> fromSeq |> toSeq</lang>
The above code implements a "merge" function so that no internal sequence generation is necessary as generation of sequence iterators is quite inefficient in F# for a combined O(n) computational time complexity but a considerable reduction in the constant factor overhead.
Other than the "merge" function, the Min Heap Priority Queue has the same time complexity as for the Binomial Heap Priority Queue above except that "push" has O(log n) performance rather than the amortized O(1) performance; however, the Binomial Heap Priority Queue is generally a constant factor slower due to more complex operations. The Binomial Heap Priority Queue is generally more suited when used where merging of large queues or frequent "push" operations are used; the Min Heap Priority Queue is more suitable for use when replacing the value at the minimum entry of the queue is frequently required, especially when the adjusted value is not displaced very far down the queue on average.
Imperative
Min Heap Priority Queue
As the Min Heap is usually implemented as a mutable array binary heap after a genealogical tree based model invented by Michael Eytzinger over 400 years ago, the following "ugly imperative" code implements the Min Heap Priority Queue this way; note that the code could be implemented not using "ugly" mutable state variables other than the contents of the array (DotNet List which implements a growable array) but in this case the code would be considerably slower as in not much faster or slower than the functional version since using mutable side effects greatly reduces the number of operations: <lang fsharp>[<RequireQualifiedAccess>] module PriorityQ =
type HeapEntry<'T> = struct val k:uint32 val v:'T new(k,v) = { k=k;v=v } end
type MinHeapTree<'T> = ResizeArray<HeapEntry<'T>>
let empty<'T> = MinHeapTree<HeapEntry<'T>>()
let isEmpty (pq: MinHeapTree<_>) = pq.Count = 0
let size (pq: MinHeapTree<_>) = let cnt = pq.Count
if cnt = 0 then 0 else cnt - 1
let peekMin (pq:MinHeapTree<_>) = if pq.Count > 1 then let kv = pq.[0]
Some (kv.k, kv.v) else None
let push k v (pq:MinHeapTree<_>) =
if pq.Count = 0 then pq.Add(HeapEntry(0xFFFFFFFFu,v)) //add an extra entry so there's always a right max node
let mutable nxtlvl = pq.Count in let mutable lvl = nxtlvl <<< 1 //1 past index of value added times 2
pq.Add(pq.[nxtlvl - 1]) //copy bottom entry then do bubble up while less than next level up
while ((lvl <- lvl >>> 1); nxtlvl <- nxtlvl >>> 1; nxtlvl <> 0) do
let t = pq.[nxtlvl - 1] in if t.k > k then pq.[lvl - 1] <- t else lvl <- lvl <<< 1; nxtlvl <- 0 //causes loop break
pq.[lvl - 1] <- HeapEntry(k,v); pq
let inline private siftdown k v ndx (pq: MinHeapTree<_>) =
let mutable i = ndx in let mutable ni = i in let cnt = pq.Count - 1
while (ni <- ni + ni + 1; ni < cnt) do
let lk = pq.[ni].k in let rk = pq.[ni + 1].k in let oi = i
let k = if k > lk then i <- ni; lk else k in if k > rk then ni <- ni + 1; i <- ni
if i <> oi then pq.[oi] <- pq.[i] else ni <- cnt //causes loop break
pq.[i] <- HeapEntry(k,v)
let replaceMin k v (pq:MinHeapTree<_>) = siftdown k v 0 pq; pq
let deleteMin (pq:MinHeapTree<_>) = let lsti = pq.Count - 2 if lsti <= 0 then pq.Clear(); pq else let lstkv = pq.[lsti] pq.RemoveAt(lsti) siftdown lstkv.k lstkv.v 0 pq; pq
let adjust f (pq:MinHeapTree<_>) = //adjust all the contents using the function, then re-heapify
let cnt = pq.Count - 1
let rec adj i =
let lefti = i + i + 1 in let righti = lefti + 1
let ckv = pq.[i] in let (nk, nv) = f ckv.k ckv.v
if righti < cnt then adj righti
if lefti < cnt then adj lefti; siftdown nk nv i pq
else pq.[i] <- HeapEntry(nk, nv)
adj 0; pq
let fromSeq sq =
if Seq.isEmpty sq then empty
else let pq = new MinHeapTree<_>(sq |> Seq.map (fun (k, v) -> HeapEntry(k, v)))
let sz = pq.Count in let lkv = pq.[sz - 1]
pq.Add(HeapEntry(UInt32.MaxValue, lkv.v))
let rec build i =
let lefti = i + i + 1
if lefti < sz then
let righti = lefti + 1 in build lefti; build righti
let ckv = pq.[i] in siftdown ckv.k ckv.v i pq
build 0; pq
let merge (pq1:MinHeapTree<_>) (pq2:MinHeapTree<_>) =
if pq2.Count = 0 then pq1 else
if pq1.Count = 0 then pq2 else
let pq = empty
pq.AddRange(pq1); pq.RemoveAt(pq.Count - 1)
pq.AddRange(pq2)
let sz = pq.Count - 1
let rec build i =
let lefti = i + i + 1
if lefti < sz then
let righti = lefti + 1 in build lefti; build righti
let ckv = pq.[i] in siftdown ckv.k ckv.v i pq
build 0; pq
let popMin pq = match peekMin pq with
| None -> None
| Some(kv) -> Some(kv, deleteMin pq)
let toSeq pq = Seq.unfold popMin pq
let sort sq = sq |> fromSeq |> toSeq</lang>
The comments for the above code are the same as for the functional version; the main difference is that the imperative code takes about two thirds of the time on average.
All of the above codes can be tested under the F# REPL using the following: <lang fsharp>> let testseq = [| (3u, "Clear drains");
(4u, "Feed cat");
(5u, "Make tea");
(1u, "Solve RC tasks");
(2u, "Tax return") |] |> Array.toSeq
let testpq = testseq |> MinHeap.fromSeq
testseq |> Seq.fold (fun pq (k, v) -> MinHeap.push k v pq) MinHeap.empty
|> MinHeap.toSeq |> Seq.iter (printfn "%A") // test slow build
printfn ""
testseq |> MinHeap.fromSeq |> MinHeap.toSeq // test fast build
|> Seq.iter (printfn "%A")
printfn ""
testseq |> MinHeap.sort |> Seq.iter (printfn "%A") // convenience function
printfn ""
MinHeap.merge testpq testpq // test merge
|> MinHeap.toSeq |> Seq.iter (printfn "%A")
printfn ""
testpq |> MinHeap.adjust (fun k v -> uint32 (MinHeap.size testpq) - k, v)
|> MinHeap.toSeq |> Seq.iter (printfn "%A") // test adjust;;</lang>
to produce the following output:
- Output:
(1u, "Solve RC tasks") (2u, "Tax return") (3u, "Clear drains") (4u, "Feed cat") (5u, "Make tea") (1u, "Solve RC tasks") (2u, "Tax return") (3u, "Clear drains") (4u, "Feed cat") (5u, "Make tea") (1u, "Solve RC tasks") (2u, "Tax return") (3u, "Clear drains") (4u, "Feed cat") (5u, "Make tea") (1u, "Solve RC tasks") (1u, "Solve RC tasks") (2u, "Tax return") (2u, "Tax return") (3u, "Clear drains") (3u, "Clear drains") (4u, "Feed cat") (4u, "Feed cat") (5u, "Make tea") (5u, "Make tea") (0u, "Make tea") (1u, "Feed cat") (2u, "Clear drains") (3u, "Tax return") (4u, "Solve RC tasks") val it : unit = ()
Note that the code using "fromSeq" instead of repeated "push" operations to build a queue is considerably faster for large random-order entry sequences.
Also note that the imperative version modifies the state of the "testpq" binding for modification operations such as "push" and "deleteMin" or operations derived from those; this means that if the last two tests were reversed then the "merge" would be passed zero sized queues since the "testpq" would have been reduced by the "toSeq" operation (which effectively uses repeated "deleteMin" functions).
Factor
Factor has priority queues implemented in the library: documentation is available at http://docs.factorcode.org/content/article-heaps.html (or by typing "heaps" help interactively in the listener). <lang factor><min-heap> [ {
{ 3 "Clear drains" }
{ 4 "Feed cat" }
{ 5 "Make tea" }
{ 1 "Solve RC tasks" }
{ 2 "Tax return" }
} swap heap-push-all
] [
[ print ] slurp-heap
] bi</lang>
output: <lang factor>Solve RC tasks Tax return Clear drains Feed cat Make tea</lang>
Fortran
<lang Fortran>module priority_queue_mod implicit none
type node
character (len=100) :: task integer :: priority
end type
type queue
type(node), allocatable :: buf(:) integer :: n = 0
contains
procedure :: top procedure :: enqueue procedure :: siftdown
end type
contains
subroutine siftdown(this, a)
class (queue) :: this
integer :: a, parent, child
associate (x => this%buf)
parent = a
do while(parent*2 <= this%n)
child = parent*2
if (child + 1 <= this%n) then
if (x(child+1)%priority > x(child)%priority ) then
child = child +1
end if
end if
if (x(parent)%priority < x(child)%priority) then
x([child, parent]) = x([parent, child])
parent = child
else
exit
end if
end do
end associate
end subroutine
function top(this) result (res)
class(queue) :: this type(node) :: res res = this%buf(1) this%buf(1) = this%buf(this%n) this%n = this%n - 1 call this%siftdown(1)
end function
subroutine enqueue(this, priority, task)
class(queue), intent(inout) :: this integer :: priority character(len=*) :: task type(node) :: x type(node), allocatable :: tmp(:) integer :: i x%priority = priority x%task = task this%n = this%n +1 if (.not.allocated(this%buf)) allocate(this%buf(1)) if (size(this%buf)<this%n) then allocate(tmp(2*size(this%buf))) tmp(1:this%n-1) = this%buf call move_alloc(tmp, this%buf) end if this%buf(this%n) = x i = this%n do i = i / 2 if (i==0) exit call this%siftdown(i) end do
end subroutine end module
program main
use priority_queue_mod
type (queue) :: q type (node) :: x
call q%enqueue(3, "Clear drains") call q%enqueue(4, "Feed cat") call q%enqueue(5, "Make Tea") call q%enqueue(1, "Solve RC tasks") call q%enqueue(2, "Tax return")
do while (q%n >0) x = q%top() print "(g0,a,a)", x%priority, " -> ", trim(x%task) end do
end program
! Output: ! 5 -> Make Tea ! 4 -> Feed cat ! 3 -> Clear drains ! 2 -> Tax return ! 1 -> Solve RC tasks </lang>
FreeBASIC
<lang freebasic>Type Tupla
Prioridad As Integer Tarea As String
End Type Dim Shared As Tupla a() Dim Shared As Integer n 'número de eltos. en la matriz, el último elto. es n-1
Function Izda(i As Integer) As Integer
Izda = 2 * i + 1
End Function
Function Dcha(i As Integer) As Integer
Dcha = 2 * i + 2
End Function
Function Parent(i As Integer) As Integer
Parent = (i - 1) \ 2
End Function
Sub Intercambio(i As Integer, j As Integer)
Dim t As Tupla t = a(i) a(i) = a(j) a(j) = t
End Sub
Sub bubbleUp(i As Integer)
Dim As Integer p = Parent(i)
Do While i > 0 And a(i).Prioridad < a(p).Prioridad
Intercambio i, p
i = p
p = Parent(i)
Loop
End Sub
Sub Annadir(fPrioridad As Integer, fTarea As String)
n += 1 If n > Ubound(a) Then Redim Preserve a(2 * n) a(n - 1).Prioridad = fPrioridad a(n - 1).Tarea = fTarea bubbleUp (n - 1)
End Sub
Sub trickleDown(i As Integer)
Dim As Integer j, l, r
Do
j = -1
r = Dcha(i)
If r < n And a(r).Prioridad < a(i).Prioridad Then
l = Izda(i)
If a(l).Prioridad < a(r).Prioridad Then
j = l
Else
j = r
End If
Else
l = Izda(i)
If l < n And a(l).Prioridad < a(i).Prioridad Then j = l
End If
If j >= 0 Then Intercambio i, j
i = j
Loop While i >= 0
End Sub
Function Remove() As Tupla
Dim As Tupla x = a(0) a(0) = a(n - 1) n = n - 1 trickleDown 0 If 3 * n < Ubound(a) Then Redim Preserve a(Ubound(a) \ 2) Remove = x
End Function
Redim a(4)
Annadir (3, "Clear drains")
Annadir (4, "Feed cat")
Annadir (5, "Make tea")
Annadir (1, "Solve RC tasks")
Annadir (2, "Tax return")
Dim t As Tupla
Do While n > 0
t = Remove Print t.Prioridad; " "; t.Tarea
Loop Sleep</lang>
- Output:
Igual que la entrada de VBA.
FunL
<lang funl>import util.ordering native scala.collection.mutable.PriorityQueue
data Task( priority, description )
def comparator( Task(a, _), Task(b, _) )
| a > b = -1 | a < b = 1 | otherwise = 0
q = PriorityQueue( ordering(comparator) )
q.enqueue(
Task(3, 'Clear drains'), Task(4, 'Feed cat'), Task(5, 'Make tea'), Task(1, 'Solve RC tasks'), Task(2, 'Tax return') )
while not q.isEmpty()
println( q.dequeue() )</lang>
- Output:
Task(1, Solve RC tasks) Task(2, Tax return) Task(3, Clear drains) Task(4, Feed cat) Task(5, Make tea)
Go
Go's standard library contains the container/heap package, which which provides operations to operate as a heap any data structure that contains the Push, Pop, Len, Less, and Swap methods.
<lang go>package main
import (
"fmt" "container/heap"
)
type Task struct {
priority int name string
}
type TaskPQ []Task
func (self TaskPQ) Len() int { return len(self) } func (self TaskPQ) Less(i, j int) bool {
return self[i].priority < self[j].priority
} func (self TaskPQ) Swap(i, j int) { self[i], self[j] = self[j], self[i] } func (self *TaskPQ) Push(x interface{}) { *self = append(*self, x.(Task)) } func (self *TaskPQ) Pop() (popped interface{}) {
popped = (*self)[len(*self)-1] *self = (*self)[:len(*self)-1] return
}
func main() {
pq := &TaskPQ{{3, "Clear drains"},
{4, "Feed cat"},
{5, "Make tea"},
{1, "Solve RC tasks"}}
// heapify heap.Init(pq)
// enqueue
heap.Push(pq, Task{2, "Tax return"})
for pq.Len() != 0 {
// dequeue
fmt.Println(heap.Pop(pq))
}
}</lang>
output:
{1 Solve RC tasks}
{2 Tax return}
{3 Clear drains}
{4 Feed cat}
{5 Make tea}
Groovy
Groovy can use the built in java PriorityQueue class <lang groovy>import groovy.transform.Canonical
@Canonical class Task implements Comparable<Task> {
int priority
String name
int compareTo(Task o) { priority <=> o?.priority }
}
new PriorityQueue<Task>().with {
add new Task(priority: 3, name: 'Clear drains') add new Task(priority: 4, name: 'Feed cat') add new Task(priority: 5, name: 'Make tea') add new Task(priority: 1, name: 'Solve RC tasks') add new Task(priority: 2, name: 'Tax return')
while (!empty) { println remove() }
}</lang>
Output:
Task(1, Solve RC tasks) Task(2, Tax return) Task(3, Clear drains) Task(4, Feed cat) Task(5, Make tea)
Haskell
One of the best Haskell implementations of priority queues (of which there are many) is pqueue, which implements a binomial heap. <lang haskell>import Data.PQueue.Prio.Min
main = print (toList (fromList [(3, "Clear drains"),(4, "Feed cat"),(5, "Make tea"),(1, "Solve RC tasks"), (2, "Tax return")]))</lang>
Although Haskell's standard library does not have a dedicated priority queue structure, one can (for most purposes) use the built-in Data.Set data structure as a priority queue, as long as no two elements compare equal (since Set does not allow duplicate elements). This is the case here since no two tasks should have the same name. The complexity of all basic operations is still O(log n) although that includes the "elemAt 0" function to retrieve the first element of the ordered sequence if that were required; "fromList" takes O(n log n) and "toList" takes O(n) time complexity. Alternatively, a Data.Map.Lazy or Data.Map.Strict can be used in the same way with the same limitations.
