Queue/Definition
You are encouraged to solve this task according to the task description, using any language you may know.
Data Structure
This illustrates a data structure, a means of storing data within a program.
- Task
Implement a FIFO queue.
Elements are added at one side and popped from the other in the order of insertion.
Operations:
- push (aka enqueue) - add element
- pop (aka dequeue) - pop first element
- empty - return truth value when empty
Errors:
- handle the error of trying to pop from an empty queue (behavior depends on the language and platform)
- See
- Queue/Usage for the built-in FIFO or queue of your language or standard library.
- See also
- Array
- Associative array: Creation, Iteration
- Collections
- Compound data type
- Doubly-linked list: Definition, Element definition, Element insertion, List Traversal, Element Removal
- Linked list
- Queue: Definition, Usage
- Set
- Singly-linked list: Element definition, Element insertion, List Traversal, Element Removal
- Stack
11l
<lang 11l>T FIFO
[Int] contents
F push(item)
.contents.append(item)
F pop()
R .contents.pop(0)
F empty()
R .contents.empty
V f = FIFO() f.push(3) f.push(2) f.push(1) L !f.empty()
print(f.pop())</lang>
- Output:
3 2 1
AArch64 Assembly
<lang AArch64 Assembly> /* ARM assembly AARCH64 Raspberry PI 3B */ /* program defqueue64.s */
/*******************************************/ /* Constantes file */ /*******************************************/ /* for this file see task include a file in language AArch64 assembly*/ .include "../includeConstantesARM64.inc"
.equ NBMAXIELEMENTS, 100
/*******************************************/ /* Structures */ /********************************************/ /* example structure for value of item */
.struct 0
value_ident: // ident
.struct value_ident + 8
value_value1: // value 1
.struct value_value1 + 8
value_value2: // value 2
.struct value_value2 + 8
value_fin: /* example structure for queue */
.struct 0
queue_ptdeb: // begin pointer of item
.struct queue_ptdeb + 8
queue_ptfin: // end pointer of item
.struct queue_ptfin + 8
queue_stvalue: // structure of value item
.struct queue_stvalue + (value_fin * NBMAXIELEMENTS)
queue_fin:
/*********************************/
/* Initialized data */
/*********************************/
.data
szMessEmpty: .asciz "Empty queue. \n"
szMessNotEmpty: .asciz "Not empty queue. \n"
szMessError: .asciz "Error detected !!!!. \n"
szMessResult: .asciz "Ident : @ value 1 : @ value 2 : @ \n" // message result
szCarriageReturn: .asciz "\n" /*********************************/ /* UnInitialized data */ /*********************************/ .bss .align 4 Queue1: .skip queue_fin // queue memory place stItem: .skip value_fin // value item memory place sZoneConv: .skip 100 /*********************************/ /* code section */ /*********************************/ .text .global main main: // entry of program
ldr x0,qAdrQueue1 // queue structure address bl isEmpty cmp x0,#0 beq 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,qAdrstItem mov x1,#1 str x1,[x0,#value_ident] mov x1,#11 str x1,[x0,#value_value1] mov x1,#12 str x1,[x0,#value_value2] ldr x0,qAdrQueue1 // queue structure address ldr x1,qAdrstItem bl pushQueue // add item in queue cmp x0,#-1 // error ? beq 99f // init item 2 ldr x0,qAdrstItem mov x1,#2 str x1,[x0,#value_ident] mov x1,#21 str x1,[x0,#value_value1] mov x1,#22 str x1,[x0,#value_value2] ldr x0,qAdrQueue1 // queue structure address ldr x1,qAdrstItem bl pushQueue // add item in queue cmp x0,#-1 beq 99f ldr x0,qAdrQueue1 // queue structure address bl isEmpty cmp x0,#0 // not empty beq 3f ldr x0,qAdrszMessEmpty bl affichageMess // display message empty b 4f
3:
ldr x0,qAdrszMessNotEmpty bl affichageMess // display message not empty
4:
ldr x0,qAdrQueue1 // queue structure address bl popQueue // return address item cmp x0,#-1 // error ? beq 99f mov x2,x0 // save item pointer ldr x0,[x2,#value_ident] ldr x1,qAdrsZoneConv // conversion ident bl conversion10S // decimal conversion ldr x0,qAdrszMessResult ldr x1,qAdrsZoneConv bl strInsertAtCharInc // insert result at first @ character mov x5,x0 ldr x0,[x2,#value_value1] ldr x1,qAdrsZoneConv // conversion value 1 bl conversion10S // decimal conversion mov x0,x5 ldr x1,qAdrsZoneConv bl strInsertAtCharInc // insert result at Second @ character mov x5,x0 ldr x0,[x2,#value_value2] ldr x1,qAdrsZoneConv // conversion value 2 bl conversion10S // decimal conversion mov x0,x5 ldr x1,qAdrsZoneConv bl strInsertAtCharInc // insert result at third @ character bl affichageMess // display message final b 4b // 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 qAdrstItem: .quad stItem qAdrszMessError: .quad szMessError qAdrszMessEmpty: .quad szMessEmpty qAdrszMessNotEmpty: .quad szMessNotEmpty qAdrszMessResult: .quad szMessResult qAdrszCarriageReturn: .quad szCarriageReturn qAdrsZoneConv: .quad sZoneConv
/******************************************************************/ /* test if queue empty */ /******************************************************************/ /* x0 contains the address of queue structure */ /* x0 returns 0 if not empty, 1 if empty */ isEmpty:
stp x1,lr,[sp,-16]! // save registers stp x2,x3,[sp,-16]! // save registers ldr x1,[x0,#queue_ptdeb] // begin pointer ldr x2,[x0,#queue_ptfin] // begin pointer cmp x1,x2 bne 1f mov x0,#1 // empty queue b 2f
1:
mov x0,#0 // not empty
2:
ldp x2,x3,[sp],16 // restaur 2 registers ldp x1,lr,[sp],16 // restaur 2 registers ret // return to address lr x30
/******************************************************************/ /* add item in queue */ /******************************************************************/ /* x0 contains the address of queue structure */ /* x1 contains the address of item */ pushQueue:
stp x1,lr,[sp,-16]! // save registers stp x2,x3,[sp,-16]! // save registers add x2,x0,#queue_stvalue // address of values structure ldr x3,[x0,#queue_ptfin] // end pointer add x2,x2,x3 // free address of queue ldr x4,[x1,#value_ident] // load ident item str x4,[x2,#value_ident] // and store in queue ldr x4,[x1,#value_value1] // idem str x4,[x2,#value_value1] ldr x4,[x1,#value_value2] str x4,[x2,#value_value2] add x3,x3,#value_fin cmp x3,#value_fin * NBMAXIELEMENTS beq 99f str x3,[x0,#queue_ptfin] // store new end pointer b 100f
99:
mov x0,#-1 // error
100:
ldp x2,x3,[sp],16 // restaur 2 registers ldp x1,lr,[sp],16 // restaur 2 registers ret // return to address lr x30
/******************************************************************/ /* pop queue */ /******************************************************************/ /* x0 contains the address of queue structure */ popQueue:
stp x1,lr,[sp,-16]! // save registers stp x2,x3,[sp,-16]! // save registers mov x1,x0 // control if empty queue bl isEmpty cmp x0,#1 // yes -> error beq 99f mov x0,x1 ldr x1,[x0,#queue_ptdeb] // begin pointer add x2,x0,#queue_stvalue // address of begin values item add x2,x2,x1 // address of item add x1,x1,#value_fin str x1,[x0,#queue_ptdeb] // store nex begin pointer mov x0,x2 // return pointer item b 100f
99:
mov x0,#-1 // error
100:
ldp x2,x3,[sp],16 // restaur 2 registers ldp x1,lr,[sp],16 // restaur 2 registers ret // return to address lr x30
/********************************************************/ /* 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. Ident : +1 value 1 : +11 value 2 : +12 Ident : +2 value 1 : +21 value 2 : +22 Error detected !!!!.
ACL2
<lang Lisp>(defun enqueue (x xs)
(cons x xs))
(defun dequeue (xs)
(declare (xargs :guard (and (consp xs)
(true-listp xs))))
(if (or (endp xs) (endp (rest xs)))
(mv (first xs) nil)
(mv-let (x ys)
(dequeue (rest xs))
(mv x (cons (first xs) ys)))))
(defun empty (xs)
(endp xs))</lang>
Ada
The first example below demonstrates a FIFO created for single-threaded computing. This version has the advantage of using a minimum of memory per FIFO element, and being very fast.
The interface specification for a FIFO is described in the package specification. <lang ada>generic
type Element_Type is private;
package Fifo is
type Fifo_Type is private; procedure Push(List : in out Fifo_Type; Item : in Element_Type); procedure Pop(List : in out Fifo_Type; Item : out Element_Type); function Is_Empty(List : Fifo_Type) return Boolean; Empty_Error : exception;
private
type Fifo_Element;
type Fifo_Ptr is access Fifo_Element;
type Fifo_Type is record
Head : Fifo_Ptr := null;
Tail : Fifo_Ptr := null;
end record;
type Fifo_Element is record
Value : Element_Type;
Next : Fifo_Ptr := null;
end record;
end Fifo;</lang> The FIFO implementation is described in the package body: <lang ada>with Ada.Unchecked_Deallocation;
package body Fifo is
---------- -- Push -- ----------
procedure Push (List : in out Fifo_Type; Item : in Element_Type) is
Temp : Fifo_Ptr := new Fifo_Element'(Item, null);
begin
if List.Tail = null then
List.Tail := Temp;
end if;
if List.Head /= null then
List.Head.Next := Temp;
end if;
List.Head := Temp;
end Push;
--------- -- Pop -- ---------
procedure Pop (List : in out Fifo_Type; Item : out Element_Type) is
procedure Free is new Ada.Unchecked_Deallocation(Fifo_Element, Fifo_Ptr);
Temp : Fifo_Ptr := List.Tail;
begin
if List.Head = null then
raise Empty_Error;
end if;
Item := List.Tail.Value;
List.Tail := List.Tail.Next;
if List.Tail = null then
List.Head := null;
end if;
Free(Temp);
end Pop;
-------------- -- Is_Empty -- --------------
function Is_Empty (List : Fifo_Type) return Boolean is
begin
return List.Head = null;
end Is_Empty;
end Fifo;</lang> A "main" procedure for this program is: <lang ada>with Fifo; with Ada.Text_Io; use Ada.Text_Io;
procedure Fifo_Test is
package Int_Fifo is new Fifo(Integer); use Int_Fifo; My_Fifo : Fifo_Type; Val : Integer;
begin
for I in 1..10 loop
Push(My_Fifo, I);
end loop;
while not Is_Empty(My_Fifo) loop
Pop(My_Fifo, Val);
Put_Line(Integer'Image(Val));
end loop;
end Fifo_Test;</lang> The following implementation produces equivalent functionality by deriving from the standard Ada Container type Doubly_Linked_Lists.
This example needs fewer lines of code on the part of the application programmer, but the implementation is less efficient than the previous example. Each element has all the data members needed for a doubly linked list. It also links in all the functionality of a doubly linked list. Most of that functionality is unneeded in a FIFO. <lang ada>
with Ada.Containers.Doubly_Linked_Lists; generic type Element_Type is private; package Generic_Fifo is type Fifo_Type is tagged private; procedure Push(The_Fifo : in out Fifo_Type; Item : in Element_Type); procedure Pop(The_Fifo : in out Fifo_Type; Item : out Element_Type); Empty_Error : Exception; private package List_Pkg is new Ada.Containers.Doubly_Linked_Lists(Element_Type); use List_Pkg; Type Fifo_Type is new List with null record; end Generic_Fifo;
</lang> <lang ada>
package body Generic_Fifo is
----------
-- Push --
----------
procedure Push (The_Fifo : in out Fifo_Type; Item : in Element_Type) is
begin
The_Fifo.Prepend(Item);
end Push;
---------
-- Pop --
---------
procedure Pop (The_Fifo : in out Fifo_Type; Item : out Element_Type) is
begin
if Is_Empty(The_Fifo) then
raise Empty_Error;
end if;
Item := The_Fifo.Last_Element;
The_Fifo.Delete_Last;
end Pop;
end Generic_Fifo;</lang>
<lang ada>with Generic_Fifo; with Ada.Text_Io; use Ada.Text_Io;
procedure Generic_Fifo_Test is
package Int_Fifo is new Generic_Fifo(Integer); use Int_Fifo; My_Fifo : Fifo_Type; Val : Integer;
begin
for I in 1..10 loop
My_Fifo.Push(I);
end loop;
while not My_Fifo.Is_Empty loop
My_Fifo.Pop(Val);
Put_Line(Integer'Image(Val));
end loop;
end Generic_Fifo_Test;</lang> The function Is_Empty is inherited from the Lists type.
The next two examples provide simple FIFO functionality for concurrent tasks. The buffer in each example holds a single value. When running concurrent tasks, one writing to the buffer, and one reading from the buffer, either the writer will be faster than the reader, or the reader will be faster than the writer. If the writer is faster a dynamic FIFO will grow to consume all available memory on the computer. If the reader is faster the FIFO will either contain a single value or it will be empty. In either case, no implementation is more efficient than a single element buffer.