<lang haskell>import qualified Data.Set as S
main = print (S.toList (S.fromList [(3, "Clear drains"),(4, "Feed cat"),(5, "Make tea"),(1, "Solve RC tasks"), (2, "Tax return")]))</lang>
- Output:
[(1,"Solve RC tasks"),(2,"Tax return"),(3,"Clear drains"),(4,"Feed cat"),(5,"Make tea")]
Alternatively, a homemade min heap implementation: <lang haskell>data MinHeap a = Nil | MinHeap { v::a, cnt::Int, l::MinHeap a, r::MinHeap a }
deriving (Show, Eq)
hPush :: (Ord a) => a -> MinHeap a -> MinHeap a hPush x Nil = MinHeap {v = x, cnt = 1, l = Nil, r = Nil} hPush x h = if x < vv -- insert element, try to keep the tree balanced
then if hLength (l h) <= hLength (r h)
then MinHeap { v=x, cnt=cc, l=hPush vv ll, r=rr }
else MinHeap { v=x, cnt=cc, l=ll, r=hPush vv rr }
else if hLength (l h) <= hLength (r h)
then MinHeap { v=vv, cnt=cc, l=hPush x ll, r=rr }
else MinHeap { v=vv, cnt=cc, l=ll, r=hPush x rr }
where (vv, cc, ll, rr) = (v h, 1 + cnt h, l h, r h)
hPop :: (Ord a) => MinHeap a -> (a, MinHeap a) hPop h = (v h, pq) where -- just pop, heed not the tree balance
pq | l h == Nil = r h
| r h == Nil = l h
| v (l h) <= v (r h) = let (vv,hh) = hPop (l h) in
MinHeap {v = vv, cnt = hLength hh + hLength (r h),
l = hh, r = r h}
| otherwise = let (vv,hh) = hPop (r h) in
MinHeap {v = vv, cnt = hLength hh + hLength (l h),
l = l h, r = hh}
hLength :: (Ord a) => MinHeap a -> Int hLength Nil = 0 hLength h = cnt h
hFromList :: (Ord a) => [a] -> MinHeap a hFromList = foldl (flip hPush) Nil
hToList :: (Ord a) => MinHeap a -> [a] hToList = unfoldr f where
f Nil = Nothing f h = Just $ hPop h
main = mapM_ print $ hToList $ hFromList [
(3, "Clear drains"), (4, "Feed cat"), (5, "Make tea"), (1, "Solve RC tasks"), (2, "Tax return")]</lang>
The above code is a Priority Queue but isn't a Min Heap based on a Binary Heap for the following reasons: 1) it does not preserve the standard tree structure of the binary heap and 2) the tree balancing can be completely destroyed by some combinations of "pop" operations. The following code is a true purely functional Min Heap implementation and as well implements the extra optional features of Min Heap's that it can build a new Min Heap from a list in O(n) amortized time rather than the O(n log n) amortized time (for a large number of randomly ordered entries) by simply using repeated "push" operations; as well as the standard "push", "peek", "delete" and "pop" (combines the previous two). As well as the "fromList", "toList", and "sort" functions (the last combines the first two), it also has an "isEmpty" function to test for the empty queue, an "adjust" function that applies a function to every entry in the queue and reheapifies in O(n) amortized time and also the "replaceMin" function which is about twice as fast on the average as combined "delete" followed by "push" operations: <lang haskell>data MinHeap kv = MinHeapEmpty
| MinHeapLeaf !kv
| MinHeapNode !kv {-# UNPACK #-} !Int !(MinHeap a) !(MinHeap a)
deriving (Show, Eq)
emptyPQ :: MinHeap kv emptyPQ = MinHeapEmpty
isEmptyPQ :: PriorityQ kv -> Bool isEmptyPQ Mt = True isEmptyPQ _ = False
sizePQ :: (Ord kv) => MinHeap kv -> Int sizePQ MinHeapEmpty = 0 sizePQ (MinHeapLeaf _) = 1 sizePQ (MinHeapNode _ cnt _ _) = cnt
peekMinPQ :: MinHeap kv -> Maybe kv peekMinPQ MinHeapEmpty = Nothing peekMinPQ (MinHeapLeaf v) = Just v peekMinPQ (MinHeapNode v _ _ _) = Just v
pushPQ :: (Ord kv) => kv -> MinHeap kv -> MinHeap kv pushPQ kv pq = insert kv 0 pq where -- insert element, keeping the tree balanced
insert kv _ MinHeapEmpty = MinHeapLeaf kv
insert kv _ (MinHeapLeaf vv) = if kv <= vv
then MinHeapNode kv 2 (MinHeapLeaf vv) MinHeapEmpty
else MinHeapNode vv 2 (MinHeapLeaf kv) MinHeapEmpty
insert kv msk (MinHeapNode vv cc ll rr) = if kv <= vv
then if nmsk >= 0
then MinHeapNode kv nc (insert vv nmsk ll) rr
else MinHeapNode kv nc ll (insert vv nmsk rr)
else if nmsk >= 0
then MinHeapNode vv nc (insert kv nmsk ll) rr
else MinHeapNode vv nc ll (insert kv nmsk rr)
where nc = cc + 1
nmsk = if msk /= 0 then msk `shiftL` 1 -- walk path to next
else let s = floor $ (log $ fromIntegral nc) / log 2 in
(nc `shiftL` ((finiteBitSize cc) - s)) .|. 1 --never 0 again
siftdown :: (Ord kv) => kv -> Int -> MinHeap kv -> MinHeap kv -> MinHeap kv siftdown kv cnt lft rght = replace cnt lft rght where
replace cc ll rr = case rr of -- adj to put kv in current left/right
MinHeapEmpty -> -- means left is a MinHeapLeaf
case ll of { (MinHeapLeaf vl) ->
if kv <= vl
then MinHeapNode kv 2 ll MinHeapEmpty
else MinHeapNode vl 2 (MinHeapLeaf kv) MinHeapEmpty }
MinHeapLeaf vr ->
case ll of
MinHeapLeaf vl -> if vl <= vr
then if kv <= vl then MinHeapNode kv cc ll rr
else MinHeapNode vl cc (MinHeapLeaf kv) rr
else if kv <= vr then MinHeapNode kv cc ll rr
else MinHeapNode vr cc ll (MinHeapLeaf kv)
MinHeapNode vl ccl lll rrl -> if vl <= vr
then if kv <= vl then MinHeapNode kv cc ll rr
else MinHeapNode vl cc (replace ccl lll rrl) rr
else if kv <= vr then MinHeapNode kv cc ll rr
else MinHeapNode vr cc ll (MinHeapLeaf kv)
MinHeapNode vr ccr llr rrr -> case ll of
(MinHeapNode vl ccl lll rrl) -> -- right is node, so is left
if vl <= vr then
if kv <= vl then MinHeapNode kv cc ll rr
else MinHeapNode vl cc (replace ccl lll rrl) rr
else if kv <= vr then MinHeapNode kv cc ll rr
else MinHeapNode vr cc ll (replace ccr llr rrr)
replaceMinPQ :: (Ord kv) => a -> MinHeap kv -> MinHeap kv replaceMinPQ _ MinHeapEmpty = MinHeapEmpty replaceMinPQ kv (MinHeapLeaf _) = MinHeapLeaf kv replaceMinPQ kv (MinHeapNode _ cc ll rr) = siftdown kv cc ll rr where
deleteMinPQ :: (Ord kv) => MinHeap kv -> MinHeap kv deleteMinPQ MinHeapEmpty = MinHeapEmpty -- remove min keeping tree balanced deleteMinPQ pq = let (dkv, npq) = delete 0 pq in
replaceMinPQ dkv npq where
delete _ (MinHeapLeaf vv) = (vv, MinHeapEmpty)
delete msk (MinHeapNode vv cc ll rr) =
if rr == MinHeapEmpty -- means left is MinHeapLeaf
then case ll of (MinHeapLeaf vl) -> (vl, MinHeapLeaf vv)
else if nmsk >= 0 -- means only deal with left
then let (dv, npq) = delete nmsk ll in
(dv, MinHeapNode vv (cc - 1) npq rr)
else let (dv, npq) = delete nmsk rr in
(dv, MinHeapNode vv (cc - 1) ll npq)
where nmsk = if msk /= 0 then msk `shiftL` 1 -- walk path to last
else let s = floor $ (log $ fromIntegral cc) / log 2 in
(cc `shiftL` ((finiteBitSize cc) - s)) .|. 1 --never 0 again
adjustPQ :: (Ord kv) => (kv -> kv) -> MinHeap kv -> MinHeap kv adjustPQ f pq = adjust pq where -- applies function to every element and reheapifies
adjust MinHeapEmpty = MinHeapEmpty adjust (MinHeapLeaf v) = MinHeapLeaf (f v) adjust (MinHeapNode vv cc ll rr) = siftdown (f vv) cc (adjust ll) (adjust rr)
fromListPQ :: (Ord kv) => [kv] -> MinHeap kv -- fromListPQ = foldl (flip pushPQ) MinHeapEmpty -- O(n log n) time; slow fromListPQ [] = MinHeapEmpty -- O(n) time using "adjust id" which is O(n) fromListPQ xs = let (_, pq) = build 1 xs in pq where
sz = length xs
szd2 = sz `div` 2
build _ [] = ([], MinHeapEmpty)
build lvl (x:xs') = if lvl > szd2 then (xs', MinHeapLeaf x)
else let nlvl = lvl + lvl in
let (xrl, pql) = build nlvl xs' in
let (xrr, pqr) = if nlvl >= sz
then (xrl, MinHeapEmpty) -- no right leaf
else build (nlvl + 1) xrl in
let cnt = sizePQ pql + sizePQ pqr + 1 in
(xrr, siftdown x cnt pql pqr)
popMinPQ :: (Ord kv) => MinHeap kv -> Maybe (kv, MinHeap kv) popMinPQ pq = case peekMinPQ pq of
Nothing -> Nothing
Just v -> Just (v, deleteMinPQ pq)
toListPQ :: (Ord kv) => MinHeap kv -> [kv] toListPQ = unfoldr f where
f MinHeapEmpty = Nothing f pq = popMinPQ pq
sortPQ :: (Ord kv) => [kv] -> [kv] sortPQ ls = toListPQ $ fromListPQ ls</lang>
If one is willing to forgo the fast O(1) "size" function and to give up strict conformance to the Heap tree structure (where rather than building each new level until each left node is full to that level before increasing level to the right, a new level is built by promoting leaves to branches only containing left leaves until all branches have left leaves before filling any right leaves of that level) although having even better tree balancing and therefore at least as high efficiency, one can use the following code adapted from the ML PRIORITY_QUEUE code by Lawrence C. Paulson including separating the key/value pairs as separate entries in the data structure for better comparison efficiency; as noted in the code comments, a "size" function to output the number of elements in the queue (fairly quickly in O((log n)^2)), an "adjust" function to apply a function to all elements and reheapify in O(n) time, and a "merge" function to merge two queues has been added to the ML code: <lang haskell>data PriorityQ k v = Mt
| Br !k v !(PriorityQ k v) !(PriorityQ k v) deriving (Eq, Ord, Read, Show)
emptyPQ :: PriorityQ k v emptyPQ = Mt
isEmptyPQ :: PriorityQ k v -> Bool isEmptyPQ Mt = True isEmptyPQ _ = False
-- The size function isn't from the ML code, but an implementation was -- suggested by Bertram Felgenhauer on Haskell Cafe, so it is included.
-- Return number of elements in the priority queue. -- /O(log(n)^2)/ sizePQ :: PriorityQ k v -> Int sizePQ Mt = 0 sizePQ (Br _ _ pl pr) = 2 * n + rest n pl pr where
n = sizePQ pr
-- rest n p q, where n = sizePQ q, and sizePQ p - sizePQ q = 0 or 1
-- returns 1 + sizePQ p - sizePQ q.
rest :: Int -> PriorityQ k v -> PriorityQ k v -> Int
rest 0 Mt _ = 1
rest 0 _ _ = 2
rest n (Br _ _ ll lr) (Br _ _ rl rr) = case r of
0 -> rest d ll rl -- subtree sizes: (d or d+1), d; d, d
1 -> rest d lr rr -- subtree sizes: d+1, (d or d+1); d+1, d
where m1 = n - 1
d = m1 `shiftR` 1
r = m1 .&. 1
peekMinPQ :: PriorityQ k v -> Maybe (k, v) peekMinPQ Mt = Nothing peekMinPQ (Br k v _ _) = Just (k, v)
pushPQ :: Ord k => k -> v -> PriorityQ k v -> PriorityQ k v pushPQ wk wv Mt = Br wk wv Mt Mt pushPQ wk wv (Br vk vv pl pr)
| wk <= vk = Br wk wv (pushPQ vk vv pr) pl
| otherwise = Br vk vv (pushPQ wk wv pr) pl
siftdown :: Ord k => k -> v -> PriorityQ k v -> PriorityQ k v -> PriorityQ k v siftdown wk wv Mt _ = Br wk wv Mt Mt siftdown wk wv (pl @ (Br vk vv _ _)) Mt
| wk <= vk = Br wk wv pl Mt | otherwise = Br vk vv (Br wk wv Mt Mt) Mt
siftdown wk wv (pl @ (Br vkl vvl pll plr)) (pr @ (Br vkr vvr prl prr))
| wk <= vkl && wk <= vkr = Br wk wv pl pr | vkl <= vkr = Br vkl vvl (siftdown wk wv pll plr) pr | otherwise = Br vkr vvr pl (siftdown wk wv prl prr)
replaceMinPQ :: Ord k => k -> v -> PriorityQ k v -> PriorityQ k v replaceMinPQ wk wv Mt = Mt replaceMinPQ wk wv (Br _ _ pl pr) = siftdown wk wv pl pr
deleteMinPQ :: (Ord k) => PriorityQ k v -> PriorityQ k v deleteMinPQ Mt = Mt deleteMinPQ (Br _ _ pr Mt) = pr deleteMinPQ (Br _ _ pl pr) = let (k, v, npl) = leftrem pl in
siftdown k v pr npl where
leftrem (Br k v Mt Mt) = (k, v, Mt)
leftrem (Br vk vv (Br k v _ _) Mt) = (k, v, Br vk vv Mt Mt)
leftrem (Br vk vv pl pr) = let (k, v, npl) = leftrem pl in
(k, v, Br vk vv pr npl)
-- the following function has been added to the ML code to apply a function -- to all the entries in the queue and reheapify in O(n) time adjustPQ :: (Ord k) => (k -> v -> (k, v)) -> PriorityQ k v -> PriorityQ k v adjustPQ f pq = adjust pq where -- applies function to every element and reheapifies
adjust Mt = Mt
adjust (Br vk vv pl pr) = let (k, v) = f vk vv in
siftdown k v (adjust pl) (adjust pr)
fromListPQ :: (Ord k) => [(k, v)] -> PriorityQ k v -- fromListPQ = foldl (flip pushPQ) Mt -- O(n log n) time; slow fromListPQ [] = Mt -- O(n) time using adjust-from-bottom which is O(n) fromListPQ xs = let (pq, _) = build (length xs) xs in pq where
build 0 xs = (Mt, xs)
build lvl ((k, v):xs') = let (pl, xrl) = build (lvl `shiftR` 1) xs'
(pr, xrr) = build ((lvl - 1) `shiftR` 1) xrl in
(siftdown k v pl pr, xrr)
-- the following function has been added to merge two queues in O(m + n) time -- where m and n are the sizes of the two queues mergePQ :: (Ord k) => PriorityQ k v -> PriorityQ k v -> PriorityQ k v mergePQ pq1 Mt = pq1 -- from concatenated "dumb" list mergePQ Mt pq2 = pq2 -- in O(m + n) time where m,n are sizes pq1,pq2 mergePQ pq1 pq2 = fromListPQ (zipper pq1 $ zipper pq2 []) where
zipper (Br wk wv Mt _) appndlst = (wk, wv) : appndlst zipper (Br wk wv pl Mt) appndlst = (wk, wv) : zipper pl appndlst zipper (Br wk wv pl pr) appndlst = (wk, wv) : zipper pl (zipper pr appndlst)
popMinPQ :: (Ord k) => PriorityQ k v -> Maybe ((k, v), PriorityQ k v) popMinPQ pq = case peekMinPQ pq of
Nothing -> Nothing
Just kv -> Just (kv, deleteMinPQ pq)
toListPQ :: (Ord k) => PriorityQ k v -> [(k, v)] toListPQ Mt = [] -- unfoldr popMinPQ toListPQ pq @ (Br vk vv _ _) = (vk, vv) : (toListPQ $ deleteMinPQ pq)
sortPQ :: (Ord k) => [(k, v)] -> [(k, v)] sortPQ ls = toListPQ $ fromListPQ ls</lang>
The above codes compile but do not run with GHC Haskell version 7.8.3 using the LLVM back end with LLVM version 3.4 and full optimization turned on under Windows 32; they were tested under Windows 64 and 32 using the Native Code Generator back end with full optimization. With GHC Haskell version 7.10.1 they compile and run with or without LLVM 3.5.1 for 32-bit Windows (64-bit GHC Haskell under Windows does not run with LLVM for version 7.10.1), with a slight execution speed advantage to using LLVM.
Min Heaps are faster than Priority Queue's based on Binomial Heaps (or Leftist or Skewed Heaps) when one mainly requires fast replacement of the head of the queue without many fresh "push" operations; Binomial Heap based versions (or Leftist or Skewed Heap based versions) are faster for merging of a series of large queues into one and for algorithms that have a lot of "push" operations of random entries. Both have O(log n) average "push" and "pop" time complexity with O(1) for "peek", but Binomial Heap based queues (and the others) tend to be somewhat slower by a constant factor due to more complex operations.
Min Heaps are also faster than the use of balanced tree Set's or Map's where many references are made to the next element in the queue (O(1) complexity rather than O(log n)) or where frequent modification and reinsertion of the next element in the queue is required (still O(log n) but faster by a constant factor greater than two on average) and generally faster by a constant factor as operations near the top of the queue don't have to traverse the entire tree structure; O(log n) is worst case time complexity for "replace" operations not average.
The above codes when tested with the following "main" function (with a slight modification for the first test when the combined kv entry is used): <lang haskell>testList = [ (3, "Clear drains"),
(4, "Feed cat"),
(5, "Make tea"),
(1, "Solve RC tasks"),
(2, "Tax return") ]
testPQ = fromListPQ testList
main = do -- slow build
mapM_ print $ toListPQ $ foldl (\pq (k, v) -> pushPQ k v pq) emptyPQ testList putStrLn "" -- fast build mapM_ print $ toListPQ $ fromListPQ testList putStrLn "" -- combined fast sort mapM_ print $ sortPQ testList putStrLn "" -- test merge mapM_ print $ toListPQ $ mergePQ testPQ testPQ putStrLn "" -- test adjust mapM_ print $ toListPQ $ adjustPQ (\x y -> (x * (-1), y)) testPQ</lang>
has the output as follows:
- Output:
(1,"Solve RC tasks") (2,"Tax return") (3,"Clear drains") (4,"Feed cat") (5,"Make tea") (1,"Solve RC tasks") (2,"Tax return") (3,"Clear drains") (4,"Feed cat") (5,"Make tea") (1,"Solve RC tasks") (2,"Tax return") (3,"Clear drains") (4,"Feed cat") (5,"Make tea") (1,"Solve RC tasks") (1,"Solve RC tasks") (2,"Tax return") (2,"Tax return") (3,"Clear drains") (3,"Clear drains") (4,"Feed cat") (4,"Feed cat") (5,"Make tea") (5,"Make tea") (-5,"Make tea") (-4,"Feed cat") (-3,"Clear drains") (-2,"Tax return") (-1,"Solve RC tasks")
but the first method uses the slower way of building a queue.
Icon and Unicon
This solution uses classes provided by the UniLib package. Heap is an implementation of a priority queue and Closure is used to allow the queue to order lists based on their first element. The solution only works in Unicon. <lang Unicon>import Utils # For Closure class import Collections # For Heap (dense priority queue) class
procedure main()
pq := Heap(, Closure("[]",Arg,1) )
pq.add([3, "Clear drains"])
pq.add([4, "Feed cat"])
pq.add([5, "Make tea"])
pq.add([1, "Solve RC tasks"])
pq.add([2, "Tax return"])
while task := pq.get() do write(task[1]," -> ",task[2])
end </lang> Output when run:
1 -> Solve RC tasks 2 -> Tax return 3 -> Clear drains 4 -> Feed cat 5 -> Make tea
J
Implementation:
<lang j>coclass 'priorityQueue'
PRI=: QUE=:
insert=:4 :0
p=. PRI,x
q=. QUE,y
assert. p -:&$ q
assert. 1 = #$q
ord=: \: p
QUE=: ord { q
PRI=: ord { p
i.0 0
)
topN=:3 :0
assert y<:#PRI
r=. y{.QUE
PRI=: y}.PRI
QUE=: y}.QUE
r
)</lang>
Efficiency is obtained by batching requests. Size of batch for insert is determined by size of arguments. Size of batch for topN is its right argument.
Example:
<lang j> Q=: conew'priorityQueue'
3 4 5 1 2 insert__Q 'clear drains';'feed cat';'make tea';'solve rc task';'tax return' >topN__Q 1
make tea
>topN__Q 4
feed cat clear drains tax return solve rc task</lang>
Java
Java has a PriorityQueue class. It requires either the elements implement Comparable, or you give it a custom Comparator to compare the elements.
<lang java>import java.util.PriorityQueue;
class Task implements Comparable<Task> {
final int priority; final String name;
public Task(int p, String n) {
priority = p;
name = n;
}
public String toString() {
return priority + ", " + name;
}
public int compareTo(Task other) {
return priority < other.priority ? -1 : priority > other.priority ? 1 : 0;
}
public static void main(String[] args) {
PriorityQueue<Task> pq = new PriorityQueue<Task>();
pq.add(new Task(3, "Clear drains"));
pq.add(new Task(4, "Feed cat"));
pq.add(new Task(5, "Make tea"));
pq.add(new Task(1, "Solve RC tasks"));
pq.add(new Task(2, "Tax return"));
while (!pq.isEmpty())
System.out.println(pq.remove());
}
}</lang>
- Output:
1, Solve RC tasks 2, Tax return 3, Clear drains 4, Feed cat 5, Make tea
jq
Since jq is a functional language, the priority queue must be represented explicitly as data; in the following, we use a JSON object with keys as priorities (strings). Since a given priority level may have more than task, we use arrays to hold the values.
The special key "priorities" is used to store the priorities in a sorted array. Since "sort" is fast we will use that rather than optimizing insertion in the priorities array.
We assume that if an item of a given priority is already in the priority queue, there is no need to add it again.<lang jq># In the following, pq stands for "priority queue".
- Add an item with the given priority (an integer,
- or a string representing an integer)
- Input: a pq
def pq_add(priority; item):
(priority|tostring) as $p
| if .priorities|index($p) then
if (.[$p] | index(item)) then . else .[$p] += [item] end
else .[$p] = [item] | .priorities = (.priorities + [$p] | sort)
end ;
- emit [ item, pq ]
- Input: a pq
def pq_pop:
.priorities as $keys
| if ($keys|length) == 0 then [ null, . ]
else
if (.[$keys[0]] | length) == 1
then .priorities = .priorities[1:]
else .
end
| [ (.[$keys[0]])[0], (.[$keys[0]] = .[$keys[0]][1:]) ]
end ;
- Emit the item that would be popped, or null if there is none
- Input: a pq
def pq_peep:
.priorities as $keys | if ($keys|length) == 0 then null else (.[$keys[0]])[0] end ;
- Add a bunch of tasks, presented as an array of arrays
- Input: a pq
def pq_add_tasks(list):
reduce list[] as $pair (.; . + pq_add( $pair[0]; $pair[1]) ) ;
- Pop all the tasks, producing a stream
- Input: a pq
def pq_pop_tasks:
pq_pop as $pair | if $pair[0] == null then empty else $pair[0], ( $pair[1] | pq_pop_tasks ) end ;
- Input: a bunch of tasks, presented as an array of arrays
def prioritize:
. as $list | {} | pq_add_tasks($list) | pq_pop_tasks ;
</lang> The specific task: <lang jq> [ [3, "Clear drains"],
[4, "Feed cat"], [5, "Make tea"], [1, "Solve RC tasks"], [2, "Tax return"] ] | prioritize
</lang>
- Output:
"Solve RC tasks" "Tax return" "Clear drains" "Feed cat" "Make tea"
Julia
Julia has built-in support for priority queues, though the PriorityQueue type is not exported by default. Priority queues are a specialization of the Dictionary type having ordered values, which serve as the priority. In addition to all of the methods of standard dictionaries, priority queues support: enqueue!, which adds an item to the queue, dequeue! which removes the lowest priority item from the queue, returning its key, and peek, which returns the (key, priority) of the lowest priority entry in the queue. The ordering behavior of the queue, which by default is its value sort order (typically low to high), can be set by passing an order directive to its constructor. For this task, Base.Order.Reverse is used to set-up the task queue to return tasks from high to low priority.
<lang Julia>
using Base.Collections
test = ["Clear drains" 3;
"Feed cat" 4;
"Make tea" 5;
"Solve RC tasks" 1;
"Tax return" 2]
task = PriorityQueue(Base.Order.Reverse) for i in 1:size(test)[1]
enqueue!(task, test[i,1], test[i,2])
end
println("Tasks, completed according to priority:") while !isempty(task)
(t, p) = peek(task)
dequeue!(task)
println(" \"", t, "\" has priority ", p)
end</lang>
- Output:
Tasks, completed according to priority:
"Make tea" has priority 5
"Feed cat" has priority 4
"Clear drains" has priority 3
"Tax return" has priority 2
"Solve RC tasks" has priority 1
Kotlin
<lang scala>import java.util.PriorityQueue
internal data class Task(val priority: Int, val name: String) : Comparable<Task> {
override fun compareTo(other: Task) = when {
priority < other.priority -> -1
priority > other.priority -> 1
else -> 0
}
}
private infix fun String.priority(priority: Int) = Task(priority, this)
fun main(args: Array<String>) {
val q = PriorityQueue(listOf("Clear drains" priority 3,
"Feed cat" priority 4,
"Make tea" priority 5,
"Solve RC tasks" priority 1,
"Tax return" priority 2))
while (q.any()) println(q.remove())
}</lang>
- Output:
Task(priority=1, name=Solve RC tasks) Task(priority=2, name=Tax return) Task(priority=3, name=Clear drains) Task(priority=4, name=Feed cat) Task(priority=5, name=Make tea)
Lasso
<lang lasso>define priorityQueue => type {
data
store = map,
cur_priority = void
public push(priority::integer, value) => {
local(store) = .`store`->find(#priority)
if(#store->isA(::array)) => {
#store->insert(#value)
return
}
.`store`->insert(#priority=array(#value))
.`cur_priority`->isA(::void) or #priority < .`cur_priority`
? .`cur_priority` = #priority
}
public pop => {
.`cur_priority` == void
? return void
local(store) = .`store`->find(.`cur_priority`)
local(retVal) = #store->first
#store->removeFirst&size > 0
? return #retVal
// Need to find next priority
.`store`->remove(.`cur_priority`)
if(.`store`->size == 0) => {
.`cur_priority` = void
else
// There are better / faster ways to do this
// The keys are actually already sorted, but the order of
// storage in a map is not actually defined, can't rely on it
.`cur_priority` = .`store`->keys->asArray->sort&first
}
return #retVal }
public isEmpty => (.`store`->size == 0)
}
local(test) = priorityQueue
- test->push(2,`e`)
- test->push(1,`H`)
- test->push(5,`o`)
- test->push(2,`l`)
- test->push(5,`!`)
- test->push(4,`l`)
while(not #test->isEmpty) => {
stdout(#test->pop)
}</lang>
- Output:
Hello!
Lua
This implementation uses a table with priorities as keys and queues as values. Queues for each priority are created when putting items as needed and are shrunk as necessary when popping items and removed when they are empty. Instead of using a plain array table for each queue, the technique shown in the Lua implementation from the Queue task is used. This avoids having to use table.remove(t, 1) to get and remove the first queue element, which is rather slow for big tables.
<lang lua>PriorityQueue = {
__index = {
put = function(self, p, v)
local q = self[p]
if not q then
q = {first = 1, last = 0}
self[p] = q
end
q.last = q.last + 1
q[q.last] = v
end,
pop = function(self)
for p, q in pairs(self) do
if q.first <= q.last then
local v = q[q.first]
q[q.first] = nil
q.first = q.first + 1
return p, v
else
self[p] = nil
end
end
end
},
__call = function(cls)
return setmetatable({}, cls)
end
}
setmetatable(PriorityQueue, PriorityQueue)
-- Usage: pq = PriorityQueue()
tasks = {
{3, 'Clear drains'},
{4, 'Feed cat'},
{5, 'Make tea'},
{1, 'Solve RC tasks'},
{2, 'Tax return'}
}
for _, task in ipairs(tasks) do
print(string.format("Putting: %d - %s", unpack(task)))
pq:put(unpack(task))
end
for prio, task in pq.pop, pq do
print(string.format("Popped: %d - %s", prio, task))
end</lang>
Output:
Putting: 3 - Clear drains Putting: 4 - Feed cat Putting: 5 - Make tea Putting: 1 - Solve RC tasks Putting: 2 - Tax return Popped: 1 - Solve RC tasks Popped: 2 - Tax return Popped: 3 - Clear drains Popped: 4 - Feed cat Popped: 5 - Make tea
The implementation is faster than the Python implementations below using queue.PriorityQueue or heapq, even when comparing the standard Lua implementation against PyPy and millions of tasks are added to the queue. With LuaJIT it is yet faster. The following code measures the time needed to add 107 tasks with a random priority between 1 and 1000 and to retrieve them from the queue again in order.
<lang lua>-- Use socket.gettime() for benchmark measurements -- since it has millisecond precision on most systems local socket = require("socket")
n = 10000000 -- number of tasks added (10^7) m = 1000 -- number different priorities
local pq = PriorityQueue()
print(string.format("Adding %d tasks with random priority 1-%d ...", n, m)) start = socket.gettime()
for i = 1, n do
pq:put(math.random(m), i)
end
print(string.format("Elapsed: %.3f ms.", (socket.gettime() - start) * 1000))
print("Retrieving all tasks in order...") start = socket.gettime()
local pp = 0 local pv = 0
for i = 1, n do
local p, task = pq:pop()
-- check that tasks are popped in ascending priority assert(p >= pp)
if pp == p then
-- check that tasks within one priority maintain the insertion order
assert(task > pt)
end
pp = p pt = task
end
print(string.format("Elapsed: %.3f ms.", (socket.gettime() - start) * 1000))</lang>
M2000 Interpreter
For these three examples, we can use same priorities, so if a priority exist then the new insertion not alter the top item (which we pop or peek from queue).
Using unordered array
<lang M2000 Interpreter> Module UnOrderedArray {
Class PriorityQueue {
Private:
Dim Item()
many=0, level=0, first
cmp = lambda->0
Module Reduce {
if .many<.first*2 then exit
If .level<.many/2 then .many/=2 : Dim .Item(.many)
}
Public:
Module Clear {
Dim .Item() \\ erase all
.many<=0 \\ default
.Level<=0
}
Module PriorityQueue {
If .many>0 then Error "Clear List First"
Read .many, .cmp
.first<=.many
Dim .Item(.many)
}
Module Add {
If .level=.many Then {
If .many=0 then Error "Define Size First"
Dim .Item(.many*2)
.many*=2
}
Read Item
If .level=0 Then {
.Item(0)=Item
} Else.if .cmp(.Item(0), Item)=-1 Then { \\ Item is max
.Item(.level)=Item
swap .Item(0), .Item(.level)
} Else .Item(.level)=Item
.level++
}
Function Peek {
If .level=0 Then error "empty"
=.Item(0)
}
Function Poll {
If .level=0 Then error "empty"
=.Item(0)
If .level=2 Then {
swap .Item(0), .Item(1)
.Item(1)=0
.Level<=1
} Else.If .level>2 Then {
.Level--
Swap .Item(.level), .Item(0)
.Item(.level)=0
For I=.level-1 to 1 {
If .cmp(.Item(I), .Item(I-1))=1 Then Swap .Item(I), .Item(I-1)
}
} else .level<=0 : .Item(0)=0
.Reduce
}
Module Remove {
If .level=0 Then error "empty"
Read Item
k=true
If .cmp(.Item(0), Item)=0 Then {
Item=.Poll()
K~ \\ k=false
} Else.If .Level>1 Then {
I2=.Level-1
For I=1 to I2 {
If k Then {
If .cmp(.Item(I), Item)=0 Then {
If I<I2 Then Swap .Item(I), .Item(I2)
.Item(I2)=0
k=false
}
} else exit
}
.Level--
}
If k Then Error "Not Found"
.Reduce
}
Function Size {
If .many=0 then Error "Define Size First"
=.Level
}
}
Class Item { X, S$
Module Item { Read .X, .S$}
}
Function PrintTop {
M=Queue.Peek() : Print "Item ";M.X, M.S$
}
Comp=Lambda -> { Read A,B : =COMPARE(A.X,B.X)}
Queue=PriorityQueue(100,Comp)
Queue.Add Item(3, "Clear drains")
Call Local PrintTop()
Queue.Add Item(4 ,"Feed cat")
Call Local PrintTop()
Queue.Add Item(5 ,"Make tea")
Call Local PrintTop()
Queue.Add Item(1 ,"Solve RC tasks")
Call Local PrintTop()
Queue.Add Item(2 ,"Tax return")
Call Local PrintTop()
Print "remove items"
While true {
MM=Queue.Poll()
Print MM.X, MM.S$
Print "Size="; Queue.Size()
If Queue.Size()=0 Then exit
Call Local PrintTop()
}
} UnOrderedArray </lang>
Using a stack with arrays as elements
Every insertion push item using binary search in proper position. Pop is very fast. <lang M2000 Interpreter> Module PriorityQueue {
a= ( (3, "Clear drains"), (4 ,"Feed cat"), ( 5 , "Make tea"), ( 1 ,"Solve RC tasks"), ( 2 , "Tax return"))
b=stack
comp=lambda (a, b) ->{
=array(a, 0)<array(b, 0)
}
module InsertPQ (a, n, &comp) {
if len(a)=0 then stack a {data n} : exit
if comp(n, stackitem(a)) then stack a {push n} : exit
stack a {
push n
t=2: b=len(a)
m=b
while t<=b {
t1=m
m=(b+t) div 2
if m=0 then m=t1 : exit
If comp(stackitem(m),n) then t=m+1: continue
b=m-1
m=b
}
if m>1 then shiftback m
}
}
n=each(a)
while n {
InsertPq b, array(n), &comp
}
n1=each(b)
while n1 {
m=stackitem(n1)
Print array(m, 0), array$(m, 1)
}
\\ Peek topitem (without popping)
Print Array$(stackitem(b), 1)
\\ Pop item
Stack b {
Read old
}
Print Array$(old, 1)
Function Peek$(a) {=Array$(stackitem(a), 1)}
Function Pop$(a) {
stack a {
=Array$(stackitem(), 1)
drop
}
}
Print Peek$(b)
Print Pop$(b)
Function IsEmpty(a) {
=len(a)=0
}
While not IsEmpty(b) {
Print pop$(b)
}
} PriorityQueue
</lang>
Using a stack with Groups as elements
This is the same as previous but now we use a group (a user object for M2000). InsertPQ is the same as before. Lambda comp has change only. We didn't use pointers to groups. All groups here works as values, so when we get a peek we get a copy of group in top position. All members of a group may not values, so if we have a pointer to group then we get a copy of that pointer, but then we can make changes and that changes happen for the group which we get the copy.