If we wish for the reader to read every value written by the writer we must synchronize the tasks. The writer can only write a new value when the buffer contains a stale value. The reader can only read a value when the value is fresh. This synchronization forces the two tasks to run at the same speed. <lang ada>generic
type Element_Type is private;
package Synchronous_Fifo is
protected type Fifo is
entry Push(Item : Element_Type);
entry Pop(Item : out Element_Type);
private
Value : Element_Type;
Is_New : Boolean := False;
end Fifo;
end Synchronous_Fifo;</lang> <lang ada>package body Synchronous_Fifo is
---------- -- Fifo -- ----------
protected body Fifo is
---------
-- Push --
---------
entry Push (Item : Element_Type) when not Is_New is
begin
Value := Item;
Is_New := True;
end Push;
---------
-- Pop --
---------
entry Pop (Item : out Element_Type) when Is_New is
begin
Item := Value;
Is_New := False;
end Pop;
end Fifo;
end Synchronous_Fifo;</lang> <lang ada>with Synchronous_Fifo; with Ada.Text_Io; use Ada.Text_Io;
procedure Synchronous_Fifo_Test is
package Int_Fifo is new Synchronous_Fifo(Integer);
use Int_Fifo;
Buffer : Fifo;
task Writer is
entry Stop;
end Writer;
task body Writer is
Val : Positive := 1;
begin
loop
select
accept Stop;
exit;
else
select
Buffer.Push(Val);
Val := Val + 1;
or
delay 1.0;
end select;
end select;
end loop;
end Writer;
task Reader is
entry Stop;
end Reader;
task body Reader is
Val : Positive;
begin
loop
select
accept Stop;
exit;
else
select
Buffer.Pop(Val);
Put_Line(Integer'Image(Val));
or
delay 1.0;
end select;
end select;
end loop;
end Reader;
begin
delay 0.1;
Writer.Stop;
Reader.Stop;
end Synchronous_Fifo_Test;</lang>
Another choice is to cause the two tasks to run independently. The writer can write whenever it is scheduled. The reader reads whenever it is scheduled, after the writer writes the first valid value.
In this example the writer writes several values before the reader reads a value. The reader will then read that same value several times before the writer is scheduled to write more values.
In a fully asynchronous system the reader only samples the values written by the writer. There is no control over the number of values not sampled by the reader, or over the number of times the reader reads the same value. <lang ada>generic
type Element_Type is private;
package Asynchronous_Fifo is
protected type Fifo is
procedure Push(Item : Element_Type);
entry Pop(Item : out Element_Type);
private
Value : Element_Type;
Valid : Boolean := False;
end Fifo;
end Asynchronous_Fifo;</lang> You may notice that the protected type specification is remarkably similar to the synchronous example above. The only important difference is that Push is declared to be an Entry in the synchronous example while it is a procedure in the asynchronous example. Entries only execute when their boundary condition evaluates to TRUE. Procedures execute unconditionally. <lang ada>package body Asynchronous_Fifo is
---------- -- Fifo -- ----------
protected body Fifo is
----------
-- Push --
----------
procedure Push (Item : Element_Type) is
begin
Value := Item;
Valid := True;
end Push;
---------
-- Pop --
---------
entry Pop (Item : out Element_Type) when Valid is
begin
Item := Value;
end Pop;
end Fifo;
end Asynchronous_Fifo;</lang> <lang ada>with Asynchronous_Fifo; with Ada.Text_Io; use Ada.Text_Io;
procedure Asynchronous_Fifo_Test is
package Int_Fifo is new Asynchronous_Fifo(Integer);
use Int_Fifo;
Buffer : Fifo;
task Writer is
entry Stop;
end Writer;
task body Writer is
Val : Positive := 1;
begin
loop
select
accept Stop;
exit;
else
Buffer.Push(Val);
Val := Val + 1;
end select;
end loop;
end Writer;
task Reader is
entry Stop;
end Reader;
task body Reader is
Val : Positive;
begin
loop
select
accept Stop;
exit;
else
Buffer.Pop(Val);
Put_Line(Integer'Image(Val));
end select;
end loop;
end Reader;
begin
delay 0.1;
Writer.Stop;
Reader.Stop;
end Asynchronous_Fifo_Test;</lang>
ALGOL 68
File: prelude/queue_base.a68<lang algol68># -*- coding: utf-8 -*- # CO REQUIRES:
MODE OBJLINK = STRUCT( REF OBJLINK next, REF OBJLINK prev, OBJVALUE value # ... etc. required # ); PROC obj link new = REF OBJLINK: ~; PROC obj link free = (REF OBJLINK free)VOID: ~
END CO
- actually a pointer to the last LINK, there ITEMs are ADDED/get #
MODE OBJQUEUE = REF OBJLINK;
OBJQUEUE obj queue empty = NIL;
BOOL obj queue par = FALSE; # make code thread safe # SEMA obj queue sema = LEVEL ABS obj queue par;
- Warning: 1 SEMA for all queues of type obj, i.e. not 1 SEMA per queue #
PROC obj queue init = (REF OBJQUEUE self)REF OBJQUEUE:
self := obj queue empty;
PROC obj queue put = (REF OBJQUEUE self, OBJVALUE obj)REF OBJQUEUE: (
REF OBJLINK out = obj link new; IF obj queue par THEN DOWN obj queue sema FI; IF self IS obj queue empty THEN out := (out, out, obj) # self referal # ELSE # join into list # out := (self, prev OF self, obj); next OF prev OF out := prev OF next OF out := out FI; IF obj queue par THEN UP obj queue sema FI; self := out
);
- define a useful prepend/put/plusto (+=:) operator... #
PROC obj queue plusto = (OBJVALUE obj, REF OBJQUEUE self)OBJQUEUE: obj queue put(self,obj); OP +=: = (OBJVALUE obj, REF OBJQUEUE self)REF OBJQUEUE: obj queue put(self,obj);
- a potential append/plusab (+:=) operator...
OP (REF OBJQUEUE, OBJVALUE)OBJQUEUE +:= = obj queue plusab;
- see if the program/coder wants the OBJ problem mended... #
PROC (REF OBJQUEUE #self#)BOOL obj queue index error mended
:= (REF OBJQUEUE self)BOOL: (abend("obj queue index error"); stop);
PROC on obj queue index error = (REF OBJQUEUE self, PROC(REF OBJQUEUE #self#)BOOL mended)VOID:
obj queue index error mended := mended;
PROC obj queue get = (REF OBJQUEUE self)OBJVALUE: (
- DOWN obj queue sema; #
IF self IS obj queue empty THEN
IF NOT obj queue index error mended(self) THEN abend("obj stack index error") FI FI;
OBJQUEUE old tail = prev OF self;
IF old tail IS self THEN # free solo member #
self := obj queue empty
ELSE # free self/tail member #
OBJQUEUE new tail = prev OF old tail;
next OF new tail := self;
prev OF self := new tail
FI;
- UP obj queue sema #
OBJVALUE out = value OF old tail;
- give a recovery hint to the garbage collector #
obj link free(old tail); out
);
PROC obj queue is empty = (REF OBJQUEUE self)BOOL:
self IS obj queue empty;
SKIP</lang>See also: Queue/Usage
ALGOL W
<lang algolw>begin
% define a Queue type that will hold StringQueueElements %
record StringQueue ( reference(StringQueueElement) front, back );
% define the StringQueueElement type %
record StringQueueElement ( string(8) element
; reference(StringQueueElement) next
);
% we would need separate types for other element types %
% adds s to the end of the StringQueue q %
procedure pushString ( reference(StringQueue) value q
; string(8) value e
) ;
begin
reference(StringQueueElement) newElement;
newElement := StringQueueElement( e, null );
if front(q) = null then begin
% adding to an empty queue %
front(q) := newElement;
back(q) := newElement
end
else begin
% the queue is not empty %
next(back(q)) := newElement;
back(q) := newElement
end
end pushString ;
% removes an element from the front of the StringQueue q %
% asserts the queue is not empty, which will stop the %
% program if it is %
string(8) procedure popString ( reference(StringQueue) value q ) ;
begin
string(8) v;
assert( not isEmptyStringQueue( q ) );
v := element(front(q));
front(q) := next(front(q));
if front(q) = null then % just popped the last element % back(q) := null;
v
end popStringQueue ;
% returns true if the StringQueue q is empty, false otherwise %
logical procedure isEmptyStringQueue ( reference(StringQueue) value q ) ; front(q) = null;
begin % test the StringQueue operations %
reference(StringQueue) q;
q := StringQueue( null, null );
pushString( q, "fred" );
pushString( q, "whilma" );
pushString( q, "betty" );
pushString( q, "barney" );
while not isEmptyStringQueue( q ) do write( popString( q ) )
end
end.</lang>
- Output:
fred whilma betty barney
ARM Assembly
<lang ARM Assembly> /* ARM assembly Raspberry PI */ /* program defqueue.s */
/* Constantes */ .equ STDOUT, 1 @ Linux output console .equ EXIT, 1 @ Linux syscall .equ WRITE, 4 @ Linux syscall
.equ NBMAXIELEMENTS, 100
/*******************************************/ /* Structures */ /********************************************/ /* example structure for value of item */
.struct 0
value_ident: @ ident
.struct value_ident + 4
value_value1: @ value 1
.struct value_value1 + 4
value_value2: @ value 2
.struct value_value2 + 4
value_fin: /* example structure for queue */
.struct 0
queue_ptdeb: @ begin pointer of item
.struct queue_ptdeb + 4
queue_ptfin: @ end pointer of item
.struct queue_ptfin + 4
queue_stvalue: @ structure of value item
.struct queue_stvalue + (value_fin * NBMAXIELEMENTS)
queue_fin:
/*********************************/
/* Initialized data */
/*********************************/
.data
szMessEmpty: .asciz "Empty queue. \n"
szMessNotEmpty: .asciz "Not empty queue. \n"
szMessError: .asciz "Error detected !!!!. \n"
szMessResult: .ascii "Ident :" @ message result
sMessIdent: .fill 11, 1, ' '
.ascii " value 1 :"
sMessValue1: .fill 11, 1, ' '
.ascii " value 2 :"
sMessValue2: .fill 11, 1, ' '
.asciz "\n"
szCarriageReturn: .asciz "\n" /*********************************/ /* UnInitialized data */ /*********************************/ .bss .align 4 Queue1: .skip queue_fin @ queue memory place stItem: .skip value_fin @ value item 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,iAdrstItem mov r1,#1 str r1,[r0,#value_ident] mov r1,#11 str r1,[r0,#value_value1] mov r1,#12 str r1,[r0,#value_value2]
ldr r0,iAdrQueue1 @ queue structure address ldr r1,iAdrstItem bl pushQueue @ add item in queue cmp r0,#-1 @ error ? beq 99f @ init item 2 ldr r0,iAdrstItem mov r1,#2 str r1,[r0,#value_ident] mov r1,#21 str r1,[r0,#value_value1] mov r1,#22 str r1,[r0,#value_value2]
ldr r0,iAdrQueue1 @ queue structure address ldr r1,iAdrstItem bl pushQueue @ add item in queue cmp r0,#-1 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:
ldr r0,iAdrQueue1 @ queue structure address bl popQueue @ return address item cmp r0,#-1 @ error ? beq 99f mov r2,r0 @ save item pointer ldr r0,[r2,#value_ident] ldr r1,iAdrsMessIdent @ display ident bl conversion10 @ decimal conversion ldr r0,[r2,#value_value1] ldr r1,iAdrsMessValue1 @ display value 1 bl conversion10 @ decimal conversion ldr r0,[r2,#value_value2] ldr r1,iAdrsMessValue2 @ display value 2 bl conversion10 @ decimal conversion ldr r0,iAdrszMessResult bl affichageMess @ display message b 4b @ 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 iAdrstItem: .int stItem iAdrszMessError: .int szMessError iAdrszMessEmpty: .int szMessEmpty iAdrszMessNotEmpty: .int szMessNotEmpty iAdrszMessResult: .int szMessResult iAdrszCarriageReturn: .int szCarriageReturn iAdrsMessIdent: .int sMessIdent iAdrsMessValue1: .int sMessValue1 iAdrsMessValue2: .int sMessValue2 /******************************************************************/ /* test if queue empty */ /******************************************************************/ /* r0 contains the address of queue structure */ isEmpty:
push {r1,r2,lr} @ save registres
ldr r1,[r0,#queue_ptdeb] @ begin pointer
ldr r2,[r0,#queue_ptfin] @ begin pointer
cmp r1,r2
moveq r0,#1 @ empty queue
movne r0,#0 @ not empty
pop {r1,r2,lr} @ restaur registers
bx lr @ return
/******************************************************************/ /* add item in queue */ /******************************************************************/ /* r0 contains the address of queue structure */ /* r1 contains the address of item */ pushQueue:
push {r1-r4,lr} @ save registres
add r2,r0,#queue_stvalue @ address of values structure
ldr r3,[r0,#queue_ptfin] @ end pointer
add r2,r3 @ free address of queue
ldr r4,[r1,#value_ident] @ load ident item
str r4,[r2,#value_ident] @ and store in queue
ldr r4,[r1,#value_value1] @ idem
str r4,[r2,#value_value1]
ldr r4,[r1,#value_value2]
str r4,[r2,#value_value2]
add r3,#value_fin
cmp r3,#value_fin * NBMAXIELEMENTS
moveq r0,#-1 @ error
beq 100f
str r3,[r0,#queue_ptfin] @ store new end pointer
100:
pop {r1-r4,lr} @ restaur registers
bx lr @ return
/******************************************************************/ /* pop queue */ /******************************************************************/ /* r0 contains the address of queue structure */ popQueue:
push {r1,r2,lr} @ save registres
mov r1,r0 @ control if empty queue
bl isEmpty
cmp r0,#1 @ yes -> error
moveq r0,#-1
beq 100f
mov r0,r1
ldr r1,[r0,#queue_ptdeb] @ begin pointer
add r2,r0,#queue_stvalue @ address of begin values item
add r2,r1 @ address of item
add r1,#value_fin
str r1,[r0,#queue_ptdeb] @ store nex begin pointer
mov r0,r2 @ return pointer item
100:
pop {r1,r2,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.
Ident :1 value 1 :11 value 2 :12
Ident :2 value 1 :21 value 2 :22
Error detected !!!!.