<lang M2000 Interpreter> Module PriorityQueueForGroups {
class obj {
x, s$
class:
module obj (.x, .s$) {}
}
Flush ' empty current stack
Data obj(3, "Clear drains"), obj(4 ,"Feed cat"), obj( 5 , "Make tea"), obj( 1 ,"Solve RC tasks"), obj( 2 , "Tax return")
b=stack
comp=lambda (a, b) ->{
=a.x<b.x
}
module InsertPQ (a, n, &comp) {
if len(a)=0 then stack a {data n} : exit
if comp(n, stackitem(a)) then stack a {push n} : exit
stack a {
push n
t=2: b=len(a)
m=b
while t<=b {
t1=m
m=(b+t) div 2
if m=0 then m=t1 : exit
If comp(stackitem(m),n) then t=m+1: continue
b=m-1
m=b
}
if m>1 then shiftback m
}
}
b=stack
While not empty {
InsertPQ b, Group, &comp ' Group pop a group from current stack
}
n1=each(b)
while n1 {
m=stackitem(n1)
Print m.x, m.s$
}
Function Peek$(a) {m=stackitem(a) : =m.s$}
Print Peek$(b)
Function Pop$(a) {
stack a {
m=stackitem()
=m.s$
drop
}
}
Function IsEmpty(a) {
=len(a)=0
}
While not isEmpty(b) {
Print Pop$(b)
}
} PriorityQueueForGroups </lang>
Mathematica/Wolfram Language
<lang mathematica>push = Function[{queue, priority, item},
queue = SortBy[Append[queue, {priority, item}], First], HoldFirst];
pop = Function[queue,
If[Length@queue == 0, Null,
With[{item = queue-1, 2}, queue = Most@queue; item]],
HoldFirst];
peek = Function[queue,
If[Length@queue == 0, Null, Max[queueAll, 1]], HoldFirst];
merge = Function[{queue1, queue2},
SortBy[Join[queue1, queue2], First], HoldAll];</lang>
Example:
<lang mathematica>queue = {}; push[queue, 3, "Clear drains"]; push[queue, 4, "Feed cat"]; push[queue, 5, "Make tea"]; push[queue, 1, "Solve RC tasks"]; push[queue, 2, "Tax return"]; Print[peek[queue]]; Print[pop[queue]]; queue1 = {}; push[queue1, 6, "Drink tea"]; Print[merge[queue, queue1]];</lang>
Output:
5
Make tea
{{1,Solve RC tasks},{2,Tax return},{3,Clear drains},{4,Feed cat},{6,Drink tea}}
Maxima
<lang maxima>/* Naive implementation using a sorted list of pairs [key, [item[1], ..., item[n]]]. The key may be any number (integer or not). Items are extracted in FIFO order. */
defstruct(pqueue(q = []))$
/* Binary search */
find_key(q, p) := block(
[i: 1, j: length(q), k, c],
if j = 0 then false
elseif (c: q[i][1]) >= p then
(if c = p then i else false)
elseif (c: q[j][1]) <= p then
(if c = p then j else false)
else catch(
while j >= i do (
k: quotient(i + j, 2),
if (c: q[k][1]) = p then throw(k)
elseif c < p then i: k + 1 else j: k - 1
),
false
)
)$
pqueue_push(pq, x, p) := block(
[q: pq@q, k], k: find_key(q, p), if integerp(k) then q[k][2]: endcons(x, q[k][2]) else pq@q: sort(cons([p, [x]], q)), 'done
)$
pqueue_pop(pq) := block(
[q: pq@q, v, x],
if emptyp(q) then 'fail else (
p: q[1][1],
v: q[1][2],
x: v[1],
if length(v) > 1 then q[1][2]: rest(v) else pq@q: rest(q),
x
)
)$
pqueue_print(pq) := block([t], while (t: pqueue_pop(pq)) # 'fail do disp(t))$
/* An example */
a: new(pqueue)$
pqueue_push(a, "take milk", 4)$ pqueue_push(a, "take eggs", 4)$ pqueue_push(a, "take wheat flour", 4)$ pqueue_push(a, "take salt", 4)$ pqueue_push(a, "take oil", 4)$ pqueue_push(a, "carry out crepe recipe", 5)$ pqueue_push(a, "savour !", 6)$ pqueue_push(a, "add strawberry jam", 5 + 1/2)$ pqueue_push(a, "call friends", 5 + 2/3)$ pqueue_push(a, "go to the supermarket and buy food", 3)$ pqueue_push(a, "take a shower", 2)$ pqueue_push(a, "get dressed", 2)$ pqueue_push(a, "wake up", 1)$ pqueue_push(a, "serve cider", 5 + 3/4)$ pqueue_push(a, "buy also cider", 3)$
pqueue_print(a); "wake up" "take a shower" "get dressed" "go to the supermarket and buy food" "buy also cider" "take milk" "take butter" "take flour" "take salt" "take oil" "carry out recipe" "add strawberry jam" "call friends" "serve cider" "savour !"</lang>
Mercury
Mercury comes with an efficient, albeit simple, priority queue in its standard library. The build_pqueue/2 predicate in the code below inserts the test data in arbitrary order. display_pqueue/3, in turn, removes one K/V pair at a time, displaying the value. Compiling and running the supplied program results in all tasks being displayed in priority order as expected.
<lang mercury>:- module test_pqueue.
- - interface.
- - import_module io.
- - pred main(io::di, io::uo) is det.
- - implementation.
- - import_module int.
- - import_module list.
- - import_module pqueue.
- - import_module string.
- - pred build_pqueue(pqueue(int,string)::in, pqueue(int,string)::out) is det.
build_pqueue(!PQ) :-
pqueue.insert(3, "Clear drains", !PQ), pqueue.insert(4, "Feed cat", !PQ), pqueue.insert(5, "Make tea", !PQ), pqueue.insert(1, "Solve RC tasks", !PQ), pqueue.insert(2, "Tax return", !PQ).
- - pred display_pqueue(pqueue(int, string)::in, io::di, io::uo) is det.
display_pqueue(PQ, !IO) :-
( pqueue.remove(K, V, PQ, PQO) ->
io.format("Key = %d, Value = %s\n", [i(K), s(V)], !IO),
display_pqueue(PQO, !IO)
;
true
).
main(!IO) :-
build_pqueue(pqueue.init, PQO), display_pqueue(PQO, !IO).</lang>
Nim
<lang nim>type
PriElem[T] = tuple data: T pri: int
PriQueue[T] = object buf: seq[PriElem[T]] count: int
- first element not used to simplify indices
proc initPriQueue[T](initialSize = 4): PriQueue[T] =
result.buf.newSeq(initialSize) result.buf.setLen(1) result.count = 0
proc add[T](q: var PriQueue[T], data: T, pri: int) =
var n = q.buf.len var m = n div 2 q.buf.setLen(n + 1)
# append at end, then up heap while m > 0 and pri < q.buf[m].pri: q.buf[n] = q.buf[m] n = m m = m div 2
q.buf[n] = (data, pri) q.count = q.buf.len - 1
proc pop[T](q: var PriQueue[T]): PriElem[T] =
assert q.buf.len > 1 result = q.buf[1]
var qn = q.buf.len - 1
var n = 1
var m = 2
while m < qn:
if m + 1 < qn and q.buf[m].pri > q.buf[m+1].pri:
inc m
if q.buf[qn].pri <= q.buf[m].pri:
break
q.buf[n] = q.buf[m] n = m m = m * 2
q.buf[n] = q.buf[qn] q.buf.setLen(q.buf.len - 1) q.count = q.buf.len - 1
var p = initPriQueue[string]() p.add("Clear drains", 3) p.add("Feed cat", 4) p.add("Make tea", 5) p.add("Solve RC tasks", 1) p.add("Tax return", 2)
while p.count > 0:
echo p.pop()</lang>
- Output:
(data: Solve RC tasks, pri: 1) (data: Tax return, pri: 2) (data: Clear drains, pri: 3) (data: Feed cat, pri: 4) (data: Make tea, pri: 5)
Using Nim HeapQueue <lang Nim>import HeapQueue
var pq = newHeapQueue[(int, string)]()
pq.push((3, "Clear drains")) pq.push((4, "Feed cat")) pq.push((5, "Make tea")) pq.push((1, "Solve RC tasks")) pq.push((2, "Tax return"))
while pq.len() > 0:
echo pq.pop()</lang>
- Output:
(Field0: 1, Field1: "Solve RC tasks") (Field0: 2, Field1: "Tax return") (Field0: 3, Field1: "Clear drains") (Field0: 4, Field1: "Feed cat") (Field0: 5, Field1: "Make tea")
Using Nim tables <lang Nim>import tables
var
pq = initTable[int, string]()
proc main() =
pq.add(3, "Clear drains") pq.add(4, "Feed cat") pq.add(5, "Make tea") pq.add(1, "Solve RC tasks") pq.add(2, "Tax return")
for i in countUp(1,5):
if pq.hasKey(i):
echo i, ": ", pq[i]
pq.del(i)
main()</lang>
- Output:
1: Solve RC tasks 2: Tax return 3: Clear drains 4: Feed cat 5: Make tea
Objective-C
The priority queue used in this example is not actually written in Objective-C. It is part of Apple's (C-based) Core Foundation library, which is included with in Cocoa on Mac OS X and iOS. Its interface is a C function interface, which makes the code very ugly. Core Foundation is not included in GNUStep or other Objective-C APIs.
<lang objc>#import <Foundation/Foundation.h>
const void *PQRetain(CFAllocatorRef allocator, const void *ptr) {
return (__bridge_retained const void *)(__bridge id)ptr;
} void PQRelease(CFAllocatorRef allocator, const void *ptr) {
(void)(__bridge_transfer id)ptr;
} CFComparisonResult PQCompare(const void *ptr1, const void *ptr2, void *unused) {
return [(__bridge id)ptr1 compare:(__bridge id)ptr2];
}
@interface Task : NSObject {
int priority; NSString *name;
} - (instancetype)initWithPriority:(int)p andName:(NSString *)n; - (NSComparisonResult)compare:(Task *)other; @end
@implementation Task - (instancetype)initWithPriority:(int)p andName:(NSString *)n {
if ((self = [super init])) {
priority = p;
name = [n copy];
}
return self;
} - (NSString *)description {
return [NSString stringWithFormat:@"%d, %@", priority, name];
} - (NSComparisonResult)compare:(Task *)other {
if (priority == other->priority) return NSOrderedSame; else if (priority < other->priority) return NSOrderedAscending; else return NSOrderedDescending;
} @end
int main (int argc, const char *argv[]) {
@autoreleasepool {
CFBinaryHeapCallBacks callBacks = {0, PQRetain, PQRelease, NULL, PQCompare};
CFBinaryHeapRef pq = CFBinaryHeapCreate(NULL, 0, &callBacks, NULL);
CFBinaryHeapAddValue(pq, [[Task alloc] initWithPriority:3 andName:@"Clear drains"]); CFBinaryHeapAddValue(pq, [[Task alloc] initWithPriority:4 andName:@"Feed cat"]); CFBinaryHeapAddValue(pq, [[Task alloc] initWithPriority:5 andName:@"Make tea"]); CFBinaryHeapAddValue(pq, [[Task alloc] initWithPriority:1 andName:@"Solve RC tasks"]); CFBinaryHeapAddValue(pq, [[Task alloc] initWithPriority:2 andName:@"Tax return"]);
while (CFBinaryHeapGetCount(pq) != 0) {
Task *task = (id)CFBinaryHeapGetMinimum(pq);
NSLog(@"%@", task);
CFBinaryHeapRemoveMinimumValue(pq);
}
CFRelease(pq);
} return 0;
}</lang>
log:
2011-08-22 07:46:19.250 Untitled[563:903] 1, Solve RC tasks 2011-08-22 07:46:19.255 Untitled[563:903] 2, Tax return 2011-08-22 07:46:19.256 Untitled[563:903] 3, Clear drains 2011-08-22 07:46:19.257 Untitled[563:903] 4, Feed cat 2011-08-22 07:46:19.258 Untitled[563:903] 5, Make tea
OCaml
Holger Arnold's OCaml base library provides a PriorityQueue module.
<lang ocaml>module PQ = Base.PriorityQueue
let () =
let tasks = [ 3, "Clear drains"; 4, "Feed cat"; 5, "Make tea"; 1, "Solve RC tasks"; 2, "Tax return"; ] in let pq = PQ.make (fun (prio1, _) (prio2, _) -> prio1 > prio2) in List.iter (PQ.add pq) tasks; while not (PQ.is_empty pq) do let _, task = PQ.first pq in PQ.remove_first pq; print_endline task done</lang>
testing:
$ ocaml -I +pcre pcre.cma base.cma pq.ml Make tea Feed cat Clear drains Tax return Solve RC tasks
Although OCaml's standard library does not have a dedicated priority queue structure, one can (for most purposes) use the built-in Set data structure as a priority queue, as long as no two elements compare equal (since Set does not allow duplicate elements). This is the case here since no two tasks should have the same name. Note that Set is a functional, persistent data structure, so we derive new priority queues from the old ones functionally, rather than modifying them imperatively; the complexity is still O(log n).
<lang ocaml>module PQSet = Set.Make
(struct
type t = int * string (* pair of priority and task name *)
let compare = compare
end);;
let () =
let tasks = [
3, "Clear drains";
4, "Feed cat";
5, "Make tea";
1, "Solve RC tasks";
2, "Tax return";
] in
let pq = PQSet.of_list tasks in
let rec aux pq' =
if not (PQSet.is_empty pq') then begin
let prio, name as task = PQSet.min_elt pq' in
Printf.printf "%d, %s\n" prio name;
aux (PQSet.remove task pq')
end
in aux pq</lang>
- Output:
1, Solve RC tasks 2, Tax return 3, Clear drains 4, Feed cat 5, Make tea
OxygenBasic
<lang> 'PRIORITY QUEUE WITH 16 LEVELS
uses console
% pl 16 'priority levels
=======
Class PriorityQueue
=======
indexbase 1 bstring buf[pl] 'buffers to hold priority queues content int bg[pl] 'buffers base offset int i[pl] 'indexers int le[pl] 'length of buffer
method constructor()
========
int p for p=1 to pl
buf[p]="" le[p]=0 bg[p]=0 i=[p]=0
next end method
method destructor()
=======
int p for p=1 to pl
del (buf[p]) le[p]=0 bg[p]=0 i=[p]=0
next end method
method Encodelength(int ls,p)
=================
int ll at i[p]+strptr(buf[p]) ll=ls i[p]+=sizeof int end method
method limit(int*p)
=======
if p>pl
p=pl
endif if p<1
p=1
endif end method
method push(string s,int p)
=================
limit p int ls ls=len s if i[p]+ls+8 > le[p] then
int e=8000+(ls*2) 'extra buffer bytes buf[p]=buf[p]+nuls e 'extend buf le[p]=len buf[p]
end if EncodeLength ls,p 'length of input s mid buf[p],i[p]+1,s 'patch in s i[p]+=ls end method
method popLength(int p) as int
==================
if bg[p]>=i[p]
return -1 'buffer empty
endif int ll at (bg[p]+strptr buf[p]) bg[p]+=sizeof int return ll end method
method pop(string *s, int *p=1, lpl=0) as int
=================================
limit p int ls do
ls=popLength p
if ls=-1
if not lpl 'lpl: lock priority level
p++ 'try next priority level
if p<=pl
continue do
endif
endif
s=""
return ls 'empty buffers
endif
exit do
loop s=mid buf[p],bg[p]+1,ls bg[p]+=ls 'cleanup buffer if bg[p]>1e6 then
buf[p]=mid buf[p],bg[p]+1 'remove old popped data le[p]=len buf[p] i[p]-=bg[p] 'shrink buf bg[p]=0
end if end method
method clear()
==
constructor end method
end class 'PriorityQueue
'====
'DEMO
'====
new PriorityQueue medo()
string s
def inp
medo.push %2,%1
end def
' Priority Task ' ══════════ ════════════════ inp 3 "Clear drains" inp 4 "Feed cat" inp 5 "Make tea" inp 1 "Solve RC tasks" inp 2 "Tax return" inp 4 "Plant beans" ' int er int p print "Priority Task" cr print "=================" cr do
er=medo.pop s,p if er=-1 print "(buffer empty)" exit do endif print p tab s cr
loop pause del medo
/* RESULTS: Priority Task
=====
1 Solve RC tasks 2 Tax return 3 Clear drains 4 Feed cat 4 Plant beans 5 Make tea (buffer empty)
- /
</lang>
Pascal
<lang Pascal> program PriorityQueueTest;
uses Classes;
Type
TItem = record Priority:Integer; Value:string; end; PItem = ^TItem;
TPriorityQueue = class(Tlist)
procedure Push(Priority:Integer;Value:string); procedure SortPriority(); function Pop():String; function Empty:Boolean;
end;
{ TPriorityQueue }
procedure TPriorityQueue.Push(Priority:Integer;Value:string); var
Item: PItem;
begin
new(Item); Item^.Priority := Priority; Item^.Value := Value; inherited Add(Item); SortPriority();
end;
procedure TPriorityQueue.SortPriority(); var
i,j:Integer;
begin
if(Count < 2) Then Exit();
for i:= 0 to Count-2 do
for j:= i+1 to Count-1 do
if ( PItem(Items[i])^.Priority > PItem(Items[j])^.Priority)then
Exchange(i,j);
end;
function TPriorityQueue.Pop():String; begin
if count = 0 then Exit(); result := PItem(First)^.value; Dispose(PItem(First)); Delete(0);
end;
function TPriorityQueue.Empty:Boolean; begin
Result := Count = 0;
end;
var
Queue : TPriorityQueue;
begin
Queue:= TPriorityQueue.Create(); Queue.Push(3,'Clear drains'); Queue.Push(4,'Feed cat'); Queue.Push(5,'Make tea'); Queue.Push(1,'Solve RC tasks'); Queue.Push(2,'Tax return'); while not Queue.Empty() do writeln(Queue.Pop()); Queue.free;
end. </lang>
Perl
There are a few implementations on CPAN. Following uses Heap::Priority[1]
<lang perl>use 5.10.0;
use strict;
use Heap::Priority;
my $h = new Heap::Priority;
$h->highest_first(); # higher or lower number is more important $h->add(@$_) for ["Clear drains", 3],
["Feed cat", 4],
["Make tea", 5],
["Solve RC tasks", 1],
["Tax return", 2];
say while ($_ = $h->pop);</lang>output<lang>Make tea Feed cat Clear drains Tax return Solve RC tasks</lang>
IBM card sorter version
<lang perl>#!/usr/bin/perl
use strict; # https://rosettacode.org/wiki/Priority_queue use warnings; # in homage to IBM card sorters :)
my $data = <<END; Priority Task
3 Clear drains 4 Feed cat 5 Make tea 1 Solve RC tasks 2 Tax return 4 Feed dog
END
insert( $1, $2 ) while $data =~ /(\d+)\h+(.*)/g; # insert all data
while( my $item = top_item_removal() ) # get in priority order
{
print "$item\n";
}
my @bins; # priorities limited to small (<1e6 maybe?) non-negative integers
sub insert { push @{ $bins[shift] }, pop } # O(1)
sub top_item_removal # O(1) (sort of, maybe?)