AutoHotkey
<lang autohotkey>push("qu", 2), push("qu", 44), push("qu", "xyz") ; TEST
MsgBox % "Len = " len("qu") ; Number of entries While !empty("qu") ; Repeat until queue is not empty
MsgBox % pop("qu") ; Print popped values (2, 44, xyz)
MsgBox Error = %ErrorLevel% ; ErrorLevel = 0: OK MsgBox % pop("qu") ; Empty MsgBox Error = %ErrorLevel% ; ErrorLevel = -1: popped too much MsgBox % "Len = " len("qu") ; Number of entries
push(queue,_) { ; push _ onto queue named "queue" (!=_), _ string not containing |
Global %queue% .= %queue% = "" ? _ : "|" _
}
pop(queue) { ; pop value from queue named "queue" (!=_,_1,_2)
Global RegExMatch(%queue%, "([^\|]*)\|?(.*)", _) Return _1, ErrorLevel := -(%queue%=""), %queue% := _2
}
empty(queue) { ; check if queue named "queue" is empty
Global Return %queue% = ""
}
len(queue) { ; number of entries in "queue"
Global StringReplace %queue%, %queue%, |, |, UseErrorLevel Return %queue% = "" ? 0 : ErrorLevel+1
}</lang>
AWK
<lang awk>#!/usr/bin/awk -f
BEGIN {
delete q
print "empty? " emptyP()
print "push " push("a")
print "push " push("b")
print "empty? " emptyP()
print "pop " pop()
print "pop " pop()
print "empty? " emptyP()
print "pop " pop()
}
function push(n) {
q[length(q)+1] = n return n
}
function pop() {
if (emptyP()) {
print "Popping from empty queue."
exit
}
r = q[length(q)]
delete q[length(q)]
return r
}
function emptyP() {
return length(q) == 0
} </lang>
- Output:
empty? 1 push a push b empty? 0 pop b pop a empty? 1 Popping from empty queue.
Batch File
This solution uses an environment variable naming convention to implement a queue as a pseudo object containing a pseudo dynamic array and head and tail attributes, as well as an empty "method" that is a sort of macro. The implementation depends on delayed expansion being enabled at the time of each call to a queue function. More complex variations can be written that remove this limitation.
<lang dos> @echo off setlocal enableDelayedExpansion
- FIFO queue usage
- Define the queue
call :newQueue myQ
- Populate the queue
for %%A in (value1 value2 value3) do call :enqueue myQ %%A
- Test if queue is empty by examining the tail "attribute"
if myQ.tail==0 (echo myQ is empty) else (echo myQ is NOT empty)
- Peek at the head of the queue
call:peekQueue myQ val && echo a peek at the head of myQueue shows !val!
- Process the first queue value
call :dequeue myQ val && echo dequeued myQ value=!val!
- Add some more values to the queue
for %%A in (value4 value5 value6) do call :enqueue myQ %%A
- Process the remainder of the queue
- processQueue
call :dequeue myQ val || goto :queueEmpty echo dequeued myQ value=!val! goto :processQueue
- queueEmpty
- Test if queue is empty using the empty "method"/"macro". Use of the
- second IF statement serves to demonstrate the negation of the empty
- "method". A single IF could have been used with an ELSE clause instead.
if %myQ.empty% echo myQ is empty if not %myQ.empty% echo myQ is NOT empty exit /b
- FIFO queue definition
- newQueue qName
set /a %~1.head=1, %~1.tail=0
- Define an empty "method" for this queue as a sort of macro
set "%~1.empty=^!%~1.tail^! == 0" exit /b
- enqueue qName value
set /a %~1.tail+=1 set %~1.!%~1.tail!=%2 exit /b
- dequeue qName returnVar
- Sets errorlevel to 0 if success
- Sets errorlevel to 1 if failure because queue was empty
if !%~1.tail! equ 0 exit /b 1 for %%N in (!%~1.head!) do (
set %~2=!%~1.%%N! set %~1.%%N=
) if !%~1.head! == !%~1.tail! (set /a "%~1.head=1, %~1.tail=0") else set /a %~1.head+=1 exit /b 0
- peekQueue qName returnVar
- Sets errorlevel to 0 if success
- Sets errorlevel to 1 if failure because queue was empty
if !%~1.tail! equ 0 exit /b 1 for %%N in (!%~1.head!) do set %~2=!%~1.%%N! exit /b 0 </lang>
BBC BASIC
<lang bbcbasic> FIFOSIZE = 1000
FOR n = 3 TO 5
PRINT "Push ";n : PROCenqueue(n)
NEXT
PRINT "Pop " ; FNdequeue
PRINT "Push 6" : PROCenqueue(6)
REPEAT
PRINT "Pop " ; FNdequeue
UNTIL FNisempty
PRINT "Pop " ; FNdequeue
END
DEF PROCenqueue(n) : LOCAL f%
DEF FNdequeue : LOCAL f% : f% = 1
DEF FNisempty : LOCAL f% : f% = 2
PRIVATE fifo(), rptr%, wptr%
DIM fifo(FIFOSIZE-1)
CASE f% OF
WHEN 0:
wptr% = (wptr% + 1) MOD FIFOSIZE
IF rptr% = wptr% ERROR 100, "Error: queue overflowed"
fifo(wptr%) = n
WHEN 1:
IF rptr% = wptr% ERROR 101, "Error: queue empty"
rptr% = (rptr% + 1) MOD FIFOSIZE
= fifo(rptr%)
WHEN 2:
= (rptr% = wptr%)
ENDCASE
ENDPROC</lang>
- Output:
Push 3 Push 4 Push 5 Pop 3 Push 6 Pop 4 Pop 5 Pop 6 Pop Error: queue empty
Bracmat
Below, queue is the name of a class with a data member list and three methods enqueue, dequeue and empty.
No special provision is implemented to "throw and exception" in case you try to dequeue from and empty queue, because, in Bracmat, evaluation of an expression, besides resulting in an evaluated expression, always also either "succeeds" or "fails". (There is, in fact, a third possibility, "ignore", telling Bracmat to close an eye even though an evaluation didn't succeed.) So in the example below, the last dequeue operation fails and the program continues on the right hand side of the bar (|) operator
<lang bracmat> ( queue
= (list=)
(enqueue=.(.!arg) !(its.list):?(its.list))
( dequeue
= x
. !(its.list):?(its.list) (.?x)
& !x
)
(empty=.!(its.list):)
)</lang>
Normally you would seldom use a class as depicted above, because the operations are so simple that you probably use them directly. Bracmat lists allow prepending as well as appending elements, and single elements can be removed from the beginning or from the end of a list.
Appending an element to a long list and removing an element from the end of a long list are quite expensive operations, with a cost O(n), where n is the number of elements in the queue.
C
Dynamic array
Dynamic array working as a circular buffer. <lang c>#include <stdio.h>
- include <stdlib.h>
- include <string.h>
typedef int DATA; /* type of data to store in queue */ typedef struct {
DATA *buf; size_t head, tail, alloc;
} queue_t, *queue;
queue q_new() {
queue q = malloc(sizeof(queue_t)); q->buf = malloc(sizeof(DATA) * (q->alloc = 4)); q->head = q->tail = 0; return q;
}
int empty(queue q) {
return q->tail == q->head;
}
void enqueue(queue q, DATA n) {
if (q->tail >= q->alloc) q->tail = 0;
q->buf[q->tail++] = n;
// Fixed bug where it failed to resizes
if (q->tail == q->alloc) { /* needs more room */
q->buf = realloc(q->buf, sizeof(DATA) * q->alloc * 2);
if (q->head) {
memcpy(q->buf + q->head + q->alloc, q->buf + q->head,
sizeof(DATA) * (q->alloc - q->head));
q->head += q->alloc;
} else
q->tail = q->alloc;
q->alloc *= 2;
}
}
int dequeue(queue q, DATA *n) {
if (q->head == q->tail) return 0;
*n = q->buf[q->head++];
if (q->head >= q->alloc) { /* reduce allocated storage no longer needed */
q->head = 0;
if (q->alloc >= 512 && q->tail < q->alloc / 2)
q->buf = realloc(q->buf, sizeof(DATA) * (q->alloc/=2));
}
return 1;
}</lang>
Doubly linked list
<lang c>#include <stdio.h>
- include <stdlib.h>
typedef struct node_t node_t, *node, *queue; struct node_t { int val; node prev, next; };
- define HEAD(q) q->prev
- define TAIL(q) q->next
queue q_new() {
node q = malloc(sizeof(node_t)); q->next = q->prev = 0; return q;
}
int empty(queue q) {
return !HEAD(q);
}
void enqueue(queue q, int n) {
node nd = malloc(sizeof(node_t)); nd->val = n; if (!HEAD(q)) HEAD(q) = nd; nd->prev = TAIL(q); if (nd->prev) nd->prev->next = nd; TAIL(q) = nd; nd->next = 0;
}
int dequeue(queue q, int *val) {
node tmp = HEAD(q); if (!tmp) return 0; *val = tmp->val;
HEAD(q) = tmp->next; if (TAIL(q) == tmp) TAIL(q) = 0; free(tmp);
return 1;
} </lang>
Test code This main function works with both implementions above. <lang c>int main() {
int i, n; queue q = q_new();
for (i = 0; i < 100000000; i++) {
n = rand();
if (n > RAND_MAX / 2) {
// printf("+ %d\n", n);
enqueue(q, n);
} else {
if (!dequeue(q, &n)) {
// printf("empty\n");
continue;
}
// printf("- %d\n", n);
}
}
while (dequeue(q, &n));// printf("- %d\n", n);
return 0;
}</lang>
Of the above two programs, for int types the array method is about twice as fast for the test code given. The doubly linked list is marginally faster than the sys/queue.h below.
sys/queue.h
Using the sys/queue.h, which is not POSIX.1-2001 (but it is BSD). The example allows to push/pop int values, but instead of int one can use void * and push/pop any kind of "object" (of course changes to the commodity functions m_queue and m_dequeue are needed)
<lang c>#include <stdio.h>
- include <stdlib.h>
- include <stdbool.h>
- include <sys/queue.h>
struct entry {
int value; TAILQ_ENTRY(entry) entries;
};
typedef struct entry entry_t;
TAILQ_HEAD(FIFOList_s, entry);
typedef struct FIFOList_s FIFOList;
bool m_enqueue(int v, FIFOList *l)
{
entry_t *val;
val = malloc(sizeof(entry_t));
if ( val != NULL ) {
val->value = v;
TAILQ_INSERT_TAIL(l, val, entries);
return true;
}
return false;
}
bool m_dequeue(int *v, FIFOList *l) {
entry_t *e = l->tqh_first;
if ( e != NULL ) {
*v = e->value;
TAILQ_REMOVE(l, e, entries);
free(e);
return true;
}
return false;
}
bool isQueueEmpty(FIFOList *l) {
if ( l->tqh_first == NULL ) return true; return false;
}</lang>
C#
Compatible with C# 3.0 specification, requires System library for exceptions (from either .Net or Mono). A FIFO class in C# using generics and nodes. <lang csharp>public class FIFO<T> {
class Node
{
public T Item { get; set; }
public Node Next { get; set; }
}
Node first = null;
Node last = null;
public void push(T item)
{
if (empty())
{
//Uses object initializers to set fields of new node
first = new Node() { Item = item, Next = null };
last = first;
}
else
{
last.Next = new Node() { Item = item, Next = null };
last = last.Next;
}
}
public T pop()
{
if (first == null)
throw new System.Exception("No elements");
if (last == first)
last = null;
T temp = first.Item;
first = first.Next;
return temp;
}
public bool empty()
{
return first == null;
}
}</lang>
C++
C++ already has a class queue in the standard library, however the following is a simple implementation based on a singly linkes list. Note that an empty queue is internally represented by head == 0, therefore it doesn't matter that the tail value is invalid in that case.
<lang cpp>namespace rosettacode
{
template<typename T> class queue
{
public:
queue();
~queue();
void push(T const& t);
T pop();
bool empty();
private:
void drop();
struct node;
node* head;
node* tail;
};
template<typename T> struct queue<T>::node
{
T data;
node* next;
node(T const& t): data(t), next(0) {}
};
template<typename T>
queue<T>::queue():
head(0)
{
}
template<typename T>
inline void queue<T>::drop()
{
node* n = head;
head = head->next;
delete n;
}
template<typename T>
queue<T>::~queue()
{
while (!empty())
drop();
}
template<typename T>
void queue<T>::push(T const& t)
{
node*& next = head? tail->next : head;
next = new node(t);
tail = next;
}
template<typename T>
T queue<T>::pop()
{
T tmp = head->data;
drop();
return tmp;
}
template<typename T>
bool queue<T>::empty()
{
return head == 0;
}
}</lang>
Clojure
Clojure has a built-in persistent FIFO queue which can be accessed by referring to clojure.lang.PersistentQueue/EMPTY. Queues are manipulated similarly to Clojure's stacks using peek and pop.
<lang clojure>
user=> (def empty-queue clojure.lang.PersistentQueue/EMPTY)
- 'user/empty-queue
user=> (def aqueue (atom empty-queue))
- 'user/aqueue
- Check if queue is empty
user=> (empty? @aqueue) true
- As with other Clojure data structures, you can add items using conj and into
user=> (swap! aqueue conj 1) user=> (swap! aqueue into [2 3 4]) user=> (pprint @aqueue) <-(1 2 3 4)-<
- You can read the head of the queue with peek
user=> (peek @aqueue) 1
- You can remove the head producing a new queue using pop
user=> (pprint (pop @aqueue)) <-(2 3 4)-<
- pop returns a new queue, the original is still intact
user=> (pprint @aqueue) <-(1 2 3 4)-<
- you can treat a queue as a sequence
user=> (into [] @aqueue) [1 2 3 4]
- but remember that using rest or next converts the queue to a seq. Compare
user=> (-> @aqueue rest (conj 5) pprint) (5 2 3 4)
- with
user=> (-> @aqueue pop (conj 5) pprint) <-(2 3 4 5)-<
</lang>
Here's a link with further documentation Queues in Clojure
CoffeeScript
<lang coffeescript>
- Implement a fifo as an array of arrays, to
- greatly amortize dequeue costs, at some expense of
- memory overhead and insertion time. The speedup
- depends on the underlying JS implementation, but
- it's significant on node.js.
Fifo = ->
max_chunk = 512 arr = [] # array of arrays count = 0
self =
enqueue: (elem) ->
if count == 0 or arr[arr.length-1].length >= max_chunk
arr.push []
count += 1
arr[arr.length-1].push elem
dequeue: (elem) ->
throw Error("queue is empty") if count == 0
val = arr[0].shift()
count -= 1
if arr[0].length == 0
arr.shift()
val
is_empty: (elem) ->
count == 0
- test
do ->
max = 5000000
q = Fifo()
for i in [1..max]
q.enqueue
number: i
console.log q.dequeue() while !q.is_empty() v = q.dequeue() console.log v
</lang>
- Output:
> time coffee fifo.coffee
{ number: 1 }
{ number: 5000000 }
real 0m2.394s
user 0m2.089s
sys 0m0.265s
Common Lisp
This defines a queue structure that stores its items in a list, and maintains a tail pointer (i.e., a pointer to the last cons in the list). Note that dequeuing the last item in the queue does not clear the tail pointer—enqueuing into the resulting empty queue will correctly reset the tail pointer.
<lang lisp>(defstruct (queue (:constructor %make-queue))
(items '() :type list) (tail '() :type list))
(defun make-queue ()
"Returns an empty queue." (%make-queue))
(defun queue-empty-p (queue)
"Returns true if the queue is empty." (endp (queue-items queue)))
(defun enqueue (item queue)
"Enqueue item in queue. Returns the queue."
(prog1 queue
(if (queue-empty-p queue)
(setf (queue-items queue) (list item)
(queue-tail queue) (queue-items queue))
(setf (cdr (queue-tail queue)) (list item)
(queue-tail queue) (cdr (queue-tail queue))))))
(defun dequeue (queue)
"Dequeues an item from queue. Signals an error if queue is empty." (if (queue-empty-p queue) (error "Cannot dequeue from empty queue.") (pop (queue-items queue))))</lang>
Component Pascal
BlackBox Component Builder <lang oberon2> MODULE Queue; IMPORT Boxes; TYPE Instance* = POINTER TO LIMITED RECORD size: LONGINT; first,last: LONGINT; _queue: POINTER TO ARRAY OF Boxes.Box; END;
PROCEDURE (self: Instance) Initialize(capacity: LONGINT),NEW; BEGIN self.size := 0; self.first := 0; self.last := 0; NEW(self._queue,capacity) END Initialize;
PROCEDURE New*(capacity: LONGINT): Instance; VAR aQueue: Instance; BEGIN NEW(aQueue);aQueue.Initialize(capacity);RETURN aQueue END New;
PROCEDURE (self: Instance) IsEmpty*(): BOOLEAN, NEW; BEGIN RETURN self.size = 0; END IsEmpty;
PROCEDURE (self: Instance) Capacity*(): LONGINT, NEW; BEGIN RETURN LEN(self._queue) END Capacity;
PROCEDURE (self: Instance) Size*(): LONGINT, NEW; BEGIN RETURN self.size END Size;
PROCEDURE (self: Instance) IsFull*(): BOOLEAN, NEW; BEGIN RETURN self.size = self.Capacity() END IsFull;
PROCEDURE (self: Instance) Push*(b: Boxes.Box), NEW; VAR i, j, newCapacity, oldSize: LONGINT; queue: POINTER TO ARRAY OF Boxes.Box; BEGIN INC(self.size); self._queue[self.last] := b; self.last := (self.last + 1) MOD self.Capacity(); IF self.IsFull() THEN (* grow queue *) newCapacity := self.Capacity() + (self.Capacity() DIV 2); (* new queue *) NEW(queue,newCapacity); (* move data from old to new queue *) i := self.first; j := 0; oldSize := self.Capacity() - self.first + self.last; WHILE (j < oldSize) & (j < newCapacity - 1) DO queue[j] := self._queue[i]; i := (i + 1) MOD newCapacity;INC(j) END; self._queue := queue;self.first := 0;self.last := j END END Push;
PROCEDURE (self: Instance) Pop*(): Boxes.Box, NEW; VAR b: Boxes.Box; BEGIN ASSERT(~self.IsEmpty()); DEC(self.size); b := self._queue[self.first]; self._queue[self.first] := NIL; self.first := (self.first + 1) MOD self.Capacity(); RETURN b END Pop;
END Queue. </lang> Interface extracted from implementation <lang oberon2> DEFINITION Queue;
IMPORT Boxes;
TYPE Instance = POINTER TO LIMITED RECORD (self: Instance) Capacity (): LONGINT, NEW; (self: Instance) IsEmpty (): BOOLEAN, NEW; (self: Instance) IsFull (): BOOLEAN, NEW; (self: Instance) Pop (): Boxes.Box, NEW; (self: Instance) Push (b: Boxes.Box), NEW; (self: Instance) Size (): LONGINT, NEW END;
PROCEDURE New (capacity: LONGINT): Instance;
END Queue.
</lang>
Cowgol
This code should be put in a file called queue.coh, to be used with the
Cowgol program at Queue/Usage. The queue is implemented by means of a linked list.
<lang cowgol>include "strings.coh"; include "malloc.coh";
- Define types. The calling code is expected to provide a QueueData type.
record QueueItem is
data: QueueData; next: [QueueItem];
end record;
record QueueMeta is
head: [QueueItem]; tail: [QueueItem];
end record;
typedef Queue is [QueueMeta]; const Q_NONE := 0 as [QueueItem];
- Allocate and free the queue datastructure.
sub MakeQueue(): (q: Queue) is
q := Alloc(@bytesof QueueMeta) as Queue; q.head := Q_NONE; q.tail := Q_NONE;
end sub;
sub FreeQueue(q: Queue) is
var cur := q.head;
while cur != Q_NONE loop
var next := cur.next;
Free(cur as [uint8]);
cur := next;
end loop;
Free(q as [uint8]);
end sub;
- Check if queue is empty.
sub QueueEmpty(q: Queue): (r: uint8) is
r := 0;
if q.head == Q_NONE then
r := 1;
end if;
end sub;
- Enqueue and dequeue data. Cowgol has no exceptions, so the calling code
- should check QueueEmpty first.
sub Enqueue(q: Queue, d: QueueData) is
var item := Alloc(@bytesof QueueItem) as [QueueItem];
item.data := d;
item.next := Q_NONE;
if q.head == Q_NONE then
q.head := item;
else
q.tail.next := item;
end if;
q.tail := item;
end sub;
sub Dequeue(q: Queue): (d: QueueData) is
d := q.head.data;
var cur := q.head;
q.head := q.head.next;
Free(cur as [uint8]);
if q.head == Q_NONE then
q.tail := Q_NONE;
end if;
end sub;</lang>
D
See code here: http://rosettacode.org/wiki/Queue/Usage#D
Déjà Vu
This uses a dictionary to have a sort of circular buffer of infinite size. <lang dejavu>queue: { :start 0 :end 0 }
enqueue q item: set-to q q!end item set-to q :end ++ q!end
dequeue q: if empty q: Raise :value-error "popping from empty queue" q! q!start delete-from q q!start set-to q :start ++ q!start
empty q: = q!start q!end</lang>
Delphi
<lang Delphi>program QueueDefinition;
{$APPTYPE CONSOLE}
uses
System.Generics.Collections;
type
TQueue = System.Generics.Collections.TQueue<Integer>;
TQueueHelper = class helper for TQueue function Empty: Boolean; function Pop: Integer; procedure Push(const NewItem: Integer); end;
{ TQueueHelper }
function TQueueHelper.Empty: Boolean; begin
Result := count = 0;
end;
function TQueueHelper.Pop: Integer; begin
Result := Dequeue;
end;
procedure TQueueHelper.Push(const NewItem: Integer); begin
Enqueue(NewItem);
end;
var
Queue: TQueue; i: Integer;
begin
Queue := TQueue.Create;
for i := 1 to 1000 do Queue.push(i);
while not Queue.Empty do Write(Queue.pop, ' '); Writeln;
Queue.Free; Readln;
end. </lang>
E
This uses a linked list representation of queues, hanging onto both ends of the list, except that the next-link reference is an E promise rather than a mutable slot.
Also, according to E design principles, the read and write ends of the queue are separate objects. This has two advantages; first, it implements POLA by allowing only the needed end of the queue to be handed out to its users; second, if the reader end is garbage collected the contents of the queue automatically will be as well (rather than accumulating if the writer continues writing).
<lang e>def makeQueue() {
def [var head, var tail] := Ref.promise()
def writer {
to enqueue(value) {
def [nh, nt] := Ref.promise()
tail.resolve([value, nh])
tail := nt
}
}
def reader {
to empty() { return !Ref.isResolved(head) }
to dequeue(whenEmpty) {
if (Ref.isResolved(head)) {
def [value, next] := head
head := next
return value
} else {
throw.eject(whenEmpty, "pop() of empty queue")
}
}
}
return [reader, writer]
}</lang>
EchoLisp
There is no native queue type in EchoLisp. make-Q implements queues in message passing style, using vector operations. Conversions from-to lists are also provided. <lang lisp>
- put info string in permanent storage for later use
(info 'make-Q "usage: (define q (make-Q)) ; (q '[top | empty? | pop | push value | to-list | from-list list])")
- make-Q
(define (make-Q)
(let ((q (make-vector 0)))
(lambda (message . args)
(case message
((empty?) (vector-empty? q))
((top) (if (vector-empty? q) (error 'Q:top:empty q) (vector-ref q 0)))
((push) (vector-push q (car args)))
((pop) (if (vector-empty? q) (error 'Q:pop:empty q) (vector-shift q)))
((to-list) (vector->list q))
((from-list) (set! q (list->vector (car args))) q )
(else (info 'make-Q) (error "Q:bad message:" message )))))) ; display info if unknown message
(define q (make-Q)) (q 'empty?) → #t (q 'push 'first) → first (q 'push 'second) → second (q 'pop) → first (q 'pop) → second (q 'top) "💬 error: Q:top:empty #()" (q 'from-list '( 6 7 8)) → #( 6 7 8) (q 'top) → 6 (q 'pop) → 6 (q 'to-list)→ (7 8) (q 'delete) "💭 error: Q:bad message: delete"
- save make-Q
(local-put 'make-Q) </lang>
Elena
ELENA 4.x : <lang elena>import extensions;
template queue<T> {
T[] theArray;
int theTop;
int theTale;
constructor()
{
theArray := new T[](8);
theTop := 0;
theTale := 0;
}
bool empty()
= theTop == theTale;
push(T object)
{
if (theTale > theArray.Length)
{
theArray := theArray.reallocate(theTale)
};
theArray[theTale] := object;
theTale += 1
}
T pop()
{
if (theTale == theTop)
{ InvalidOperationException.new:"Queue is empty".raise() };
T item := theArray[theTop];
theTop += 1;
^ item
}
}
public program() {
queue<int> q := new queue<int>();
q.push(1);
q.push(2);
q.push(3);
console.printLine(q.pop());
console.printLine(q.pop());
console.printLine(q.pop());
console.printLine("a queue is ", q.empty().iif("empty","not empty"));
console.print("Trying to pop:");
try
{
q.pop()
}
catch(Exception e)
{
console.printLine(e.Message)
}
}</lang>
- Output:
1 2 3 a queue is empty Trying to pop:Queue is empty
Elisa
This is a generic Queue component based on bi-directional lists. See how in Elisa these lists are defined.
<lang Elisa> component GenericQueue ( Queue, Element );
type Queue;
Queue (MaxLength = integer) -> Queue;
Length( Queue ) -> integer;
Empty ( Queue ) -> boolean;
Full ( Queue ) -> boolean;
Push ( Queue, Element) -> nothing;
Pull ( Queue ) -> Element;
begin
Queue (MaxLength) = Queue:[ MaxLength; length:=0; list=alist(Element) ];
Length ( queue ) = queue.length;
Empty ( queue ) = (queue.length <= 0);
Full ( queue ) = (queue.length >= queue.MaxLength);
Push ( queue, element ) =
[ exception (Full(queue), "Queue Overflow");
queue.length:= queue.length + 1;
add (queue.list, element)];
Pull ( queue ) =
[ exception (Empty(queue), "Queue Underflow");
queue.length:= queue.length - 1;
remove(first(queue.list))];
end component GenericQueue; </lang> In the following tests we will also show how the internal structure of the queue can be made visible to support debugging. <lang Elisa> use GenericQueue (QueueofPersons, Person); type Person = text; Q = QueueofPersons(25);
Push (Q, "Peter"); Push (Q, "Alice"); Push (Q, "Edward"); Q? QueueofPersons:[MaxLength = 25;
length = 3;
list = { "Peter",
"Alice",
"Edward"}]
Pull (Q)? "Peter"
Pull (Q)? "Alice"
Pull (Q)? "Edward"
Q? QueueofPersons:[MaxLength = 25;
length = 0;
list = { }]
Pull (Q)?