{
delete $bins[-1] while @bins and @{ $bins[-1] // [] } == 0;
shift @{ $bins[-1] // [] };
}</lang>
- Output:
Make tea Feed cat Feed dog Clear drains Tax return Solve RC tasks
Phix
Dictionary based solution. Allows duplicate tasks, FIFO within priority, and uses a callback-style method of performing tasks.
Assumes 5 is the highest priority and should be done first, for 1 first just delete the ",true" on traverse_dict calls.
with javascript_semantics constant tasklist = new_dict() procedure add_task(integer priority, string desc) integer k = getd_index(priority,tasklist) if k=0 then putd(priority,{desc},tasklist) else sequence descs = getd_by_index(k,tasklist) putd(priority,append(descs,desc),tasklist) end if end procedure function list_task_visitor(integer priority, sequence descs, integer /*user_data*/) ?{priority,descs} return true -- continue end function procedure list_tasks() traverse_dict(list_task_visitor, 0, tasklist, true) end procedure function pop_task_visitor(integer priority, sequence descs, integer rid) string desc = descs[1] descs = descs[2..$] if length(descs)=0 then deld(priority,tasklist) else putd(priority,descs,tasklist) end if rid(priority,desc) return false -- stop end function procedure pop_task(integer rid) if dict_size(tasklist)!=0 then traverse_dict(pop_task_visitor, rid, tasklist, true) end if end procedure add_task(3,"Clear drains") add_task(4,"Feed cat") add_task(5,"Make tea") add_task(1,"Solve RC tasks") add_task(2,"Tax return") procedure do_task(integer priority, string desc) ?{priority,desc} end procedure list_tasks() ?"===" pop_task(do_task) ?"===" list_tasks()
- Output:
{5,{"Make tea"}}
{4,{"Feed cat"}}
{3,{"Clear drains"}}
{2,{"Tax return"}}
{1,{"Solve RC tasks"}}
"==="
{5,"Make tea"}
"==="
{4,{"Feed cat"}}
{3,{"Clear drains"}}
{2,{"Tax return"}}
{1,{"Solve RC tasks"}}
trans nim
(I needed this for Taxicab_numbers)
The bulk of this code now forms builtins/pqueue.e (not properly documented at the time, but now is, see below)
with javascript_semantics sequence pq = {} constant PRIORITY = 2 procedure pqAdd(sequence item) -- item is {object data, object priority} integer n = length(pq)+1, m = floor(n/2) pq &= 0 -- append at end, then up heap while m>0 and item[PRIORITY]<pq[m][PRIORITY] do pq[n] = pq[m] n = m m = floor(m/2) end while pq[n] = item end procedure function pqPop() sequence result = pq[1] integer qn = length(pq), n = 1, m = 2 while m<qn do if m+1<qn and pq[m][PRIORITY]>pq[m+1][PRIORITY] then m += 1 end if if pq[qn][PRIORITY]<=pq[m][PRIORITY] then exit end if pq[n] = pq[m] n = m m = m * 2 end while pq[n] = pq[qn] pq = pq[1..$-1] return result end function set_rand(iff(platform()=JS?5749: -- (optional!) iff(machine_bits()=32?4601:97))) constant set = shuffle({{"Clear drains", 3}, {"Feed cat", 4}, {"Make tea", 5}, {"Solve RC tasks", 1}, {"Tax return", 2}}) for i=1 to length(set) do pqAdd(set[i]) pqAdd(set[rand(length(set))]) end for sequence res = {} while length(pq) do ?pqPop() end while
- Output:
The optional initial set_rand() makes it slightly more amusing.
As shown set_rand() achieves consistency per-platform, not cross-platform, the numbers above were found using a brute-force outer loop stopping on the desired result, since deleted.
{"Solve RC tasks",1}
{"Tax return",2}
{"Clear drains",3}
{"Feed cat",4}
{"Feed cat",4}
{"Feed cat",4}
{"Feed cat",4}
{"Feed cat",4}
{"Feed cat",4}
{"Make tea",5}
builtin
If you omit MAX_HEAP or (same thing) specify MIN_HEAP, the output'll be 1..5
with javascript_semantics constant tasklist = pq_new(MAX_HEAP) pq_add({"Clear drains",3},tasklist) pq_add({"Feed cat",4},tasklist) pq_add({"Make tea",5},tasklist) pq_add({"Solve RC tasks",1},tasklist) pq_add({"Tax return",2},tasklist) while pq_size(tasklist) do {string task, integer priority} = pq_pop(tasklist) printf(1,"%d: %s\n",{priority,task}) end while
- Output:
5: Make tea 4: Feed cat 3: Clear drains 2: Tax return 1: Solve RC tasks
Phixmonti
<lang Phixmonti>/# Rosetta Code problem: http://rosettacode.org/wiki/Priority_queue by Galileo, 05/2022 #/
include ..\Utilitys.pmt
( ) ( 3 "Clear drains" ) 0 put ( 4 "Feed cat" ) 0 put ( 5 "Make tea" ) 0 put ( 1 "Solve RC tasks" ) 0 put ( 2 "Tax return" ) 0 put sort pop swap print pstack </lang>
- Output:
[1, "Solve RC tasks"] [[[2, "Tax return"], [3, "Clear drains"], [4, "Feed cat"], [5, "Make tea"]]] === Press any key to exit ===
PHP
PHP's SplPriorityQueue class implements a max-heap. PHP also separately has SplHeap, SplMinHeap, and SplMaxHeap classes.
<lang php><?php
$pq = new SplPriorityQueue;
$pq->insert('Clear drains', 3); $pq->insert('Feed cat', 4); $pq->insert('Make tea', 5); $pq->insert('Solve RC tasks', 1); $pq->insert('Tax return', 2);
// This line causes extract() to return both the data and priority (in an associative array), // Otherwise it would just return the data $pq->setExtractFlags(SplPriorityQueue::EXTR_BOTH);
while (!$pq->isEmpty()) {
print_r($pq->extract());
} ?></lang>
Output:
Array
(
[data] => Make tea
[priority] => 5
)
Array
(
[data] => Feed cat
[priority] => 4
)
Array
(
[data] => Clear drains
[priority] => 3
)
Array
(
[data] => Tax return
[priority] => 2
)
Array
(
[data] => Solve RC tasks
[priority] => 1
)
The difference between SplHeap and SplPriorityQueue is that SplPriorityQueue takes the data and the priority as two separate arguments, and the comparison is only made on the priority; whereas SplHeap takes only one argument (the element), and the comparison is made on that directly. In all of these classes it is possible to provide a custom comparator by subclassing the class and overriding its compare method.
<lang php><?php
$pq = new SplMinHeap;
$pq->insert(array(3, 'Clear drains')); $pq->insert(array(4, 'Feed cat')); $pq->insert(array(5, 'Make tea')); $pq->insert(array(1, 'Solve RC tasks')); $pq->insert(array(2, 'Tax return'));
while (!$pq->isEmpty()) {
print_r($pq->extract());
} ?></lang>
Output:
Array
(
[0] => 1
[1] => Solve RC tasks
)
Array
(
[0] => 2
[1] => Tax return
)
Array
(
[0] => 3
[1] => Clear drains
)
Array
(
[0] => 4
[1] => Feed cat
)
Array
(
[0] => 5
[1] => Make tea
)
Picat
Picat has built-in support for min and max heaps. <lang Picat>main =>
Tasks = [[3,"Clear drains"],
[4,"Feed cat"],
[5,"Make tea"],
[1,"Solve RC tasks"],
[2,"Tax return"]],
Heap = new_min_heap([]),
foreach(Task in Tasks)
Heap.heap_push(Task),
println(top=Heap.heap_top())
end,
nl,
println(Heap),
println(size=Heap.heap_size),
nl,
println("Pop the elements from the queue:"),
println([Heap.heap_pop() : _ in 1..Heap.heap_size]).</lang>
- Output:
top = [3,Clear drains]
top = [3,Clear drains]
top = [3,Clear drains]
top = [1,Solve RC tasks]
top = [1,Solve RC tasks]
_$heap(5,{[1,Solve RC tasks],[2,Tax return],[5,Make tea],[4,Feed cat],[3,Clear drains],_33c8},min)
size = 5
Pop the elements from the queue:
[[1,Solve RC tasks],[2,Tax return],[3,Clear drains],[4,Feed cat],[5,Make tea]]
The heaps creation functions can take the task list as argument: <lang Picat> Tasks = [[3,"Clear drains"],
[4,"Feed cat"],
[5,"Make tea"],
[1,"Solve RC tasks"],
[2,"Tax return"]],
Heap = new_min_heap(Tasks),
println([Heap.heap_pop() : _ in 1..Heap.heap_size]).</lang>
PicoLisp
The following implementation imposes no limits. It uses a binary tree for storage. The priority levels may be numeric, or of any other type. <lang PicoLisp># Insert item into priority queue (de insertPQ (Queue Prio Item)
(idx Queue (cons Prio Item) T) )
- Remove and return top item from priority queue
(de removePQ (Queue)
(cdar (idx Queue (peekPQ Queue) NIL)) )
- Find top element in priority queue
(de peekPQ (Queue)
(let V (val Queue)
(while (cadr V)
(setq V @) )
(car V) ) )
- Merge second queue into first
(de mergePQ (Queue1 Queue2)
(balance Queue1 (sort (conc (idx Queue1) (idx Queue2)))) )</lang>
Test: <lang PicoLisp># Two priority queues (off Pq1 Pq2)
- Insert into first queue
(insertPQ 'Pq1 3 '(Clear drains)) (insertPQ 'Pq1 4 '(Feed cat))
- Insert into second queue
(insertPQ 'Pq2 5 '(Make tea)) (insertPQ 'Pq2 1 '(Solve RC tasks)) (insertPQ 'Pq2 2 '(Tax return))
- Merge second into first queue
(mergePQ 'Pq1 'Pq2)
- Remove and print all items from first queue
(while Pq1
(println (removePQ 'Pq1)) )</lang>
Output:
(Solve RC tasks) (Tax return) (Clear drains) (Feed cat) (Make tea)
Alternative version using a pairing heap:
<lang PicoLisp> (de heap-first (H) (car H))
(de heap-merge (H1 H2)
(cond
((= H1 NIL) H2)
((= H2 NIL) H1)
((< (car H1) (car H2))
(cons (car H1) (cons H2 (cdr H1))))
(T
(cons (car H2) (cons H1 (cdr H2))))))
(de heap-insert (Item Heap)
(heap-merge (list Item) Heap))
(de "merge-pairs" (H)
(if (= (cdr H) NIL)
(car H) # also handles NIL (H = NIL -> NIL)
(heap-merge
(heap-merge (car H) (cadr H))
("merge-pairs" (cddr H)))))
(de heap-rest (H)
("merge-pairs" (cdr H)))
</lang> Test: <lang PicoLisp> (setq H NIL) (for
Task '(
(3 . "Clear drains.")
(4 . "Feed cat.")
(5 . "Make tea.")
(1 . "Solve RC tasks.")
(2 . "Tax Return."))
(setq H (heap-insert Task H)))
(while H
(prinl (caar H) ". " (cdar H)) (setq H (heap-rest H)))
(bye) </lang>
- Output:
1. Solve RC tasks. 2. Tax Return. 3. Clear drains. 4. Feed cat. 5. Make tea.
Prolog
SWI-Prolog has a library heaps.pl, written by Lars Buitinck that implements priority queues.
Informations here : http://www.swi-prolog.org/pldoc/doc/swi/library/heaps.pl
Example of use : <lang Prolog>priority-queue :-
TL0 = [3-'Clear drains',
4-'Feed cat'],
% we can create a priority queue from a list list_to_heap(TL0, Heap0), % alternatively we can start from an empty queue % get from empty_heap/1. % now we add the other elements add_to_heap(Heap0, 5, 'Make tea', Heap1), add_to_heap(Heap1, 1, 'Solve RC tasks', Heap2), add_to_heap(Heap2, 2, 'Tax return', Heap3),
% we list the content of the heap:
heap_to_list(Heap3, TL1),
writeln('Content of the queue'), maplist(writeln, TL1),
nl,
% now we retrieve the minimum-priority pair
get_from_heap(Heap3, Priority, Key, Heap4),
format('Retrieve top of the queue : Priority ~w, Element ~w~n', [Priority, Key]),
nl,
% we list the content of the heap:
heap_to_list(Heap4, TL2),
writeln('Content of the queue'), maplist(writeln, TL2).
</lang> The output :
1 ?- priority-queue. Content of the queue 1-Solve RC tasks 2-Tax return 3-Clear drains 4-Feed cat 5-Make tea Retrieve top of the queue : Priority 1, Element Solve RC tasks Content of the queue 2-Tax return 3-Clear drains 4-Feed cat 5-Make tea true.
PureBasic
The priority queue is implemented using a binary heap array and a map. The map stores the elements of a given priority in a FIFO list. Priorities can be any signed 32 value. <lang purebasic>Structure taskList
List description.s() ;implements FIFO queue
EndStructure
Structure task
*tl.tList ;pointer to a list of task descriptions Priority.i ;tasks priority, lower value has more priority
EndStructure
Structure priorityQueue
maxHeapSize.i ;increases as needed heapItemCount.i ;number of elements currently in heap Array heap.task(0) ;elements hold FIFO queues ordered by priorities, lowest first map heapMap.taskList() ;holds lists of tasks with the same priority that are FIFO queues
EndStructure
Procedure insertPQ(*PQ.priorityQueue, description.s, p)
If FindMapElement(*PQ\heapMap(), Str(p))
LastElement(*PQ\heapMap()\description())
AddElement(*PQ\heapMap()\description())
*PQ\heapMap()\description() = description
Else
Protected *tl.taskList = AddMapElement(*PQ\heapMap(), Str(p))
AddElement(*tl\description())
*tl\description() = description
Protected pos = *PQ\heapItemCount
*PQ\heapItemCount + 1
If *PQ\heapItemCount > *PQ\maxHeapSize
Select *PQ\maxHeapSize
Case 0
*PQ\maxHeapSize = 128
Default
*PQ\maxHeapSize * 2
EndSelect
Redim *PQ\heap.task(*PQ\maxHeapSize)
EndIf
While pos > 0 And p < *PQ\heap((pos - 1) / 2)\Priority
*PQ\heap(pos) = *PQ\heap((pos - 1) / 2)
pos = (pos - 1) / 2
Wend
*PQ\heap(pos)\tl = *tl
*PQ\heap(pos)\Priority = p
EndIf
EndProcedure
Procedure.s removePQ(*PQ.priorityQueue)
Protected *tl.taskList = *PQ\heap(0)\tl, description.s
FirstElement(*tl\description())
description = *tl\description()
If ListSize(*tl\description()) > 1
DeleteElement(*tl\description())
Else
DeleteMapElement(*PQ\heapMap(), Str(*PQ\heap(0)\Priority))
*PQ\heapItemCount - 1
*PQ\heap(0) = *PQ\heap(*PQ\heapItemCount)
Protected pos
Repeat
Protected child1 = 2 * pos + 1
Protected child2 = 2 * pos + 2
If child1 >= *PQ\heapItemCount
Break
EndIf
Protected smallestChild
If child2 >= *PQ\heapItemCount
smallestChild = child1
ElseIf *PQ\heap(child1)\Priority <= *PQ\heap(child2)\Priority
smallestChild = child1
Else
smallestChild = child2
EndIf
If (*PQ\heap(smallestChild)\Priority >= *PQ\heap(pos)\Priority)
Break
EndIf
Swap *PQ\heap(pos)\tl, *PQ\heap(smallestChild)\tl
Swap *PQ\heap(pos)\Priority, *PQ\heap(smallestChild)\Priority
pos = smallestChild
ForEver
EndIf
ProcedureReturn description
EndProcedure
Procedure isEmptyPQ(*PQ.priorityQueue) ;returns 1 if empty, otherwise returns 0
If *PQ\heapItemCount ProcedureReturn 0 EndIf ProcedureReturn 1
EndProcedure
If OpenConsole()
Define PQ.priorityQueue insertPQ(PQ, "Clear drains", 3) insertPQ(PQ, "Answer Phone 1", 8) insertPQ(PQ, "Feed cat", 4) insertPQ(PQ, "Answer Phone 2", 8) insertPQ(PQ, "Make tea", 5) insertPQ(PQ, "Sleep", 9) insertPQ(PQ, "Check email", 3) insertPQ(PQ, "Solve RC tasks", 1) insertPQ(PQ, "Answer Phone 3", 8) insertPQ(PQ, "Exercise", 9) insertPQ(PQ, "Answer Phone 4", 8) insertPQ(PQ, "Tax return", 2) While Not isEmptyPQ(PQ) PrintN(removePQ(PQ)) Wend Print(#CRLF$ + #CRLF$ + "Press ENTER to exit"): Input() CloseConsole()
EndIf</lang>
- Output:
Solve RC tasks Tax return Clear drains Check email Feed cat Make tea Answer Phone 1 Answer Phone 2 Answer Phone 3 Answer Phone 4 Sleep Exercise
Python
Using PriorityQueue
Python has the class queue.PriorityQueue in its standard library.