- Exception: Queue Underflow
</lang>
Elixir
<lang elixir>defmodule Queue do
def new, do: {Queue, [], []}
def push({Queue, input, output}, x), do: {Queue, [x|input], output}
def pop({Queue, [], []}), do: (raise RuntimeError, message: "empty Queue")
def pop({Queue, input, []}), do: pop({Queue, [], Enum.reverse(input)})
def pop({Queue, input, [h|t]}), do: {h, {Queue, input, t}}
def empty?({Queue, [], []}), do: true
def empty?({Queue, _, _}), do: false
end</lang>
Example:
iex(1)> c("queue.ex")
[Queue]
iex(2)> q = Queue.new
{Queue, [], []}
iex(3)> Queue.empty?(q)
true
iex(4)> q2 = Queue.push(q,1)
{Queue, [1], []}
iex(5)> q3 = Queue.push(q2,2)
{Queue, [2, 1], []}
iex(6)> Queue.empty?(q3)
false
iex(7)> Queue.pop(q3)
{1, {Queue, [], [2]}}
iex(8)> {popped, ^q} = Queue.pop(q2)
{1, {Queue, [], []}}
iex(9)> Queue.pop(Queue.new)
** (RuntimeError) empty Queue
queue.ex:6: Queue.pop/1
Erlang
The standard way to manage fifo in functional programming is to use a pair of list for the fifo queue, one is the input, the other is the output. When the output is empty just take the input list and reverse it. <lang Erlang>-module(fifo). -export([new/0, push/2, pop/1, empty/1]).
new() -> {fifo, [], []}.
push({fifo, In, Out}, X) -> {fifo, [X|In], Out}.
pop({fifo, [], []}) -> erlang:error('empty fifo'); pop({fifo, In, []}) -> pop({fifo, [], lists:reverse(In)}); pop({fifo, In, [H|T]}) -> {H, {fifo, In, T}}.
empty({fifo, [], []}) -> true; empty({fifo, _, _}) -> false.</lang>
Note that there exists a 'queue' module in the standard library handling this for you in the first place
ERRE
With ERRE 3.0 you can use a class to define the task (in C-64 version you can simply use procedures): <lang ERRE>PROGRAM CLASS_DEMO
CLASS QUEUE
LOCAL SP LOCAL DIM STACK[100]
FUNCTION ISEMPTY()
ISEMPTY=(SP=0)
END FUNCTION
PROCEDURE INIT
SP=0
END PROCEDURE
PROCEDURE POP(->XX)
XX=STACK[SP]
SP=SP-1
END PROCEDURE
PROCEDURE PUSH(XX)
SP=SP+1
STACK[SP]=XX
END PROCEDURE
END CLASS
NEW PILA:QUEUE
BEGIN
PILA_INIT ! constructor
FOR N=1 TO 4 DO ! push 4 numbers
PRINT("Push";N)
PILA_PUSH(N)
END FOR
FOR I=1 TO 5 DO ! pop 5 numbers
IF NOT PILA_ISEMPTY() THEN
PILA_POP(->N)
PRINT("Pop";N)
ELSE
PRINT("Queue is empty!")
END IF
END FOR
PRINT("* End *")
END PROGRAM</lang>
- Output:
Push 1 Push 2 Push 3 Push 4 Pop 4 Pop 3 Pop 2 Pop 1 Queue is empty! * End *
Factor
<lang factor>USING: accessors kernel ; IN: rosetta-code.queue-definition
TUPLE: queue head tail ; TUPLE: node value next ;
- <queue> ( -- queue ) queue new ;
- <node> ( obj -- node ) node new swap >>value ;
- empty? ( queue -- ? ) head>> >boolean not ;
- enqueue ( obj queue -- )
[ <node> ] dip 2dup dup empty? [ head<< ] [ tail>> next<< ] if tail<< ;
- dequeue ( queue -- obj )
dup empty? [ "Cannot dequeue empty queue." throw ] when [ head>> value>> ] [ head>> next>> ] [ head<< ] tri ;</lang>
Fantom
<lang fantom> class Queue {
List queue := [,]
public Void push (Obj obj)
{
queue.add (obj) // add to right of list
}
public Obj pop ()
{
if (queue.isEmpty)
throw (Err("queue is empty"))
else
{
return queue.removeAt(0) // removes left-most item
}
}
public Bool isEmpty ()
{
queue.isEmpty
}
} </lang>
Forth
This is a FIFO implemented as a circular buffer, as is often found between communicating processes such the interrupt and user parts of a device driver. In practice, the get/put actions would block instead of aborting if the queue is empty/full.
<lang forth>1024 constant size create buffer size cells allot here constant end variable head buffer head ! variable tail buffer tail ! variable used 0 used !
- empty? used @ 0= ;
- full? used @ size = ;
- next ( ptr -- ptr )
cell+ dup end = if drop buffer then ;
- put ( n -- )
full? abort" buffer full" \ begin full? while pause repeat tail @ ! tail @ next tail ! 1 used +! ;
- get ( -- n )
empty? abort" buffer empty" \ begin empty? while pause repeat head @ @ head @ next head ! -1 used +! ;</lang>
Linked list version
Using Forth-2012 structure words and ALLOCATE/FREE. In spirit quite similar to the Java variant below, with one difference: Here we use addresses of fields (not possible in Java), which often makes things simpler than in Java (fewer special cases at boundaries), but in this case it does not. Where the Java version has a special case on enqueue, this version has a special case on dequeue:
<lang forth> 0
field: list-next field: list-val
constant list-struct
- insert ( x list-addr -- )
list-struct allocate throw >r swap r@ list-val ! dup @ r@ list-next ! r> swap ! ;
- remove ( list-addr -- x )
>r r@ @ ( list-node ) r@ @ dup list-val @ ( list-node x ) swap list-next @ r> ! swap free throw ;
0
field: queue-last \ points to the last entry (head of the list) field: queue-nextaddr \ points to the pointer to the next-inserted entry
constant queue-struct
- init-queue ( queue -- )
>r 0 r@ queue-last ! r@ queue-last r> queue-nextaddr ! ;
- make-queue ( -- queue )
queue-struct allocate throw dup init-queue ;
- empty? ( queue -- f )
queue-last @ 0= ;
- enqueue ( x queue -- )
dup >r queue-nextaddr @ insert r@ queue-nextaddr @ @ list-next r> queue-nextaddr ! ;
- dequeue ( queue -- x )
dup empty? abort" dequeue applied to an empty queue"
dup queue-last remove ( queue x )
over empty? if
over init-queue then
nip ;
</lang>
Fortran
See FIFO (usage) for an example of fifo_nodes
<lang fortran>module FIFO
use fifo_nodes
! fifo_nodes must define the type fifo_node, with the two field ! next and valid, for queue handling, while the field datum depends ! on the usage (see FIFO (usage) for an example) ! type fifo_node ! integer :: datum ! ! the next part is not variable and must be present ! type(fifo_node), pointer :: next ! logical :: valid ! end type fifo_node
type fifo_head
type(fifo_node), pointer :: head, tail
end type fifo_head
contains
subroutine new_fifo(h) type(fifo_head), intent(out) :: h nullify(h%head) nullify(h%tail) end subroutine new_fifo
subroutine fifo_enqueue(h, n) type(fifo_head), intent(inout) :: h type(fifo_node), intent(inout), target :: n
if ( associated(h%tail) ) then
h%tail%next => n
h%tail => n
else
h%tail => n
h%head => n
end if
nullify(n%next) end subroutine fifo_enqueue
subroutine fifo_dequeue(h, n) type(fifo_head), intent(inout) :: h type(fifo_node), intent(out), target :: n
if ( associated(h%head) ) then
n = h%head
if ( associated(n%next) ) then
h%head => n%next
else
nullify(h%head)
nullify(h%tail)
end if
n%valid = .true.
else
n%valid = .false.
end if
nullify(n%next)
end subroutine fifo_dequeue
function fifo_isempty(h) result(r)
logical :: r
type(fifo_head), intent(in) :: h
if ( associated(h%head) ) then
r = .false.
else
r = .true.
end if
end function fifo_isempty
end module FIFO</lang>
Free Pascal
<lang pascal>program queue;
{$IFDEF FPC}{$MODE DELPHI}{$IFDEF WINDOWS}{$APPTYPE CONSOLE}{$ENDIF}{$ENDIF}
{$ASSERTIONS ON}
uses Generics.Collections;
var
lQueue: TQueue<Integer>;
begin
lQueue := TQueue<Integer>.Create; try lQueue.EnQueue(1); lQueue.EnQueue(2); lQueue.EnQueue(3); Write(lQueue.DeQueue:2); // 1 Write(lQueue.DeQueue:2); // 2 Writeln(lQueue.DeQueue:2); // 3 Assert(lQueue.Count = 0, 'Queue is not empty'); // should be empty finally lQueue.Free; end;
end.</lang>
Output: 1 2 3
FreeBASIC
We first use a macro to define a generic Queue type : <lang freebasic>' FB 1.05.0 Win64
' queue_rosetta.bi ' simple generic Queue type
- Define Queue(T) Queue_##T
- Macro Declare_Queue(T)
Type Queue(T)
Public: Declare Constructor() Declare Destructor() Declare Property capacity As Integer Declare Property count As Integer Declare Property empty As Boolean Declare Property front As T Declare Function pop() As T Declare Sub push(item As T) Private: a(any) As T count_ As Integer = 0 Declare Function resize(size As Integer) As Integer
End Type
Constructor Queue(T)()
Redim a(0 To 0) create a default T instance for various purposes
End Constructor
Destructor Queue(T)()
Erase a
End Destructor
Property Queue(T).capacity As Integer
Return UBound(a)
End Property
Property Queue(T).count As Integer
Return count_
End Property
Property Queue(T).empty As Boolean
Return count_ = 0
End Property
Property Queue(T).front As T
If count_ > 0 Then Return a(1) End If Print "Error: Attempted to access 'front' element of an empty queue" Return a(0) return default element
End Property
Function Queue(T).pop() As T
If count_ > 0 Then
Dim value As T = a(1)
If count_ > 1 Then move remaining elements to fill space vacated
For i As Integer = 2 To count_
a(i - 1) = a(i)
Next
End If
a(count_) = a(0) zero last element
count_ -= 1
Return value
End If
Print "Error: Attempted to remove 'front' element of an empty queue"
Return a(0) return default element
End Function
Sub Queue(T).push(item As T)
Dim size As Integer = UBound(a) count_ += 1 If count_ > size Then size = resize(size) Redim Preserve a(0 to size) End If a(count_) = item
End Sub
Function Queue(T).resize(size As Integer) As Integer
If size = 0 Then size = 4 ElseIf size <= 32 Then size = 2 * size Else size += 32 End If Return size
End Function
- EndMacro</lang>
We now use this type to create a Queue of Cat instances : <lang freebasic>' FB 1.05.0 Win64
- Include "queue_rosetta.bi"
Type Cat
name As String age As Integer Declare Constructor Declare Constructor(name_ As string, age_ As integer) Declare Operator Cast() As String
end type
Constructor Cat default constructor End Constructor
Constructor Cat(name_ As String, age_ As Integer)
name = name_ age = age_
End Constructor
Operator Cat.Cast() As String
Return "[" + name + ", " + Str(age) + "]"
End Operator
Declare_Queue(Cat) expand Queue type for Cat instances
Dim CatQueue As Queue(Cat)
Var felix = Cat("Felix", 8) Var sheba = Cat("Sheba", 4) Var fluffy = Cat("Fluffy", 2) With CatQueue push these Cat instances into the Queue
.push(felix) .push(sheba) .push(fluffy)
End With Print "Number of Cats in the Queue :" ; CatQueue.count Print "Capacity of Cat Queue :" ; CatQueue.capacity Print "Front Cat : "; CatQueue.front CatQueue.pop() Print "Front Cat now : "; CatQueue.front Print "Number of Cats in the Queue :" ; CatQueue.count CatQueue.pop() Print "Front Cat now : "; CatQueue.front Print "Number of Cats in the Queue :" ; CatQueue.count Print "Is Queue empty now : "; CatQueue.empty catQueue.pop() Print "Number of Cats in the Queue :" ; CatQueue.count Print "Is Queue empty now : "; CatQueue.empty catQueue.pop() Print Print "Press any key to quit" Sleep</lang>
- Output:
Number of Cats in the Queue : 3 Capacity of Cat Queue : 4 Front Cat : [Felix, 8] Front Cat now : [Sheba, 4] Number of Cats in the Queue : 2 Front Cat now : [Fluffy, 2] Number of Cats in the Queue : 1 Is Queue empty now : false Number of Cats in the Queue : 0 Is Queue empty now : true Error: Attempted to remove 'front' element of an empty queue
GAP
<lang gap>Enqueue := function(v, x)
Add(v[1], x);
end;
Dequeue := function(v)
if IsEmpty(v[2]) then
if IsEmpty(v[1]) then
return fail;
else
v[2] := Reversed(v[1]);
v[1] := [];
fi;
fi;
return Remove(v[2]);
end;
- a new queue
v := [[], []];
Enqueue(v, 3); Enqueue(v, 4); Enqueue(v, 5); Dequeue(v);
- 3
Enqueue(v, 6); Dequeue(v);
- 4
Dequeue(v);
- 5
Dequeue(v);
- 6
Dequeue(v);
- fail</lang>
Go
Hard coded to be a queue of strings. Implementation is a circular buffer which grows as needed. <lang go> package queue
// int queue // the zero object is a valid queue ready to be used. // items are pushed at tail, popped at head. // tail = -1 means queue is full type Queue struct {
b []string head, tail int
}
func (q *Queue) Push(x string) {
switch {
// buffer full. reallocate.
case q.tail < 0:
next := len(q.b)
bigger := make([]string, 2*next)
copy(bigger[copy(bigger, q.b[q.head:]):], q.b[:q.head])
bigger[next] = x
q.b, q.head, q.tail = bigger, 0, next+1
// zero object. make initial allocation.
case len(q.b) == 0:
q.b, q.head, q.tail = make([]string, 4), 0 ,1
q.b[0] = x
// normal case
default:
q.b[q.tail] = x
q.tail++
if q.tail == len(q.b) {
q.tail = 0
}
if q.tail == q.head {
q.tail = -1
}
}
}
func (q *Queue) Pop() (string, bool) {
if q.head == q.tail {
return "", false
}
r := q.b[q.head]
if q.tail == -1 {
q.tail = q.head
}
q.head++
if q.head == len(q.b) {
q.head = 0
}
return r, true
}
func (q *Queue) Empty() bool {
return q.head == q.tail
} </lang>
Groovy
Solution: <lang groovy>class Queue {
private List buffer
public Queue(List buffer = new LinkedList()) {
assert buffer != null
assert buffer.empty
this.buffer = buffer
}
def push (def item) { buffer << item }
final enqueue = this.&push
def pop() {
if (this.empty) throw new NoSuchElementException('Empty Queue')
buffer.remove(0)
}
final dequeue = this.&pop
def getEmpty() { buffer.empty }
String toString() { "Queue:${buffer}" }
}</lang>
Test: <lang groovy>def q = new Queue() assert q.empty
['Crosby', 'Stills'].each { q.push(it) } assert !q.empty ['Nash', 'Young'].each { q.enqueue(it) } println q assert !q.empty assert q.pop() == 'Crosby' println q assert !q.empty assert q.dequeue() == 'Stills' println q assert !q.empty assert q.pop() == 'Nash' println q assert !q.empty q.push('Crazy Horse') println q assert q.dequeue() == 'Young' println q assert !q.empty assert q.pop() == 'Crazy Horse' println q assert q.empty try { q.pop() } catch (NoSuchElementException e) { println e } try { q.dequeue() } catch (NoSuchElementException e) { println e }</lang>
- Output:
Queue:[Crosby, Stills, Nash, Young] Queue:[Stills, Nash, Young] Queue:[Nash, Young] Queue:[Young] Queue:[Young, Crazy Horse] Queue:[Crazy Horse] Queue:[] java.util.NoSuchElementException: Empty Queue java.util.NoSuchElementException: Empty Queue
Haskell
The standard way to manage fifo in functional programming is to use a pair of list for the fifo queue, one is the input, the other is the output. When the output is empty just take the input list and reverse it.
<lang haskell>data Fifo a = F [a] [a]
emptyFifo :: Fifo a emptyFifo = F [] []
push :: Fifo a -> a -> Fifo a push (F input output) item = F (item:input) output
pop :: Fifo a -> (Maybe a, Fifo a) pop (F input (item:output)) = (Just item, F input output) pop (F [] [] ) = (Nothing, F [] []) pop (F input [] ) = pop (F [] (reverse input))
isEmpty :: Fifo a -> Bool isEmpty (F [] []) = True isEmpty _ = False </lang>
Icon and Unicon
Icon
The following works in both Icon and Unicon:
<lang icon>
- Use a record to hold a Queue, using a list as the concrete implementation
record Queue(items)
procedure make_queue ()
return Queue ([])
end
procedure queue_push (queue, item)
put (queue.items, item)
end
- if the queue is empty, this will 'fail' and return nothing
procedure queue_pop (queue)
return pop (queue.items)
end
procedure queue_empty (queue)
return *queue.items = 0
end
- procedure to test class
procedure main ()
queue := make_queue()
# add the numbers 1 to 5
every (item := 1 to 5) do
queue_push (queue, item)
# pop them in the added order, and show a message when queue is empty
every (1 to 6) do {
write ("Popped value: " || queue_pop (queue))
if (queue_empty (queue)) then write ("empty queue")
}
end </lang>
- Output:
Popped value: 1 Popped value: 2 Popped value: 3 Popped value: 4 Popped value: 5 empty queue empty queue
Unicon
Unicon also provides classes:
<lang Unicon>
- Use a class to hold a Queue, with a list as the concrete implementation
class Queue (items)
method push (item) put (items, item) end
# if the queue is empty, this will 'fail' and return nothing method take () return pop (items) end
method is_empty () return *items = 0 end
initially () # initialises the field on creating an instance items := []
end
procedure main ()
queue := Queue ()
every (item := 1 to 5) do
queue.push (item)
every (1 to 6) do {
write ("Popped value: " || queue.take ())
if queue.is_empty () then write ("empty queue")
}
end </lang>
Produces the same output as above.
J
Object oriented technique, using mutable state:
<lang J>queue_fifo_=:
pop_fifo_=: verb define
r=. {. ::] queue
queue=: }.queue
r
)
push_fifo_=: verb define
queue=: queue,y y
)
isEmpty_fifo_=: verb define
0=#queue
)</lang>
Function-level technique, with no reliance on mutable state:
<lang J>pop =: ( {.^:notnull ; }. )@: > @: ] / push =: ( ; ,~ )& > / tell_atom =: >& {. tell_queue =: >& {: is_empty =: -: 1 tell_queue
make_empty =: a: , a: [ ] onto =: [ ; }.@]
notnull =: 0 ~: #</lang>
See also FIFO (usage)#J
Java
This task could be done using a LinkedList from java.util, but here is a user-defined version with generics: <lang java>public class Queue<E>{
Node<E> head = null, tail = null;
static class Node<E>{
E value;
Node<E> next;
Node(E value, Node<E> next){
this.value= value;
this.next= next;
}
}
public Queue(){
}
public void enqueue(E value){ //standard queue name for "push"
Node<E> newNode= new Node<E>(value, null);
if(empty()){
head= newNode;
}else{
tail.next = newNode;
}
tail= newNode;
}
public E dequeue() throws java.util.NoSuchElementException{//standard queue name for "pop"
if(empty()){
throw new java.util.NoSuchElementException("No more elements.");
}
E retVal= head.value;
head= head.next;
return retVal;
}
public boolean empty(){
return head == null;
}
}</lang>
JavaScript
Most of the time, the built-in Array suffices. However, if you explicitly want to limit the usage to FIFO operations, it's easy to implement such a constructor.
Using built-in Array
<lang javascript>var fifo = []; fifo.push(42); // Enqueue. fifo.push(43); var x = fifo.shift(); // Dequeue. alert(x); // 42</lang>
Custom constructor function
<lang javascript>function FIFO() {
this.data = new Array();
this.push = function(element) {this.data.push(element)}
this.pop = function() {return this.data.shift()}
this.empty = function() {return this.data.length == 0}
this.enqueue = this.push; this.dequeue = this.pop;
}</lang>
jq
Note that since jq is a purely functional language, the entity representing a queue must be presented as an input to any function that is to operate on it.
The definition of pop as given below is idiomatic in jq but implies that popping an empty queue yields [null, []] rather than an error. An alternative definition, pop_or_error, is also given to illustrate how an error condition can be generated. <lang jq># An empty queue: def fifo: [];
def push(e): [e] + .;
def pop: [.[0], .[1:]];
def pop_or_error: if length == 0 then error("pop_or_error") else pop end;
def empty: length == 0;</lang> Examples: <lang jq>fifo | pop # produces [null,[]]
fifo
| push(42) # enqueue | push(43) # enqueue | pop # dequeue | .[0] # the value
- produces 43
fifo|push(1) as $q1 | fifo|push(2) as $q2 | [($q1|pop|.[0]), ($q2|pop|.[0])]
- produces: [1, 2]</lang>
Julia
Julia provides a variety of queue-like methods for vectors, making the solution to this task rather straightforward. Define a Queue in terms of a one dimensional array, and provide its methods using the appropriate vector operations. To adhere to Julia naming conventions, the queue operations are named "push!", "pop!" and "isempty" rather than "push", "pop" and "empty". <lang Julia> type Queue{T}
a::Array{T,1}
end
Queue() = Queue(Any[]) Queue(a::DataType) = Queue(a[]) Queue(a) = Queue(typeof(a)[])
Base.isempty(q::Queue) = isempty(q.a)
function Base.pop!{T}(q::Queue{T})
!isempty(q) || error("queue must be non-empty")
pop!(q.a)
end
function Base.push!{T}(q::Queue{T}, x::T)
unshift!(q.a, x) return q
end
function Base.push!{T}(q::Queue{Any}, x::T)
unshift!(q.a, x) return q
end </lang>
- Output:
It is easiest to use the REPL to show a Queue in action.
julia> q = Queue()
Queue{Any}({})
julia> isempty(q)
true
julia> push!(q, 1)
Queue{Any}({1})
julia> isempty(q)
false
julia> push!(q, "two")
Queue{Any}({"two",1})
julia> push!(q, 3.0)
Queue{Any}({3.0,"two",1})
julia> push!(q, false)
Queue{Any}({false,3.0,"two",1})
julia> pop!(q)
1
julia> pop!(q)
"two"
julia> pop!(q)
3.0
julia> pop!(q)
false
julia> pop!(q)
ERROR: queue must be non-empty
in pop! at none:2
Klingphix
<lang Klingphix>{ include ..\Utilitys.tlhy } "..\Utilitys.tlhy" load
- push! { l i -- l&i }
0 put
- empty? { l -- flag }
len not { len 0 equal }
- pop! { l -- l-1 }
empty? (
["Empty"]
[pop swap]
) if
( ) { empty queue }
1 push! 2 push! 3 push! pop! ? pop! ? pop! ? pop! ?
"End " input</lang>
- Output:
1 2 3 Empty End
Kotlin
<lang scala>// version 1.1.2
import java.util.LinkedList
class Queue<E> {
private val data = LinkedList<E>()
val size get() = data.size
val empty get() = size == 0
fun push(element: E) = data.add(element)
fun pop(): E {
if (empty) throw RuntimeException("Can't pop elements from an empty queue")
return data.removeFirst()
}
val top: E
get() {
if (empty) throw RuntimeException("Empty queue can't have a top element")
return data.first()
}
fun clear() = data.clear()
override fun toString() = data.toString()
}
fun main(args: Array<String>) {
val q = Queue<Int>()
(1..5).forEach { q.push(it) }
println(q)
println("Size of queue = ${q.size}")
print("Popping: ")
(1..3).forEach { print("${q.pop()} ") }
println("\nRemaining in queue: $q")
println("Top element is now ${q.top}")
q.clear()
println("After clearing, queue is ${if(q.empty) "empty" else "not empty"}")
try {
q.pop()
}
catch (e: Exception) {
println(e.message)
}
}</lang>
- Output:
[1, 2, 3, 4, 5] Size of queue = 5 Popping: 1 2 3 Remaining in queue: [4, 5] Top element is now 4 After clearing, queue is empty Can't pop elements from an empty queue
LabVIEW
This image is a VI Snippet, an executable image of LabVIEW code. The LabVIEW version is shown on the top-right hand corner. You can download it, then drag-and-drop it onto the LabVIEW block diagram from a file browser, and it will appear as runnable, editable code.
Lasso
Definition: <lang lasso>define myqueue => type {
data store = list
public onCreate(...) => {
if(void != #rest) => {
with item in #rest do .`store`->insert(#item)
}
}
public push(value) => .`store`->insertLast(#value)
public pop => {
handle => {
.`store`->removefirst
}
return .`store`->first }
public isEmpty => (.`store`->size == 0)
}</lang>
Usage: <lang lasso>local(q) = myqueue('a')
- q->isEmpty
// => false
- q->push('b')
- q->pop
// => a
- q->pop
// => b
- q->isEmpty
// => true
- q->pop
// => void</lang>
Lua
<lang lua>Queue = {}
function Queue.new()
return { first = 0, last = -1 }
end
function Queue.push( queue, value )
queue.last = queue.last + 1 queue[queue.last] = value
end
function Queue.pop( queue )
if queue.first > queue.last then
return nil
end
local val = queue[queue.first]
queue[queue.first] = nil
queue.first = queue.first + 1
return val
end
function Queue.empty( queue )
return queue.first > queue.last
end</lang>
M2000 Interpreter
A Stack object can be used as LIFO or FIFO. Data push to bottom of stack. Read pop a value to a variable from top of stack. <lang M2000 Interpreter> Module Checkit {
a=Stack
Stack a {
Data 100,200, 300
}
Stack a {
While not empty {
Read N
Print N
}
}
} Checkit </lang>
Mathematica
<lang Mathematica>EmptyQ[a_] := Length[a] == 0 SetAttributes[Push, HoldAll]; Push[a_, elem_] := AppendTo[a, elem] SetAttributes[Pop, HoldAllComplete]; Pop[a_] := If[EmptyQ[a], False, b = First[a]; Set[a, Most[a]]; b]</lang>
MATLAB / Octave
Here is a simple implementation of a queue, that works in Matlab and Octave. <lang matlab>myfifo = {};
% push myfifo{end+1} = x;
% pop x = myfifo{1}; myfifo{1} = [];
% empty isempty(myfifo)</lang>
Below is another solution, that encapsulates the fifo within the object-orientated "class" elements supported by Matlab. For this to work it must be saved in a file named "FIFOQueue.m" in a folder named "@FIFOQueue" in your current Matlab directory. <lang MATLAB>%This class impliments a standard FIFO queue. classdef FIFOQueue
properties
queue
end
methods
%Class constructor
function theQueue = FIFOQueue(varargin)
if isempty(varargin) %No input arguments
%Initialize the queue state as empty
theQueue.queue = {};
elseif (numel(varargin) > 1) %More than 1 input arg
%Make the queue the list of input args
theQueue.queue = varargin;
elseif iscell(varargin{:}) %If the only input is a cell array
%Make the contents of the cell array the elements in the queue
theQueue.queue = varargin{:};
else %There is one input argument that is not a cell
%Make that one arg the only element in the queue
theQueue.queue = varargin;
end
end
%push() - pushes a new element to the end of the queue
function push(theQueue,varargin)
if isempty(varargin)
theQueue.queue(end+1) = {[]};
elseif (numel(varargin) > 1) %More than 1 input arg
%Make the queue the list of input args
theQueue.queue( end+1:end+numel(varargin) ) = varargin;
elseif iscell(varargin{:}) %If the only input is a cell array
%Make the contents of the cell array the elements in the queue
theQueue.queue( end+1:end+numel(varargin{:}) ) = varargin{:};
else %There is one input argument that is not a cell
%Make that one arg the only element in the queue
theQueue.queue{end+1} = varargin{:};
end
%Makes changes to the queue permanent
assignin('caller',inputname(1),theQueue);
end
%pop() - pops the first element off the queue
function element = pop(theQueue)
if empty(theQueue)
error 'The queue is empty'
else
%Returns the first element in the queue
element = theQueue.queue{1};
%Removes the first element from the queue
theQueue.queue(1) = [];
%Makes changes to the queue permanent
assignin('caller',inputname(1),theQueue);
end
end
%empty() - Returns true if the queue is empty
function trueFalse = empty(theQueue)
trueFalse = isempty(theQueue.queue);
end
end %methods
end</lang>
Sample usage: <lang MATLAB>>> myQueue = FIFOQueue({'hello'})
myQueue =
FIFOQueue
>> push(myQueue,'jello') >> pop(myQueue)
ans =
hello
>> pop(myQueue)
ans =
jello
>> pop(myQueue) ??? Error using ==> FIFOQueue.FIFOQueue>FIFOQueue.pop at 61 The queue is empty</lang>
Maxima
<lang maxima>defstruct(queue(in=[], out=[]))$
enqueue(x, q) := (q@in: cons(x, q@in), done)$
dequeue(q) := (if not emptyp(q@out) then first([first(q@out), q@out: rest(q@out)]) elseif emptyp(q@in) then 'fail else (q@out: reverse(q@in), q@in: [], first([first(q@out), q@out: rest(q@out)])))$
q:new(queue); /* queue([], []) */ enqueue(1, q)$ enqueue(2, q)$ enqueue(3, q)$ dequeue(q); /* 1 */ enqueue(4, q)$ dequeue(q); /* 2 */ dequeue(q); /* 3 */ dequeue(q); /* 4 */ dequeue(q); /* fail */</lang>
Nanoquery
This is a fully-featured FIFO queue class definition. In addition to the functions required by the task, it also demonstrates redefining operators for Nanoquery classes by redefining +, *, and =. <lang Nanoquery>class FIFO declare contents
// define constructors for FIFO objects def FIFO() this.contents = {} end def FIFO(contents) this.contents = contents end
// define methods for this class def push(value) contents.append(value) end def pop() if !this.empty() value = contents[len(contents) - 1] contents.remove(len(contents) - 1) return value else // we could throw our own exception here but // we'll return null instead return null end end def length() return len(contents) end def extend(itemlist) contents += itemlist end def empty() return len(contents) = 0 end
// define operators for this class def toString() return str(contents) end def operator+(other) return this.contents + other.contents end def operator*(n) return this.contents * n end def operator=(other) return this.contents = other.contents end end</lang>
NetRexx
Unlike Rexx, NetRexx does not include built–in support for queues but the language's ability to access the Java SDK permits use of any number of Java's "Collection" classes.