The data structures in the "queue" module are synchronized multi-producer, multi-consumer queues for multi-threaded use. They can however handle this task: <lang python>>>> import queue >>> pq = queue.PriorityQueue() >>> for item in ((3, "Clear drains"), (4, "Feed cat"), (5, "Make tea"), (1, "Solve RC tasks"), (2, "Tax return")):
pq.put(item)
>>> while not pq.empty():
print(pq.get_nowait())
(1, 'Solve RC tasks')
(2, 'Tax return')
(3, 'Clear drains')
(4, 'Feed cat')
(5, 'Make tea')
>>> </lang>
- Help text for queue.PriorityQueue
<lang python>>>> import queue >>> help(queue.PriorityQueue) Help on class PriorityQueue in module queue:
class PriorityQueue(Queue)
| Variant of Queue that retrieves open entries in priority order (lowest first).
|
| Entries are typically tuples of the form: (priority number, data).
|
| Method resolution order:
| PriorityQueue
| Queue
| builtins.object
|
| Methods inherited from Queue:
|
| __init__(self, maxsize=0)
|
| empty(self)
| Return True if the queue is empty, False otherwise (not reliable!).
|
| This method is likely to be removed at some point. Use qsize() == 0
| as a direct substitute, but be aware that either approach risks a race
| condition where a queue can grow before the result of empty() or
| qsize() can be used.
|
| To create code that needs to wait for all queued tasks to be
| completed, the preferred technique is to use the join() method.
|
| full(self)
| Return True if the queue is full, False otherwise (not reliable!).
|
| This method is likely to be removed at some point. Use qsize() >= n
| as a direct substitute, but be aware that either approach risks a race
| condition where a queue can shrink before the result of full() or
| qsize() can be used.
|
| get(self, block=True, timeout=None)
| Remove and return an item from the queue.
|
| If optional args 'block' is true and 'timeout' is None (the default),
| block if necessary until an item is available. If 'timeout' is
| a positive number, it blocks at most 'timeout' seconds and raises
| the Empty exception if no item was available within that time.
| Otherwise ('block' is false), return an item if one is immediately
| available, else raise the Empty exception ('timeout' is ignored
| in that case).
|
| get_nowait(self)
| Remove and return an item from the queue without blocking.
|
| Only get an item if one is immediately available. Otherwise
| raise the Empty exception.
|
| join(self)
| Blocks until all items in the Queue have been gotten and processed.
|
| The count of unfinished tasks goes up whenever an item is added to the
| queue. The count goes down whenever a consumer thread calls task_done()
| to indicate the item was retrieved and all work on it is complete.
|
| When the count of unfinished tasks drops to zero, join() unblocks.
|
| put(self, item, block=True, timeout=None)
| Put an item into the queue.
|
| If optional args 'block' is true and 'timeout' is None (the default),
| block if necessary until a free slot is available. If 'timeout' is
| a positive number, it blocks at most 'timeout' seconds and raises
| the Full exception if no free slot was available within that time.
| Otherwise ('block' is false), put an item on the queue if a free slot
| is immediately available, else raise the Full exception ('timeout'
| is ignored in that case).
|
| put_nowait(self, item)
| Put an item into the queue without blocking.
|
| Only enqueue the item if a free slot is immediately available.
| Otherwise raise the Full exception.
|
| qsize(self)
| Return the approximate size of the queue (not reliable!).
|
| task_done(self)
| Indicate that a formerly enqueued task is complete.
|
| Used by Queue consumer threads. For each get() used to fetch a task,
| a subsequent call to task_done() tells the queue that the processing
| on the task is complete.
|
| If a join() is currently blocking, it will resume when all items
| have been processed (meaning that a task_done() call was received
| for every item that had been put() into the queue).
|
| Raises a ValueError if called more times than there were items
| placed in the queue.
|
| ----------------------------------------------------------------------
| Data descriptors inherited from Queue:
|
| __dict__
| dictionary for instance variables (if defined)
|
| __weakref__
| list of weak references to the object (if defined)
>>> </lang>
Using heapq
Python has the heapq module in its standard library.
Although one can use the heappush method to add items individually to a heap similar to the method used in the PriorityQueue example above, we will instead transform the list of items into a heap in one go then pop them off one at a time as before. <lang python>>>> from heapq import heappush, heappop, heapify >>> items = [(3, "Clear drains"), (4, "Feed cat"), (5, "Make tea"), (1, "Solve RC tasks"), (2, "Tax return")] >>> heapify(items) >>> while items:
print(heappop(items))
(1, 'Solve RC tasks')
(2, 'Tax return')
(3, 'Clear drains')
(4, 'Feed cat')
(5, 'Make tea')
>>> </lang>
- Help text for module heapq
<lang python>>>> help('heapq') Help on module heapq:
NAME
heapq - Heap queue algorithm (a.k.a. priority queue).
DESCRIPTION
Heaps are arrays for which a[k] <= a[2*k+1] and a[k] <= a[2*k+2] for
all k, counting elements from 0. For the sake of comparison,
non-existing elements are considered to be infinite. The interesting
property of a heap is that a[0] is always its smallest element.
Usage:
heap = [] # creates an empty heap
heappush(heap, item) # pushes a new item on the heap
item = heappop(heap) # pops the smallest item from the heap
item = heap[0] # smallest item on the heap without popping it
heapify(x) # transforms list into a heap, in-place, in linear time
item = heapreplace(heap, item) # pops and returns smallest item, and adds
# new item; the heap size is unchanged
Our API differs from textbook heap algorithms as follows:
- We use 0-based indexing. This makes the relationship between the
index for a node and the indexes for its children slightly less
obvious, but is more suitable since Python uses 0-based indexing.
- Our heappop() method returns the smallest item, not the largest.
These two make it possible to view the heap as a regular Python list
without surprises: heap[0] is the smallest item, and heap.sort()
maintains the heap invariant!
FUNCTIONS
heapify(...)
Transform list into a heap, in-place, in O(len(heap)) time.
heappop(...)
Pop the smallest item off the heap, maintaining the heap invariant.
heappush(...)
Push item onto heap, maintaining the heap invariant.
heappushpop(...)
Push item on the heap, then pop and return the smallest item
from the heap. The combined action runs more efficiently than
heappush() followed by a separate call to heappop().
heapreplace(...)
Pop and return the current smallest value, and add the new item.
This is more efficient than heappop() followed by heappush(), and can be
more appropriate when using a fixed-size heap. Note that the value
returned may be larger than item! That constrains reasonable uses of
this routine unless written as part of a conditional replacement:
if item > heap[0]:
item = heapreplace(heap, item)
merge(*iterables)
Merge multiple sorted inputs into a single sorted output.
Similar to sorted(itertools.chain(*iterables)) but returns a generator,
does not pull the data into memory all at once, and assumes that each of
the input streams is already sorted (smallest to largest).
>>> list(merge([1,3,5,7], [0,2,4,8], [5,10,15,20], [], [25]))
[0, 1, 2, 3, 4, 5, 5, 7, 8, 10, 15, 20, 25]
nlargest(n, iterable, key=None)
Find the n largest elements in a dataset.
Equivalent to: sorted(iterable, key=key, reverse=True)[:n]
nsmallest(n, iterable, key=None)
Find the n smallest elements in a dataset.
Equivalent to: sorted(iterable, key=key)[:n]
DATA
__about__ = 'Heap queues\n\n[explanation by François Pinard]\n\nH... t... __all__ = ['heappush', 'heappop', 'heapify', 'heapreplace', 'merge', '...
FILE
c:\python32\lib\heapq.py
>>> </lang>
Quackery
For more examples uf usage, see Sorting algorithms/Heapsort#Quackery and Huffman coding#Quackery
<lang Quackery>( ***** priotity queue ***** )
[ stack ] is heap.pq ( --> s )
[ stack ] is comp.pq ( --> s )
[ unpack comp.pq put heap.pq put ] is pq->stacks ( p --> )
[ heap.pq take comp.pq take 2 pack ] is stacks->pq ( --> p )
[ heap.pq share swap peek ] is heapeek ( n --> x )
[ heap.pq take swap poke heap.pq put ] is heapoke ( n x --> )
[ 1 - 1 >> ] is parent ( n --> n )
[ 0 > ] is has-parent ( n --> b )
[ 1 << 1+ ] is child ( n --> n )
[ child heap.pq share size < ] is has-child ( n --> b )
[ 1+ ] is sibling ( n --> n )
[ sibling heap.pq share size < ] is has-sibling ( n --> b )
[ comp.pq share do ] is compare ( x x --> b )
[ 0 peek size ] is pqsize ( p --> n )
[ swap pq->stacks
heap.pq take tuck size
rot 0 join heap.pq put
[ dup has-parent while
dup parent
rot over heapeek
2dup compare iff
[ 2swap unrot heapoke ]
again
rot 2drop swap ]
heapoke
stacks->pq ] is topq ( p x --> p )
[ dup heapeek swap
[ dup has-child while
dup child
dup has-sibling if
[ dup sibling heapeek
over heapeek
compare if sibling ]
dip over dup heapeek
rot dip dup compare iff
[ rot heapoke ] again
2drop ]
heapoke ] is heapify.pq ( n --> )
[ dup pqsize 0 = if
[ $ "Unexpectedly empty priority queue."
fail ]
pq->stacks heap.pq share
behead
over [] = iff
[ swap heap.pq replace ]
else
[ swap -1 split
swap join heap.pq put
0 heapify.pq ]
stacks->pq swap ] is frompq ( p --> p x )
[ ]'[ 2 pack pq->stacks
heap.pq share
size 1 >> times
[ i heapify.pq ]
stacks->pq ] is pqwith ( [ --> p )
( ***** task ***** )
[ 2 pack topq ] is insert ( p n $ --> p )
[ frompq unpack ] is dequeue ( p --> p n $ )
[] pqwith [ 0 peek dip [ 0 peek ] < ]
3 $ "Clear drains" insert 4 $ "Feed cat" insert 5 $ "Make tea" insert 1 $ "Solve RC tasks" insert 2 $ "Tax return" insert
5 times
[ dequeue
swap echo say ": "
echo$ cr ]
drop</lang>
- Output:
1: Solve RC tasks 2: Tax return 3: Clear drains 4: Feed cat 5: Make tea
R
Using closures: <lang R>PriorityQueue <- function() {
keys <- values <- NULL
insert <- function(key, value) {
ord <- findInterval(key, keys)
keys <<- append(keys, key, ord)
values <<- append(values, value, ord)
}
pop <- function() {
head <- list(key=keys[1],value=values1)
values <<- values[-1]
keys <<- keys[-1]
return(head)
}
empty <- function() length(keys) == 0
environment()
}
pq <- PriorityQueue() pq$insert(3, "Clear drains") pq$insert(4, "Feed cat") pq$insert(5, "Make tea") pq$insert(1, "Solve RC tasks") pq$insert(2, "Tax return") while(!pq$empty()) {
with(pq$pop(), cat(key,":",value,"\n"))
}</lang>With output:<lang R>1 : Solve RC tasks 2 : Tax return 3 : Clear drains 4 : Feed cat 5 : Make tea</lang>A similar implementation using R5 classes:<lang R>PriorityQueue <-
setRefClass("PriorityQueue",
fields = list(keys = "numeric", values = "list"),
methods = list(
insert = function(key,value) {
insert.order <- findInterval(key, keys)
keys <<- append(keys, key, insert.order)
values <<- append(values, value, insert.order)
},
pop = function() {
head <- list(key=keys[1],value=values1)
keys <<- keys[-1]
values <<- values[-1]
return(head)
},
empty = function() length(keys) == 0
))</lang>The only change in the example would be in the instantiation:<lang R>pq <- PriorityQueue$new()</lang>.
Racket
This solution implements priority queues on top of heaps. <lang racket>
- lang racket
(require data/heap)
(define pq (make-heap (λ(x y) (<= (second x) (second y)))))
(define (insert! x pri)
(heap-add! pq (list pri x)))
(define (remove-min!)
(begin0 (first (heap-min pq)) (heap-remove-min! pq)))
(insert! 3 "Clear drains") (insert! 4 "Feed cat") (insert! 5 "Make tea") (insert! 1 "Solve RC tasks") (insert! 2 "Tax return")
(remove-min!) (remove-min!) (remove-min!) (remove-min!) (remove-min!) </lang> Output: <lang racket> "Solve RC tasks" "Tax return" "Clear drains" "Feed cat" "Make tea" </lang>
Raku
(formerly Perl 6) This is a rather simple implementation. It requires the priority to be a positive integer value, with lower values being higher priority. There isn't a hard limit on how many priority levels you can have, though more than a few dozen is probably not practical.
The tasks are stored internally as an array of FIFO buffers, so multiple tasks of the same priority level will be returned in the order they were stored.
<lang perl6>class PriorityQueue {
has @!tasks;
method insert (Int $priority where * >= 0, $task) {
@!tasks[$priority].push: $task;
}
method get { @!tasks.first(?*).shift }
method is-empty { ?none @!tasks }
}
my $pq = PriorityQueue.new;
for (
3, 'Clear drains', 4, 'Feed cat', 5, 'Make tea', 9, 'Sleep', 3, 'Check email', 1, 'Solve RC tasks', 9, 'Exercise', 2, 'Do taxes'
) -> $priority, $task {
$pq.insert( $priority, $task );
}
say $pq.get until $pq.is-empty;</lang>
- Output:
Solve RC tasks Do taxes Clear drains Check email Feed cat Make tea Sleep Exercise
REXX
version 1
Programming note: this REXX version allows any number (with or without decimals, say, 5.7) for the priority, including negative numbers. <lang rexx>/*REXX program implements a priority queue with insert/display/delete the top task.*/
- =0; @.= /*0 tasks; nullify the priority queue.*/
say '══════ inserting tasks.'; call .ins 3 "Clear drains"
call .ins 4 "Feed cat"
call .ins 5 "Make tea"
call .ins 1 "Solve RC tasks"
call .ins 2 "Tax return"
call .ins 6 "Relax"
call .ins 6 "Enjoy"
say '══════ showing tasks.'; call .show say '══════ deletes top task.'; say .del() /*delete the top task. */ exit /*stick a fork in it, we're all done. */ /*──────────────────────────────────────────────────────────────────────────────────────*/ .del: procedure expose @. #; arg p; if p= then p=.top(); y=@.p; @.p=; return y .ins: procedure expose @. #; #=#+1; @.#=arg(1); return # /*entry, P, task.*/ .show: procedure expose @. #; do j=1 for #; _=@.j; if _\== then say _; end; return /*──────────────────────────────────────────────────────────────────────────────────────*/ .top: procedure expose @. #; top=; top#=
do j=1 for #; _=word(@.j, 1); if _== then iterate
if top== | _>top then do; top=_; top#=j; end
end /*j*/
return top#</lang>
- output:
══════ inserting tasks. ══════ showing tasks. 3 Clear drains 4 Feed cat 5 Make tea 1 Solve RC tasks 2 Tax return 6 Relax 6 Enjoy ══════ deletes top task. 6 Relax
version 2
<lang rexx>/*REXX pgm implements a priority queue; with insert/show/delete top task*/ n=0 task.=0 /* for the sake of task.0done.* */ say '------ inserting tasks.'; call ins_task 3 'Clear drains'
call ins_task 4 'Feed cat'
call ins_task 5 'Make tea'
call ins_task 1 'Solve RC tasks'
call ins_task 2 'Tax return'
call ins_task 6 'Relax'
call ins_task 6 'Enjoy'
say '------ Showing tasks.'; call show_tasks say '------ Show and delete top task.' todo=n /* tasks to be done */ do While todo>0
Say top() End
exit
ins_task: procedure expose n task. n=n+1 Parse Arg task.0pri.n task.0txt.n Return
show_tasks: procedure expose task. n do i=1 To n
Say task.0pri.i task.0txt.i End
Return
top: procedure expose n task. todo /* get top task and mark it 'done' */ high=0 Do i=1 To n
If task.0pri.i>high &,
task.0done.i=0 Then Do
j=i
high=task.0pri.i
End
End
res=task.0pri.j task.0txt.j task.0done.j=1 todo=todo-1 return res</lang>
- Output:
------ inserting tasks. ------ Showing tasks. 3 Clear drains 4 Feed cat 5 Make tea 1 Solve RC tasks 2 Tax return 6 Relax 6 Enjoy ------ Show and delete top task. 6 Relax 6 Enjoy 5 Make tea 4 Feed cat 3 Clear drains 2 Tax return 1 Solve RC tasks
Ruby
A naive, inefficient implementation <lang ruby>class PriorityQueueNaive
def initialize(data=nil)
@q = Hash.new {|h, k| h[k] = []}
data.each {|priority, item| @q[priority] << item} if data
@priorities = @q.keys.sort
end
def push(priority, item)
@q[priority] << item
@priorities = @q.keys.sort
end
def pop
p = @priorities[0]
item = @q[p].shift
if @q[p].empty?
@q.delete(p)
@priorities.shift
end
item
end
def peek
unless empty?
@q[@priorities[0]][0]
end
end
def empty?