The following sample implements a stack via the ArrayDeque double–ended queue.
<lang NetRexx>/* NetRexx */
options replace format comments java crossref savelog symbols nobinary
mqueue = ArrayDeque()
viewQueue(mqueue)
a = "Fred" mqueue.push() /* Puts an empty line onto the queue */ mqueue.push(a 2) /* Puts "Fred 2" onto the queue */ viewQueue(mqueue)
a = "Toft" mqueue.add(a 2) /* Enqueues "Toft 2" */ mqueue.add() /* Enqueues an empty line behind the last */ viewQueue(mqueue)
loop q_ = 1 while mqueue.size > 0
parse mqueue.pop.toString line say q_.right(3)':' line end q_
viewQueue(mqueue)
return
method viewQueue(mqueue = Deque) private static
If mqueue.size = 0 then do Say 'Queue is empty' end else do Say 'There are' mqueue.size 'elements in the queue' end
return
</lang>
Queue is empty There are 2 elements in the queue There are 4 elements in the queue 1: Fred 2 2: 3: Toft 2 4: Queue is empty
Nim
<lang nim>import queues
- defining push & pop (obviously optional)
proc push*[T](q: var TQueue[T]; item: T) =
add(q,item)
proc pop*[T](q: var TQueue[T]): T =
result = dequeue(q)
var fifo: TQueue[int] = initQueue[int]()
fifo.push(26) fifo.push(99) fifo.push(2) echo("Fifo size: ", fifo.len()) echo("Popping: ", fifo.pop()) echo("Popping: ", fifo.pop()) echo("Popping: ", fifo.pop())
- echo("Popping: ", fifo.pop()) # popping an empty stack raises [EAssertionFailed]</lang>
- Output:
Fifo size: 3 Popping: 26 Popping: 99 Popping: 2
OCaml
The standard way to manage fifo in functional programming is to use a pair of list for the fifo queue, one is the input, the other is the output. When the output is empty just take the input list and reverse it.
<lang ocaml>module FIFO : sig
type 'a fifo val empty: 'a fifo val push: fifo:'a fifo -> item:'a -> 'a fifo val pop: fifo:'a fifo -> 'a * 'a fifo val is_empty: fifo:'a fifo -> bool
end = struct
type 'a fifo = 'a list * 'a list let empty = [], [] let push ~fifo:(input,output) ~item = (item::input,output) let is_empty ~fifo = match fifo with | [], [] -> true | _ -> false let rec pop ~fifo = match fifo with | input, item :: output -> item, (input,output) | [], [] -> failwith "empty fifo" | input, [] -> pop ([], List.rev input)
end</lang>
and a session in the top-level:
<lang ocaml># open FIFO;;
- let q = empty ;;
val q : '_a FIFO.fifo = <abstr>
- is_empty q ;;
- : bool = true
- let q = push q 1 ;;
val q : int FIFO.fifo = <abstr>
- is_empty q ;;
- : bool = false
- let q =
List.fold_left push q [2;3;4] ;;
val q : int FIFO.fifo = <abstr>
- let v, q = pop q ;;
val v : int = 1 val q : int FIFO.fifo = <abstr>
- let v, q = pop q ;;
val v : int = 2 val q : int FIFO.fifo = <abstr>
- let v, q = pop q ;;
val v : int = 3 val q : int FIFO.fifo = <abstr>
- let v, q = pop q ;;
val v : int = 4 val q : int FIFO.fifo = <abstr>
- let v, q = pop q ;;
Exception: Failure "empty fifo".</lang>
The standard ocaml library also provides a FIFO module, but it is imperative, unlike the implementation above which is functional.
Oforth
If queue is empty, null is returned.
<lang Oforth>Object Class new: Queue(mutable l)
Queue method: initialize ListBuffer new := l ; Queue method: empty @l isEmpty ; Queue method: push @l add ; Queue method: pop @l removeFirst ;</lang>
OxygenBasic
This buffer pushes any primitive data (auto converted to strings), and pops strings. The buffer can expand or contract according to usage. <lang oxygenbasic> 'FIRST IN FIRST OUT
'========== Class Queue '==========
string buf sys bg,i,le
method Encodelength(sys ls) int p at (i+strptr buf) p=ls i+=sizeof int end method
method push(string s) ls=len s if i+ls+8>le then
buf+=nuls 8000+ls*2 : le=len buf 'expand buf
end if EncodeLength ls mid buf,i+1,s i+=ls 'EncodeLength ls end method
method GetLength() as sys
if bg>=i then return -1 'buffer empty int p at (bg+strptr buf) bg+=sizeof int return p
end method
method pop(string *s) as sys sys ls=GetLength if ls<0 then s="" : return ls 'empty buffer s=mid buf,bg+1,ls bg+=ls if bg>1e6 then
buf=mid buf,bg+1 : bg=0 : le=len buf : i-=bg 'shrink buf
end if end method
method clear()
buf="" : le="" : bg=0 : i=0
end method
end class
'==== 'TEST '====
Queue fifo string s ' fifo.push "HumptyDumpty" fifo.push "Sat on a wall" ' sys er do
er=fifo.pop s if er then print "(buffer empty)" : exit do print s
end do </lang>
Oz
The semantics of the built-in "Port" datatype is essentially that of a thread-safe queue. We can implement the specified queue type as operations on a pair of a port and a mutable reference to the current read position of the associated stream.
It seems natural to make "Pop" a blocking operation (i.e. it waits for a new value if the queue is currently empty).
The implementation is thread-safe if there is only one reader thread. When multiple reader threads exist, it is possible that a value is popped more than once.
<lang oz>declare
fun {NewQueue}
Stream
WritePort = {Port.new Stream}
ReadPos = {NewCell Stream}
in
WritePort#ReadPos
end
proc {Push WritePort#_ Value}
{Port.send WritePort Value}
end
fun {Empty _#ReadPos}
%% the queue is empty if the value at the current
%% read position is not determined
{Not {IsDet @ReadPos}}
end
fun {Pop _#ReadPos}
%% blocks if empty
case @ReadPos of X|Xr then
ReadPos := Xr
X
end
end
Q = {NewQueue}
in
{Show {Empty Q}}
{Push Q 42}
{Show {Empty Q}}
{Show {Pop Q}}
{Show {Empty Q}}</lang>
There is also a queue datatype in the Mozart standard library.
Pascal
This program should be Standard Pascal compliant (i.e. it doesn't make use of the advanced/non-standard features of FreePascal or GNU Pascal).
<lang pascal>program fifo(input, output);
type
pNode = ^tNode;
tNode = record
value: integer;
next: pNode;
end;
tFifo = record
first, last: pNode;
end;
procedure initFifo(var fifo: tFifo);
begin fifo.first := nil; fifo.last := nil end;
procedure pushFifo(var fifo: tFifo; value: integer);
var node: pNode; begin new(node); node^.value := value; node^.next := nil; if fifo.first = nil then fifo.first := node else fifo.last^.next := node; fifo.last := node end;
function popFifo(var fifo: tFifo; var value: integer): boolean;
var
node: pNode;
begin
if fifo.first = nil
then
popFifo := false
else
begin
node := fifo.first;
fifo.first := fifo.first^.next;
value := node^.value;
dispose(node);
popFifo := true
end
end;
procedure testFifo;
var
fifo: tFifo;
procedure testpop(expectEmpty: boolean; expectedValue: integer);
var
i: integer;
begin
if popFifo(fifo, i)
then
if expectEmpty
then
writeln('Error! Expected empty, got ', i, '.')
else
if i = expectedValue
then
writeln('Ok, got ', i, '.')
else
writeln('Error! Expected ', expectedValue, ', got ', i, '.')
else
if expectEmpty
then
writeln('Ok, fifo is empty.')
else
writeln('Error! Expected ', expectedValue, ', found fifo empty.')
end;
begin
initFifo(fifo);
pushFifo(fifo, 2);
pushFifo(fifo, 3);
pushFifo(fifo, 5);
testpop(false, 2);
pushFifo(fifo, 7);
testpop(false, 3);
testpop(false, 5);
pushFifo(fifo, 11);
testpop(false, 7);
testpop(false, 11);
pushFifo(fifo, 13);
testpop(false, 13);
testpop(true, 0);
pushFifo(fifo, 17);
testpop(false, 17);
testpop(true, 0)
end;
begin
writeln('Testing fifo implementation ...');
testFifo;
writeln('Testing finished.')
end.</lang>
Perl
Lists are a central part of Perl. To implement a FIFO using OO will to many Perl programmers seem a bit awkward.
<lang perl>use Carp; sub mypush (\@@) {my($list,@things)=@_; push @$list, @things} sub mypop (\@) {my($list)=@_; @$list or croak "Empty"; shift @$list } sub empty (@) {not @_}</lang>
Example:
<lang perl>my @fifo=qw(1 2 3 a b c);
mypush @fifo, 44, 55, 66; mypop @fifo for 1 .. 6+3; mypop @fifo; #empty now</lang>
Phix
<lang Phix>sequence queue = {}
procedure push(object what)
queue = append(queue,what)
end procedure
function pop()
object what = queue[1] queue = queue[2..$] return what
end function
function empty()
return length(queue)=0
end function</lang>
Phixmonti
<lang Phixmonti>include ..\Utilitys.pmt
def push /# l i -- l&i #/
0 put
enddef
def empty? /# l -- flag #/
len 0 ==
enddef
def pop /# l -- l-1 #/
empty? if
"Empty"
else
head swap tail nip swap
endif
enddef
( ) /# empty queue #/
1 push 2 push 3 push pop ? pop ? pop ? pop ?</lang>
PHP
<lang PHP>class Fifo {
private $data = array();
public function push($element){
array_push($this->data, $element);
}
public function pop(){
if ($this->isEmpty()){
throw new Exception('Attempt to pop from an empty queue');
}
return array_shift($this->data);
}
//Alias functions
public function enqueue($element) { $this->push($element); }
public function dequeue() { return $this->pop(); }
//Note: PHP prevents a method name of 'empty'
public function isEmpty(){
return empty($this->data);
}
}</lang>
Example:
<lang PHP>$foo = new Fifo(); $foo->push('One'); $foo->enqueue('Two'); $foo->push('Three');
echo $foo->pop(); //Prints 'One' echo $foo->dequeue(); //Prints 'Two' echo $foo->pop(); //Prints 'Three' echo $foo->pop(); //Throws an exception </lang>
PicoLisp
The built-in function 'fifo' maintains a queue in a circular list, with direct access to the first and the last cell <lang PicoLisp>(off Queue) # Clear Queue (fifo 'Queue 1) # Store number '1' (fifo 'Queue 'abc) # an internal symbol 'abc' (fifo 'Queue "abc") # a transient symbol "abc" (fifo 'Queue '(a b c)) # and a list (a b c) Queue # Show the queue</lang>
- Output:
->((a b c) 1 abc "abc" .)