@priorities.empty?
end
def each
@q.each do |priority, items|
items.each {|item| yield priority, item}
end
end
def dup
@q.each_with_object(self.class.new) do |(priority, items), obj|
items.each {|item| obj.push(priority, item)}
end
end
def merge(other)
raise TypeError unless self.class == other.class
pq = dup
other.each {|priority, item| pq.push(priority, item)}
pq # return a new object
end
def inspect
@q.inspect
end
end
test = [
[6, "drink tea"], [3, "Clear drains"], [4, "Feed cat"], [5, "Make tea"], [6, "eat biscuit"], [1, "Solve RC tasks"], [2, "Tax return"],
]
pq = PriorityQueueNaive.new test.each {|pr, str| pq.push(pr, str) } until pq.empty?
puts pq.pop
end
puts test2 = test.shift(3) p pq1 = PriorityQueueNaive.new(test) p pq2 = PriorityQueueNaive.new(test2) p pq3 = pq1.merge(pq2) puts "peek : #{pq3.peek}" until pq3.empty?
puts pq3.pop
end puts "peek : #{pq3.peek}"</lang>
- Output:
Solve RC tasks
Tax return
Clear drains
Feed cat
Make tea
drink tea
eat biscuit
{5=>["Make tea"], 6=>["eat biscuit"], 1=>["Solve RC tasks"], 2=>["Tax return"]}
{6=>["drink tea"], 3=>["Clear drains"], 4=>["Feed cat"]}
{5=>["Make tea"], 6=>["eat biscuit", "drink tea"], 1=>["Solve RC tasks"], 2=>["Tax return"], 3=>["Clear drains"], 4=>["Feed cat"]}
peek : Solve RC tasks
Solve RC tasks
Tax return
Clear drains
Feed cat
Make tea
eat biscuit
drink tea
peek :
Run BASIC
<lang runbasic>sqliteconnect #mem, ":memory:"
- mem execute("CREATE TABLE queue (priority float,descr text)")
' -------------------------------------------------------------- ' Insert items into the que ' --------------------------------------------------------------
- mem execute("INSERT INTO queue VALUES (3,'Clear drains')")
- mem execute("INSERT INTO queue VALUES (4,'Feed cat')")
- mem execute("INSERT INTO queue VALUES (5,'Make tea')")
- mem execute("INSERT INTO queue VALUES (1,'Solve RC tasks')")
- mem execute("INSERT INTO queue VALUES (2,'Tax return')")
'--------------- insert priority between 4 and 5 -----------------
- mem execute("INSERT INTO queue VALUES (4.5,'My Special Project')")
what$ = " -------------- Find first priority ---------------------" mem$ = "SELECT * FROM queue ORDER BY priority LIMIT 1" gosub [getQueue]
what$ = " -------------- Find last priority ---------------------" mem$ = "SELECT * FROM queue ORDER BY priority desc LIMIT 1" gosub [getQueue]
what$ = " -------------- Delete Highest Priority ---------------------" mem$ = "DELETE FROM queue WHERE priority = (select max(q.priority) FROM queue as q)"
- mem execute(mem$)
what$ = " -------------- List Priority Sequence ---------------------" mem$ = "SELECT * FROM queue ORDER BY priority" gosub [getQueue] end
[getQueue]
print what$
- mem execute(mem$)
rows = #mem ROWCOUNT() print "Priority Description" for i = 1 to rows
#row = #mem #nextrow() priority = #row priority() descr$ = #row descr$()
print priority;" ";descr$ next i RETURN</lang>
- Output:
-------------- Find first priority --------------------- Priority Description 1.0 Solve RC tasks -------------- Find last priority --------------------- Priority Description 5.0 Make tea -------------- List Priority Sequence --------------------- Priority Description 1.0 Solve RC tasks 2.0 Tax return 3.0 Clear drains 4.0 Feed cat 4.5 My Special Project
Rust
<lang rust>use std::collections::BinaryHeap; use std::cmp::Ordering; use std::borrow::Cow;
- [derive(Eq, PartialEq)]
struct Item<'a> {
priority: usize, task: Cow<'a, str>, // Takes either borrowed or owned string
}
impl<'a> Item<'a> {
fn new<T>(p: usize, t: T) -> Self
where T: Into<Cow<'a, str>>
{
Item {
priority: p,
task: t.into(),
}
}
}
// Manually implpement Ord so we have a min heap impl<'a> Ord for Item<'a> {
fn cmp(&self, other: &Self) -> Ordering {
other.priority.cmp(&self.priority)
}
}
// PartialOrd is required by Ord impl<'a> PartialOrd for Item<'a> {
fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
Some(self.cmp(other))
}
}
fn main() {
let mut queue = BinaryHeap::with_capacity(5); queue.push(Item::new(3, "Clear drains")); queue.push(Item::new(4, "Feed cat")); queue.push(Item::new(5, "Make tea")); queue.push(Item::new(1, "Solve RC tasks")); queue.push(Item::new(2, "Tax return"));
for item in queue {
println!("{}", item.task);
}
}</lang>
- Output:
Solve RC tasks Tax return Make tea Feed cat Clear drains
SAS
Using macros in a SAS data step: <lang sas>%macro HeapInit(size=1000,nchar=20); do; _len = 0; _size = &size; array _times(&size) _temporary_ ; array _kinds(&size) $ &nchar _temporary_; drop _size _len; end; %mend; %macro HeapSwapItem(index1, index2); do; _tempN = _times[&index1]; _times[&index1] = _times[&index2]; _times[&index2] = _tempN; _tempC = _kinds[&index1]; _kinds[&index1] = _kinds[&index2]; _kinds[&index2]= _tempC; drop _tempN _tempC; end; %mend; %macro HeapEmpty; (_len=0) %mend; %macro HeapCompare(index1, index2); (_times[&index1] < _times[&index2]) %mend; %macro HeapSiftdown(index); do; _parent = &index; _done = 0; do while (_parent*2 <= _len & ^_done); _child = _parent*2; if (_child+1 <= _len and %HeapCompare(_child+1,_child)) then _child = _child + 1; if %HeapCompare(_child,_parent) then do; %HeapSwapItem(_child,_parent); _parent = _child; end; else _done = 1; end; drop _done _parent _child; end; %mend; %macro HeapSiftup(index); do; _child = &index; _done = 0; do while(_child>1 and ^_done); _parent = floor(_child/2); if %HeapCompare(_parent,_child) then _done = 1; else do; %HeapSwapItem(_child,_parent); _tempN = _child; _child = _parent; _parent = _tempN; end; end; drop _parent _child _done _tempN; end; %mend; %macro HeapPush(time, kind); do; if _len >= _size then do; put "ERROR: exceeded size of heap. Consider changing size argument to %HeapInit."; stop; end; _len = _len + 1; _times[_len] = &time; _kinds[_len] = &kind; %HeapSiftup(_len); end; %mend; %macro HeapPop; do; _len = _len - 1; if (_len>0) then do; _times[1] = _times[_len+1]; _kinds[1] = _kinds[_len+1]; %HeapSiftdown(1); end; end; %mend; %macro HeapPeek; time = _times[1]; kind = _kinds[1]; put time kind; %mend; data _null_; %HeapInit; %HeapPush(3, "Clear drains"); %HeapPush(4, "Feed cat"); %HeapPush(5, "Make tea"); %HeapPush(1, "Solve RC tasks"); %HeapPush(2, "Tax return"); do while(^%HeapEmpty); %HeapPeek; %HeapPop; end; run;</lang>
- Output:
1 Solve RC tasks 2 Tax return 3 Clear drains 4 Feed cat 5 Make tea
An implementation using proc ds2 may be more structured:
<lang sas>proc ds2;
package Heap / overwrite=yes;
dcl int _entryorder _size _len _entryOrders[1000];
dcl double _times[1000];
dcl varchar(20) _kinds[1000];
method init();
_entryOrder = 1;
_size = 1000;
_len = 0;
end;
method empty() returns int;
return(_len=0);
end;
method compare(int index1, int index2) returns int;
return(_times[index1] < _times[index2] or (_times[index1] = _times[index2] and _entryOrders[index1] < _entryOrders[index2]));
end;
method SetItem(int index, double _time, int entryOrder, varchar(20) kind);
_times[index] = _time;
_entryOrders[index] = entryOrder;
_kinds[index] = kind;
end;
method CopyItem(int ito, int ifrom);
_times[ito] = _times[ifrom];
_entryOrders[ito] = _entryOrders[ifrom];
_kinds[ito] = _kinds[ifrom];
end;
method SwapItem(int index1, int index2);
dcl double tempD;
dcl int tempI;
dcl varchar(20) tempC;
tempD = _times[index1]; _times[index1] = _times[index2]; _times[index2]= tempD;
tempI = _entryorders[index1]; _entryorders[index1] = _entryorders[index2]; _entryorders[index2]= tempI;
tempC = _kinds[index1]; _kinds[index1] = _kinds[index2]; _kinds[index2]= tempC;
end;
method Siftdown(int index);
dcl int parent done child;
parent = index;
done = 0;
do while (parent*2 <= _len and ^done);
child = parent*2;
if (child+1 <= _len and Compare(child+1,child)) then
child = child + 1;
if Compare(child,parent) then do;
SwapItem(child,parent);
parent = child;
end;
else done = 1;
end;
end;
method Siftup(int index);
dcl int parent child done tempI;
child = index;
done = 0;
do while(child>1 and ^done);
parent = floor(child/2);
if Compare(parent,child) then
done = 1;
else do;
SwapItem(child,parent);
tempI = child;
child = parent;
parent = tempI;
end;
end;
end;
method Pop();
_len = _len - 1;
if (_len>0) then do;
CopyItem(1,_len+1);
Siftdown(1);
end;
end;
method PeekTime() returns double;
return _times[1];
end;
method PeekKind() returns varchar(20);
return _kinds[1];
end;
method Push(double _time, varchar(20) kind);
if _len >= _size then do;
put 'ERROR: exceeded size of heap.';
stop;
end;
_len = _len + 1;
_entryOrder = _entryOrder + 1;
SetItem(_len, _time, _entryOrder, kind);
Siftup(_len);
end;
endpackage;
run;
data _null_;
dcl package Heap h();
method init();
dcl double _time;
dcl varchar(20) _kind;
h.init();
h.Push(3, 'Clear drains');
h.Push(4, 'Feed cat');
h.Push(5, 'Make tea');
h.Push(1, 'Solve RC tasks');
h.Push(2, 'Tax return');
do while(^h.empty());
_time = h.PeekTime();
_kind = h.PeekKind();
put _time _kind;
h.Pop();
end;
end;
enddata;
run;</lang>
Scala
Scala has a class PriorityQueue in its standard library. <lang scala>import scala.collection.mutable.PriorityQueue case class Task(prio:Int, text:String) extends Ordered[Task] {
def compare(that: Task)=that.prio compare this.prio
}
//test var q=PriorityQueue[Task]() ++ Seq(Task(3, "Clear drains"), Task(4, "Feed cat"),
Task(5, "Make tea"), Task(1, "Solve RC tasks"), Task(2, "Tax return"))
while(q.nonEmpty) println(q dequeue)</lang> Output:
Task(1,Solve RC tasks) Task(2,Tax return) Task(3,Clear drains) Task(4,Feed cat) Task(5,Make tea)
Instead of deriving the class from Ordering an implicit conversion could be provided. <lang scala>case class Task(prio:Int, text:String) implicit def taskOrdering=new Ordering[Task] {
def compare(t1:Task, t2:Task):Int=t2.prio compare t1.prio
}</lang>
SenseTalk
We use New to create an object instance -- in this case (for simplicity) based on the script itself, which is named PriorityQueue. The Tell command or dot notation are then used to send messages directly to that object. <lang sensetalk> // PriorityQueue.script set Tasks to a new PriorityQueue
tell Tasks to add 3,"Clear drains" tell Tasks to add 4,"Feed cat" tell Tasks to add 5,"Make tea" tell Tasks to add 1,"Solve RC tasks" tell Tasks to add 2,"Tax return"
put "Initial tasks:" put Tasks.report & return
put Tasks.getTop into topItem put "Top priority: " & topItem & return
put "Remaining:" & return & Tasks.report
properties queue:[] end properties
to add newPriority, newTask repeat with each item of my queue if newPriority comes before the priority of it then insert {priority:newPriority, task:newTask} before item the counter of my queue return end if end repeat insert {priority:newPriority, task:newTask} into my queue -- add at end if last priority end add
to getTop pull my queue into firstItem return firstItem end getTop
to report return (priority of each && task of each for each item of my queue) joined by return end report </lang>
- Output:
Initial tasks:
1 Solve RC tasks
2 Tax return
3 Clear drains
4 Feed cat
5 Make tea
Top priority: {priority:1, task:"Solve RC tasks"}
Remaining:
2 Tax return
3 Clear drains
4 Feed cat
5 Make tea
Sidef
<lang ruby>class PriorityQueue {
has tasks = []
method insert (Number priority { _ >= 0 }, task) {
for n in range(tasks.len, priority) {
tasks[n] = []
}
tasks[priority].append(task)
}
method get { tasks.first { !.is_empty } -> shift }
method is_empty { tasks.all { .is_empty } }
}
var pq = PriorityQueue()
[
[3, 'Clear drains'], [4, 'Feed cat'], [5, 'Make tea'], [9, 'Sleep'], [3, 'Check email'], [1, 'Solve RC tasks'], [9, 'Exercise'], [2, 'Do taxes'],
].each { |pair|
pq.insert(pair...)
}
say pq.get while !pq.is_empty</lang>
- Output:
Solve RC tasks Do taxes Clear drains Check email Feed cat Make tea Sleep Exercise
Standard ML
Note: this is a max-heap
<lang sml>structure TaskPriority = struct
type priority = int val compare = Int.compare type item = int * string val priority : item -> int = #1
end
structure PQ = LeftPriorityQFn (TaskPriority)
let
val tasks = [
(3, "Clear drains"),
(4, "Feed cat"),
(5, "Make tea"),
(1, "Solve RC tasks"),
(2, "Tax return")]
val pq = foldl PQ.insert PQ.empty tasks
(* or val pq = PQ.fromList tasks *)
fun aux pq' =
case PQ.next pq' of
NONE => ()
| SOME ((prio, name), pq) => (
print (Int.toString prio ^ ", " ^ name ^ "\n");
aux pq
)
in
aux pq
end</lang>
testing:
5, Make tea 4, Feed cat 3, Clear drains 2, Tax return 1, Solve RC tasks
Stata
Using mata, which has 1-based arrays:
<lang stata>mata
struct Node {
real scalar time
transmorphic data
}
class Heap {
public:
struct Node rowvector nodes
real scalar len
real scalar size
real scalar minHeap
void new()
void push()
void siftup()
void siftdown()
struct Node scalar pop()
real scalar empty()
real scalar compare()
}
real scalar Heap::compare(a,b) {
struct Node scalar left, right
left = nodes[a]
right = nodes[b]
return(minHeap ? left.time<right.time : left.time>right.time)
}
void Heap::new() {
len = 0
size = 4
nodes = Node(1,size)
minHeap = 1 // defaults to min heap
}
real scalar Heap::empty() {
return(len==0)
}
void Heap::siftdown(real scalar index) {
parent = index
while (parent*2 <= len) {
child = parent*2
if (child+1 <= len ? compare(child+1,child) : 0) {
child++
}
if (compare(child,parent)) {
nodes[(child,parent)] = nodes[(parent,child)]
parent = child
} else break
}
}
void Heap::siftup(real scalar index) {
child = index
while(child>1) {
parent = floor(child/2)
if (compare(parent,child)) {
break
}
nodes[(child,parent)] = nodes[(parent,child)]
temp = child
child = parent
parent = temp
}
}
void Heap::push (real scalar time, transmorphic data) {
if (len + 1 >= size) {
nodes = (nodes, nodes)
size = size*2
}
len++
nodes[len].time = time
nodes[len].data = data
siftup(len)
}
struct Node scalar Heap::pop () {
if (len==0) {
_error(3000,"empty heap")
}
len--
struct Node scalar newnode
newnode.time = nodes[1].time
newnode.data = nodes[1].data
if (len>0) {
nodes[1] = nodes[len+1]
siftdown(1)
}
return(newnode)
}
void testHeap(real scalar minHeap) {
class Heap scalar h
struct Node scalar node
h = Heap()
h.minHeap = minHeap
h.push(3, "Clear drains")
h.push(4, "Feed cat")
h.push(5, "Make tea")
h.push(1, "Solve RC tasks")
h.push(2, "Tax return")
while (!h.empty()) {
node = h.pop()
printf("%f -> %s\n", node.time, node.data)
}
}
testHeap(1)
testHeap(0)
end
</lang>
- Output:
: testHeap(1) 1 -> Solve RC tasks 2 -> Tax return 3 -> Clear drains 4 -> Feed cat 5 -> Make tea : testHeap(0) 5 -> Make tea 4 -> Feed cat 3 -> Clear drains 2 -> Tax return 1 -> Solve RC tasks
Note: the Fortran version was simpler and did not use the siftup() method.
Swift
You can use CFBinaryHeap from Core Foundation, but it is super ugly due to the fact that CFBinaryHeap operates on generic pointers, and you need to convert back and forth between that and objects.
<lang swift>class Task : Comparable, CustomStringConvertible {
var priority : Int
var name: String
init(priority: Int, name: String) {
self.priority = priority
self.name = name
}
var description: String {
return "\(priority), \(name)"
}
} func ==(t1: Task, t2: Task) -> Bool {
return t1.priority == t2.priority
} func <(t1: Task, t2: Task) -> Bool {
return t1.priority < t2.priority
}
struct TaskPriorityQueue {
let heap : CFBinaryHeapRef = {
var callBacks = CFBinaryHeapCallBacks(version: 0, retain: {
UnsafePointer(Unmanaged<Task>.fromOpaque(COpaquePointer($1)).retain().toOpaque())
}, release: {
Unmanaged<Task>.fromOpaque(COpaquePointer($1)).release()
}, copyDescription: nil, compare: { (ptr1, ptr2, _) in
let t1 : Task = Unmanaged<Task>.fromOpaque(COpaquePointer(ptr1)).takeUnretainedValue()
let t2 : Task = Unmanaged<Task>.fromOpaque(COpaquePointer(ptr2)).takeUnretainedValue()
return t1 == t2 ? CFComparisonResult.CompareEqualTo : t1 < t2 ? CFComparisonResult.CompareLessThan : CFComparisonResult.CompareGreaterThan
})
return CFBinaryHeapCreate(nil, 0, &callBacks, nil)
}()
var count : Int { return CFBinaryHeapGetCount(heap) }
mutating func push(t: Task) {
CFBinaryHeapAddValue(heap, UnsafePointer(Unmanaged.passUnretained(t).toOpaque()))
}
func peek() -> Task {
return Unmanaged<Task>.fromOpaque(COpaquePointer(CFBinaryHeapGetMinimum(heap))).takeUnretainedValue()
}
mutating func pop() -> Task {
let result = Unmanaged<Task>.fromOpaque(COpaquePointer(CFBinaryHeapGetMinimum(heap))).takeUnretainedValue()
CFBinaryHeapRemoveMinimumValue(heap)
return result
}
}
var pq = TaskPriorityQueue()
pq.push(Task(priority: 3, name: "Clear drains")) pq.push(Task(priority: 4, name: "Feed cat")) pq.push(Task(priority: 5, name: "Make tea")) pq.push(Task(priority: 1, name: "Solve RC tasks")) pq.push(Task(priority: 2, name: "Tax return"))
while pq.count != 0 {
print(pq.pop())
}</lang>
- Output:
1, Solve RC tasks 2, Tax return 3, Clear drains 4, Feed cat 5, Make tea
Tcl
<lang tcl>package require struct::prioqueue
set pq [struct::prioqueue] foreach {priority task} {
3 "Clear drains" 4 "Feed cat" 5 "Make tea" 1 "Solve RC tasks" 2 "Tax return"
} {
# Insert into the priority queue $pq put $task $priority
}
- Drain the queue, in priority-sorted order
while {[$pq size]} {
# Remove the front-most item from the priority queue puts [$pq get]
}</lang> Which produces this output:
Make tea Feed cat Clear drains Tax return Solve RC tasks
uBasic/4tH
This implementation inserts items using a binary search. Hence, no sorting is required, since all entries are always in order of priority. It also allows for listing of valid entries. <lang>b = -1 ' b points to last entry on the queue
q = FUNC(_Grab) ' now grab the top value
' and display it
Print Peek(q, 0) - Ord("0"), Show(Chop(q, 1)) Print
Proc _Insert(3, Dup("Clear drains")) ' insert the whole bunch Proc _Insert(4, Dup("Feed cat")) Proc _Insert(5, Dup("Make tea")) Proc _Insert(1, Dup("Solve RC tasks")) Proc _Insert(2, Dup("Tax return"))
For x = 0 To b: Proc _List(x) : Next ' list all entries
q = FUNC(_Grab) ' now grab the top value
Print ' and display it Print Peek(q, 0) - Ord("0"), Show(Chop(q, 1)) Print
For x = 0 To b: Proc _List(x) : Next ' list all entries End
_Grab ' return and remove top entry
Local (2)
' return dummy on error
If b < 0 Then Return (Dup("0No such entry"))
a@ = @(0)) ' save the top entry
For b@ = 0 To Set(b, b - 1) : @(b@) = @(b@+1): Next
Return (a@)
_List ' list any (valid) position on queue
Param (1)
If (a@ > b) = 0 Then Print Peek(@(a@), 0) - Ord("0"), Show(Chop(@(a@), 1))
Return
_Insert ' insert an entry
Param (2) ' priority, task Local (1) c@ = FUNC(_binarySearch(a@, 0, b)) ' search the position Proc _MakeRoom (c@) ' make room @(c@) = Join(Str(a@), b@) ' assign the entry
Return
_binarySearch ' perform a binary search
Param(3) ' value, start index, end index
Local(1) ' The middle of the array
' Ok, signal we didn't find it
If c@ < b@ Then Return (b@) ' first entry on start
d@ = SHL(b@ + c@, -1) ' prevent overflow (LOL!)
If a@ < Peek(@(d@), 0) - Ord("0") Then Return (FUNC(_binarySearch (a@, b@, d@-1)))
If a@ > Peek(@(d@), 0) - Ord("0") Then Return (FUNC(_binarySearch (a@, d@+1, c@)))
If a@ = Peek(@(d@), 0) - Ord("0") Then Return (d@)
Return (-1) ' -1 on error
_MakeRoom ' make some space
Param (1) ' starting position
Local (1)
' from bottom to top
For b@ = Set(b, b+1) To a@ + 1 Step -1: @(b@) = @(b@-1) : Next
Return </lang>
- Output:
First an entry is dequeued from an empty queue. Then all entries are inserted and listed. Finally, another entry is dequeued and the remainder of the entries is listed again.
0 No such entry 1 Solve RC tasks 2 Tax return 3 Clear drains 4 Feed cat 5 Make tea 1 Solve RC tasks 2 Tax return 3 Clear drains 4 Feed cat 5 Make tea 0 OK, 0:750
VBA
<lang VB>Type Tuple
Priority As Integer Data As String
End Type Dim a() As Tuple Dim n As Integer 'number of elements in array, last element is n-1 Private Function Left(i As Integer) As Integer
Left = 2 * i + 1
End Function Private Function Right(i As Integer) As Integer
Right = 2 * i + 2
End Function Private Function Parent(i As Integer) As Integer
Parent = (i - 1) \ 2
End Function Private Sub Add(fPriority As Integer, fData As String)
n = n + 1 If n > UBound(a) Then ReDim Preserve a(2 * n) a(n - 1).Priority = fPriority a(n - 1).Data = fData bubbleUp (n - 1)
End Sub Private Sub Swap(i As Integer, j As Integer)
Dim t As Tuple t = a(i) a(i) = a(j) a(j) = t
End Sub Private Sub bubbleUp(i As Integer)
Dim p As Integer
p = Parent(i)
Do While i > 0 And a(i).Priority < a(p).Priority
Swap i, p
i = p
p = Parent(i)
Loop
End Sub Private Function Remove() As Tuple
Dim x As Tuple x = a(0) a(0) = a(n - 1) n = n - 1 trickleDown 0 If 3 * n < UBound(a) Then ReDim Preserve a(UBound(a) \ 2) Remove = x
End Function Private Sub trickleDown(i As Integer)
Dim j As Integer, l As Integer, r As Integer
Do
j = -1
r = Right(i)
If r < n And a(r).Priority < a(i).Priority Then
l = Left(i)
If a(l).Priority < a(r).Priority Then
j = l
Else
j = r
End If
Else
l = Left(i)
If l < n And a(l).Priority < a(i).Priority Then j = l
End If
If j >= 0 Then Swap i, j
i = j
Loop While i >= 0
End Sub Public Sub PQ()
ReDim a(4)
Add 3, "Clear drains"
Add 4, "Feed cat"
Add 5, "Make tea"
Add 1, "Solve RC tasks"
Add 2, "Tax return"
Dim t As Tuple
Do While n > 0
t = Remove
Debug.Print t.Priority, t.Data
Loop
End Sub</lang>
- Output:
1 Solve RC tasks2 Tax return 3 Clear drains 4 Feed cat 5 Make tea
VBScript
I wrote this priority queue in a class. It uses a dynamic array to implement the class. Function out_of_order must be adapted to the data. <lang vb> option explicit
Class prio_queue private size private q 'adapt this function to your data private function out_of_order(f1,f2): out_of_order = f1(0)>f2(0):end function function peek peek=q(1) end function property get qty qty=size end property
property get isempty
isempty=(size=0)
end property
function remove
dim x
x=q(1)
q(1)=q(size)
size=size-1
sift_down
remove=x
end function
sub add (x)
size=size+1
if size>ubound(q) then redim preserve q(ubound(q)*1.1)
q(size)=x
sift_up
end sub
Private sub swap (i,j)
dim x
x=q(i):q(i)=q(j):q(j)=x
end sub
private sub sift_up
dim h,p
h=size
p=h\2
if p=0 then exit sub
while out_of_order(q(p),q(h))
swap h,p
h=p
p=h\2
if p=0 then exit sub
wend
end sub
end sub
private sub sift_down
dim p,h
p=1
do
if p>=size then exit do
h =p*2
if h >size then exit do
if h+1<=size then if out_of_order(q(h),q(h+1)) then h=h+1
if out_of_order(q(p),q(h)) then swap h,p
p=h
loop
end sub
'Al instanciar objeto con New
Private Sub Class_Initialize( )
redim q(100)
size=0
End Sub
'When Object is Set to Nothing
Private Sub Class_Terminate( )
erase q
End Sub
End Class '------------------------------------- 'test program '--------------------------------- dim tasks:tasks=array(_
array(3,"Clear drains"),_ array(4,"Feed cat"),_ array(5,"Make tea"),_ array(1,"Solve RC tasks"),_ array(2,"Tax return"))
dim queue,i,x set queue=new prio_queue for i=0 to ubound(tasks)
queue.add(tasks(i))
next
wscript.echo "Done: " & queue.qty() &" items in queue. "& queue.peek()(1)& " is at the top." & vbcrlf
while not queue.isempty()
x=queue.remove() wscript.echo x(0),x(1)
wend
set queue= nothing
</lang> Output:
Done: 5 items in queue. Item Solve RC tasks is at the top. 1 Solve RC tasks 2 Tax return 3 Clear drains 4 Feed cat 5 Make tea
Wren
The above module contains a PriorityQueue class. Unlike some other languages here, the higher the number, the higher the priority. So the 'Make tea' task has the highest priority and, thankfully, the cat has a good chance of being fed! <lang ecmascript>import "/queue" for PriorityQueue
var tasks = PriorityQueue.new() tasks.push("Clear drains", 3) tasks.push("Feed cat", 4) tasks.push("Make tea", 5) tasks.push("Solve RC tasks", 1) tasks.push("Tax return", 2) while (!tasks.isEmpty) {
var t = tasks.pop() System.print(t)
}</lang>
- Output:
[Make tea, 5] [Feed cat, 4] [Clear drains, 3] [Tax return, 2] [Solve RC tasks, 1]
XLISP
It does not seem necessary that every queue should support arbitrarily many distinct priority levels, so long as each particular queue supports as many levels as the user anticipates needing. We therefore store a priority queue as a fixed-length vector of queues and allow the user to pass the least urgent level needed (counting from 0 as the most urgent) as a parameter when a new priority queue is instantiated.
A vector can be efficiently indexed into, and we can eliminate a lot of searching by providing for each priority queue to know its most urgent priority level at any given time. The 'POP method can then return the first item stored at that level, without needing to search. If this operation leaves that level empty, however, it does need to search for the next non-empty level. The worst case would be popping from a queue that contained only one item, at the most urgent priority level: the program would have to search down all the levels looking for one that wasn't empty. In the nature of a priority queue, however, this case is probably unusual.
The 'PUSH method never needs to search down the levels. The efficiency bottleneck here is probably the implementation of NCONC (used for adding the new item to the end of the queue at the relevant level). A priority stack, with first in / last out at each priority level rather than first in / first out, would be faster.
<lang lisp>(define-class priority-queue (instance-variables queue lowest-priority most-urgent) )
(define-method (priority-queue 'initialize limit) (defun setup (x) (vector-set! queue x nil) (if (< x limit) (setup (+ x 1)) ) ) (setq lowest-priority limit) (setq most-urgent limit) (setq queue (make-vector (+ limit 1))) (setup 0) self )
(define-method (priority-queue 'push item priority) (if (and (integerp priority) (>= priority 0) (<= priority lowest-priority)) (progn (setq most-urgent (min priority most-urgent)) (vector-set! queue priority (nconc (vector-ref queue priority) (cons item nil))) ) ) )
(define-method (priority-queue 'pop) (defun find-next (q) (if (or (= q lowest-priority) (not (null (vector-ref queue q)))) q (find-next (+ q 1)) ) ) (define item (car (vector-ref queue most-urgent))) (vector-set! queue most-urgent (cdr (vector-ref queue most-urgent))) (setq most-urgent (find-next most-urgent)) item )
(define-method (priority-queue 'peek) (car (vector-ref queue most-urgent)) )
(define-method (priority-queue 'emptyp) (and (= most-urgent lowest-priority) (null (vector-ref queue most-urgent))) )</lang>
The example uses strings, but the data items stored in the priority queue can be of any type (including the empty list—or even other priority queues). <lang lisp>(define pq (priority-queue 'new 5))
(pq 'push "Clear drains" 3) (pq 'push "Feed cat" 4) (pq 'push "Make tea" 5) (pq 'push "Solve RC tasks" 1) (pq 'push "Tax return" 2)</lang>
- Output:
Items are popped beginning from the most urgent: <lang lisp>[1] (pq 'pop)
"Solve RC tasks" [2] (pq 'pop)
"Tax return"</lang> Within each priority level, new items are pushed onto the end and popped from the beginning of the list (a queue is a first in / first out data structure): <lang lisp>[3] (pq 'push "Answer emails" 4)
("Feed cat" "Answer emails")</lang> Attempting to push with an invalid priority value returns the empty list, i.e. false: <lang lisp>[4] (pq 'push "Weed garden" 17)
()</lang>
'EMPTYP returns false if the priority queue is not empty:
<lang lisp>[5] (pq 'emptyp)
()</lang>
'PEEK non-destructively returns the item that would be popped if you called 'POP:
<lang lisp>[6] (pq 'peek)
"Clear drains"</lang>
If you want to examine a whole priority queue, the built-in 'SHOW method allows you to do so:
<lang scheme>[7] (pq 'show)
Object is #<Object:PRIORITY-QUEUE #x4e2cba8>, Class is #<Class:PRIORITY-QUEUE #x4e254c8> Instance variables:
QUEUE = #(() () () ("Clear drains") ("Feed cat" "Answer emails") ("Make tea"))
LOWEST-PRIORITY = 5
MOST-URGENT = 3
- <Object:PRIORITY-QUEUE #x4e2cba8></lang>
Once all the items have been popped, the priority queue is empty and 'EMPTYP then returns true:
<lang lisp>[8] (pq 'pop)
"Clear drains" [9] (pq 'pop)
"Feed cat" [10] (pq 'pop)
"Answer emails" [11] (pq 'pop)
"Make tea" [12] (pq 'emptyp)
- T</lang>
Attempting to pop from an empty priority queue returns false: <lang lisp>[13] (pq 'pop)
()</lang>
Zig
Zig's standard library has a built-in implementation of the Priority Queue data structure.
Save the following code as priority_queue.zig, and run the tests using zig test priority_queue.zig.
<lang Zig> const std = @import("std"); const PriorityQueue = std.PriorityQueue; const Allocator = std.mem.Allocator; const testing = std.testing;
/// wrapper for the task - stores task priority /// along with the task name const Task = struct {
const Self = @This();
priority: i32, name: []const u8,
pub fn init(priority: i32, name: []const u8) Self {
return Self{
.priority = priority,
.name = name,
};
}
};
/// Simple wrapper for the comparator function. /// Each comparator function has the following signature: /// /// fn(T, T) bool const Comparator = struct {
fn maxCompare(a: Task, b: Task) bool {
return a.priority > b.priority;
}
fn minCompare(a: Task, b: Task) bool {
return a.priority < b.priority;
}
};
test "priority queue (max heap)" {
var arena = std.heap.ArenaAllocator.init(std.heap.page_allocator); defer arena.deinit(); var allocator = &arena.allocator;
var pq = PriorityQueue(Task).init(allocator, Comparator.maxCompare); defer pq.deinit();
try pq.add(Task.init(3, "Clear drains")); try pq.add(Task.init(4, "Feed Cat")); try pq.add(Task.init(5, "Make tea")); try pq.add(Task.init(1, "Solve RC tasks")); try pq.add(Task.init(2, "Tax returns")); testing.expectEqual(pq.count(), 5);
std.debug.print("\n", .{});
// execute the tasks in decreasing order of priority
while (pq.count() != 0) {
const task = pq.remove();
std.debug.print("Executing: {} (priority {})\n", .{ task.name, task.priority });
}
std.debug.print("\n", .{});
}
test "priority queue (min heap)" {
var arena = std.heap.ArenaAllocator.init(std.heap.page_allocator); defer arena.deinit(); var allocator = &arena.allocator;
var pq = PriorityQueue(Task).init(allocator, Comparator.minCompare); defer pq.deinit();
try pq.add(Task.init(3, "Clear drains")); try pq.add(Task.init(4, "Feed Cat")); try pq.add(Task.init(5, "Make tea")); try pq.add(Task.init(1, "Solve RC tasks")); try pq.add(Task.init(2, "Tax returns")); testing.expectEqual(pq.count(), 5);
std.debug.print("\n", .{});
// execute the tasks in increasing order of priority
while (pq.count() != 0) {
const task = pq.remove();
std.debug.print("Executing: {} (priority {})\n", .{ task.name, task.priority });
}
std.debug.print("\n", .{});
}
</lang>
Sample output:
<lang Zig> $ zig test priority_queue.zig Test [1/2] test "priority queue (max heap)"... Executing: Make tea (priority 5) Executing: Feed Cat (priority 4) Executing: Clear drains (priority 3) Executing: Tax returns (priority 2) Executing: Solve RC tasks (priority 1)
Test [2/2] test "priority queue (min heap)"... Executing: Solve RC tasks (priority 1) Executing: Tax returns (priority 2) Executing: Clear drains (priority 3) Executing: Feed Cat (priority 4) Executing: Make tea (priority 5)
All 2 tests passed. </lang>
zkl
This solution uses a [hopefully small] fixed number of priorities, each of which has an unordered list of tasks. This allows O(1) insertions, O(p) for retrievals (p is the number of priorities). <lang zkl>class PQ{
fcn init(numLevels=10){ // 0..numLevels, bigger # == lower priorty
var [const] queue=(1).pump(numLevels+1,List.createLong(numLevels).write,L().copy);
}
fcn add(item,priorty){ queue[priorty].append(item); }
fcn peek{ if(q:=queue.filter1()) return(q[-1]); Void }// -->Void if empty
fcn pop { if(q:=queue.filter1()) return(q.pop()); Void }// -->Void if empty
var [private] state=L();
fcn [private] next{ // iterate
qi,ii:=state;
foreach n in ([qi..queue.len()-1]){
q:=queue[n];
if(ii>=q.len()) ii=0; else{ state.clear().append(n,ii+1); return(q[ii]) }
}
Void.Stop
}
fcn walker{ state.clear().append(0,0); Walker(next) } // iterator front end
fcn toString{ "PQ(%d) items".fmt(queue.reduce(fcn(sum,q){ sum+q.len() },0)) }
}</lang> <lang zkl>pq:=PQ(); foreach x in
(T("Clear drains",3, "Feed cat",4, "Make tea",5, "Solve RC tasks",1, "Tax return",2,
"Clean room",10,"Wash cat",10)){
pq.add(x,__xWalker.next())
} pq.println(); println("Number 1 thing to do: ",pq.peek()); println("Top 2 things to do: ",pq.walker().walk(2)); println("Do this next year: ",pq.walker().walk()[-1]); println("ToDo list:"); foreach item in (pq){ item.println() } pq.println();</lang>
- Output:
PQ(7) items
Number 1 thing to do: Solve RC tasks
Top 2 things to do: L("Solve RC tasks","Tax return")
Do this next year: Wash cat
ToDo list:
Solve RC tasks
Tax return
Clear drains
Feed cat
Make tea
Clean room
Wash cat
PQ(7) items