PL/I
<lang pli> /* To push a node onto the end of the queue. */ push: procedure (tail);
declare tail handle (node), t handle (node);
t = new(:node:);
get (t => value);
if tail ^= bind(:null, node:) then
tail => link = t;
/* If the queue was non-empty, points the tail of the queue */
/* to the new node. */
tail = t; /* Point "tail" at the end of the queue. */
tail => link = bind(:node, null:);
end push;
/* To pop a node from the head of the queue. */ pop: procedure (head, val);
declare head handle (node), val fixed binary;
if head = bind(:node, null:) then signal error;
val = head => value;
head = head => pointer; /* pops the top node. */
if head = bind(:node, null:) then tail = head;
/* (If the queue is now empty, make tail null also.) */
end pop;
/* Queue status: the EMPTY function, returns true for empty queue. */ empty: procedure (h) returns (bit(1));
declare h handle (Node); return (h = bind(:Node, null:) );
end empty; </lang>
PostScript
<lang postscript> % our queue is just [] and empty? is already defined. /push {exch tadd}. /pop {uncons exch}. </lang>
PowerShell
PowerShell can natively use the .Net Queue class. <lang PowerShell> $Q = New-Object System.Collections.Queue
$Q.Enqueue( 1 ) $Q.Enqueue( 2 ) $Q.Enqueue( 3 )
$Q.Dequeue() $Q.Dequeue()
$Q.Count -eq 0 $Q.Dequeue() $Q.Count -eq 0
try { $Q.Dequeue() } catch [System.InvalidOperationException] { If ( $_.Exception.Message -eq 'Queue empty.' ) { 'Caught error' } }</lang>
- Output:
1 2 False 3 True Caught error
Prolog
Works with SWI-Prolog. One can push any data in queue. <lang Prolog>empty(U-V) :-
unify_with_occurs_check(U, V).
push(Queue, Value, NewQueue) :-
append_dl(Queue, [Value|X]-X, NewQueue).
% when queue is empty pop fails. pop([X|V]-U, X, V-U) :-
\+empty([X|V]-U).
append_dl(X-Y, Y-Z, X-Z). </lang>
PureBasic
For FIFO function PureBasic normally uses linked lists. Usage as described above could look like; <lang PureBasic>NewList MyStack()
Procedure Push(n)
Shared MyStack() LastElement(MyStack()) AddElement(MyStack()) MyStack()=n
EndProcedure
Procedure Pop()
Shared MyStack() Protected n If FirstElement(MyStack()) ; e.g. Stack not empty n=MyStack() DeleteElement(MyStack(),1) Else Debug "Pop(), out of range. Error at line "+str(#PB_Compiler_Line) EndIf ProcedureReturn n
EndProcedure
Procedure Empty()
Shared MyStack() If ListSize(MyStack())=0 ProcedureReturn #True EndIf ProcedureReturn #False
EndProcedure
- ---- Example of implementation ----
Push(3) Push(1) Push(4) While Not Empty()
Debug Pop()
Wend
- ---- Now an extra Pop(), e.g. one to many ----
Debug Pop()</lang>
- Output:
3 1 4 Pop(), out of range. Error at line 17 0
Python
A python list can be used as a simple FIFO by simply using only it's .append() and .pop() methods and only using .pop(0) to consistently pull the head off the list. (The default .pop() pulls off the tail, and using that would treat the list as a stack.
To encapsulate this behavior into a class and provide the task's specific API we can simply use:
<lang python> class FIFO(object):
def __init__(self, *args):
self.contents = list(args)
def __call__(self):
return self.pop()
def __len__(self):
return len(self.contents)
def pop(self):
return self.contents.pop(0)
def push(self, item):
self.contents.append(item)
def extend(self,*itemlist):
self.contents += itemlist
def empty(self):
return bool(self.contents)
def __iter__(self):
return self
def next(self):
if self.empty():
raise StopIteration
return self.pop()
if __name__ == "__main__":
# Sample usage:
f = FIFO()
f.push(3)
f.push(2)
f.push(1)
while not f.empty():
print f.pop(),
# >>> 3 2 1
# Another simple example gives the same results:
f = FIFO(3,2,1)
while not f.empty():
print f(),
# Another using the default "truth" value of the object
# (implicitly calls on the length() of the object after
# checking for a __nonzero__ method
f = FIFO(3,2,1)
while f:
print f(),
# Yet another, using more Pythonic iteration:
f = FIFO(3,2,1)
for i in f:
print i,</lang>
This example does add to a couple of features which are easy in Python and allow this FIFO class to be used in ways that Python programmers might find more natural. Our __init__ accepts and optional list of initial values, we add __len__ and extend methods which simply wrap the corresponding list methods; we define a __call__ method to show how one can make objects "callable" as functions, and we define __iter__ and next() methods to facilitate using these FIFO objects with Python's prevalent iteration syntax (the for loop). The empty method could be implemented as simply an alias for __len__ --- but we've chosen to have it more strictly conform to the task specification. Implementing the __len__ method allows code using this object to test of emptiness using normal Python idioms for "truth" (any non-empty container is considered to be "true" and any empty container evaluates as "false").
These additional methods could be omitted and some could have been dispatched to the "contents" object by defining a __getattr__ method. (All methods that are note defined could be relayed to the contained list). This would allow us to skip our definitions of extend, __iter__, and __len__, and would allow contents of these objects to be access by indexes and slices as well as supporting all other list methods.
That sort of wrapper looks like:
<lang python>class FIFO: ## NOT a new-style class, must not derive from "object"
def __init__(self,*args):
self.contents = list(args)
def __call__(self):
return self.pop()
def empty(self):
return bool(self.contents)
def pop(self):
return self.contents.pop(0)
def __getattr__(self, attr):
return getattr(self.contents,attr)
def next(self):
if not self:
raise StopIteration
return self.pop()</lang>
As noted in the contents this must NOT be a new-style class, it must NOT but sub-classed from object nor any of its descendents. (A new-style implementation using __getattribute__ would be possible)
Python 2.4 and later includes a deque class, supporting thread-safe, memory efficient appends and pops from either side of the deque with approximately the same O(1) performance in either direction. For other options see Python Cookbook.
<lang python>from collections import deque fifo = deque() fifo. appendleft(value) # push value = fifo.pop() not fifo # empty fifo.pop() # raises IndexError when empty</lang>
R
Simple functional implementation
This simple implementation provides three functions that act on a variable in the global environment (user workspace) named l. the push and pop functions display the new status of l, but return NULL silently. <lang R>empty <- function() length(l) == 0 push <- function(x) {
l <<- c(l, list(x)) print(l) invisible()
} pop <- function() {
if(empty()) stop("can't pop from an empty list")
l1 <<- NULL
print(l)
invisible()
} l <- list() empty()
- [1] TRUE
push(3)
- 1
- [1] 3
push("abc")
push(matrix(1:6, nrow=2))
empty()
- [1] FALSE
pop()
pop()
- 1
- [1] 3
pop()
- list()
pop()
- Error in pop() : can't pop from an empty list</lang>
The problem with this is that the functions aren't related to the FIFO object (the list l), and they require the list to exist in the global environment. (This second problem is possible to get round by passing l into the function and then returning it, but that is extra work.)
Message passing
<lang r># The usual Scheme way : build a function that takes commands as parameters (it's like message passing oriented programming) queue <- function() {
v <- list()
f <- function(cmd, val=NULL) {
if(cmd == "push") {
v <<- c(v, val)
invisible()
} else if(cmd == "pop") {
if(length(v) == 0) {
stop("empty queue")
} else {
x <- v1
v1 <<- NULL
x
}
} else if(cmd == "length") {
length(v)
} else if(cmd == "empty") {
length(v) == 0
} else {
stop("unknown command")
}
}
f
}
- Create two queues
a <- queue() b <- queue() a("push", 1) a("push", 2) b("push", 3) a("push", 4) b("push", 5)
a("pop")
- [1] 1
b("pop")
- [1] 3</lang>
Object oriented implementation
A better solution is to use the object oriented facility in the proto package. (R does have it's own native object oriented code, though the proto package is often nicer to use.)
<lang R>library(proto)
fifo <- proto(expr = {
l <- list()
empty <- function(.) length(.$l) == 0
push <- function(., x)
{
.$l <- c(.$l, list(x))
print(.$l)
invisible()
}
pop <- function(.)
{
if(.$empty()) stop("can't pop from an empty list")
.$l1 <- NULL
print(.$l)
invisible()
}
})
- The following code provides output that is the same as the previous example.
fifo$empty() fifo$push(3) fifo$push("abc") fifo$push(matrix(1:6, nrow=2)) fifo$empty() fifo$pop() fifo$pop() fifo$pop() fifo$pop()</lang>
Racket
Racket comes with a queue implementation in the data/queue library. Here's an explicit implementation:
<lang Racket>
- lang racket
(define (make-queue) (mcons #f #f)) (define (push! q x)
(define new (mcons x #f)) (if (mcar q) (set-mcdr! (mcdr q) new) (set-mcar! q new)) (set-mcdr! q new))
(define (pop! q)
(define old (mcar q))
(cond [(eq? old (mcdr q)) (set-mcar! q #f) (set-mcdr! q #f)]
[else (set-mcar! q (mcdr old))])
(mcar old))
(define (empty? q)
(not (mcar q)))
(define Q (make-queue)) (empty? Q) ; -> #t (push! Q 'x) (empty? Q) ; -> #f (for ([x 3]) (push! Q x)) (pop! Q) ; -> 'x (list (pop! Q) (pop! Q) (pop! Q)) ; -> '(0 1 2) </lang>
And this is an implementation of a functional queue. <lang racket>
- lang racket
- Invariants
- The elements in the queue are (append front (reverse back)).
- Front is always non-empty (except for the empty queue).
(struct queue (front back))
(define empty (queue '() '()))
(define (push x q)
(if (null? (queue-front q))
(queue (reverse (cons x (queue-back q))) '())
(queue (queue-front q) (cons x (queue-back q)))))
(define (empty? q)
(null? (queue-front q)))
(define (pop q)
(cond [(empty? q) (error 'pop "the queue is empty")]
[(not (null? (queue-front q)))
(if (null? (rest (queue-front q)))
(queue (reverse (queue-back q)) '())
(queue (rest (queue-front q)) (queue-back q)))]
[else (queue (reverse (queue-back q)) '())]))
(define (first q)
(cond [(empty? q) (error 'first "the queue is empty")]
[(car (queue-front q))]))
- Example
(first (pop (pop (for/fold ([q empty]) ([x '(1 2 3 4)])
(push x q)))))
- => 3
</lang>
Raku
(formerly Perl 6)
We could build a new container class to do FIFO pretty easily, but Arrays already do everything needed by a FIFO queue, so it is easier to just compose a Role on the existing Array class. <lang perl6>role FIFO {
method enqueue ( *@values ) { # Add values to queue, returns the number of values added.
self.push: @values;
return @values.elems;
}
method dequeue ( ) { # Remove and return the first value from the queue.
# Return Nil if queue is empty.
return self.elems ?? self.shift !! Nil;
}
method is-empty ( ) { # Check to see if queue is empty. Returns Boolean value.
return self.elems == 0;
}
}
- Example usage:
my @queue does FIFO;
say @queue.is-empty; # -> Bool::True for <A B C> -> $i { say @queue.enqueue: $i } # 1 \n 1 \n 1 say @queue.enqueue: Any; # -> 1 say @queue.enqueue: 7, 8; # -> 2 say @queue.is-empty; # -> Bool::False say @queue.dequeue; # -> A say @queue.elems; # -> 4 say @queue.dequeue; # -> B say @queue.is-empty; # -> Bool::False say @queue.enqueue('OHAI!'); # -> 1 say @queue.dequeue until @queue.is-empty; # -> C \n Any() \n [7 8] \n OHAI! say @queue.is-empty; # -> Bool::True say @queue.dequeue; # -></lang>
REBOL
<lang REBOL>rebol [
Title: "FIFO" URL: http://rosettacode.org/wiki/FIFO
]
- Define fifo class
fifo: make object! [
queue: copy []
push: func [x][append queue x]
pop: func [/local x][ ; Make 'x' local so it won't pollute global namespace.
if empty [return none]
x: first queue remove queue x]
empty: does [empty? queue]
]
- Create and populate a FIFO
q: make fifo [] q/push 'a q/push 2 q/push USD$12.34 ; Did I mention that REBOL has 'money!' datatype? q/push [Athos Porthos Aramis] ; List elements pushed on one by one. q/push Huey Dewey Lewey ; This list is preserved as a list.
- Dump it out, with narrative
print rejoin ["Queue is " either q/empty [""]["not "] "empty."] while [not q/empty][print [" " q/pop]] print rejoin ["Queue is " either q/empty [""]["not "] "empty."] print ["Trying to pop an empty queue yields:" q/pop]</lang>
- Output:
Queue is not empty. a 2 USD$12.34 Athos Porthos Aramis Huey Dewey Lewey Queue is empty. Trying to pop an empty queue yields: none
REXX
Support for LIFO & FIFO queues is built into the Rexx language.
The following are supported in REXX:
- PUSH (lifo)
- QUEUE (fifo)
- PULL --- which is a short version of:
- PARSE UPPER PULL
- PARSE LOWER PULL --- supported by some newer REXXes
- PARSE PULL
- QUEUED() [a BIF which returns the number of queued entries.]
<lang rexx>/*REXX program to demonstrate FIFO queue usage by some simple operations*/
call viewQueue
a="Fred"
push /*puts a "null" on top of queue.*/
push a 2 /*puts "Fred 2" on top of queue.*/
call viewQueue
queue "Toft 2" /*put "Toft 2" on queue bottom.*/ queue /*put a "null" on queue bottom.*/ call viewQueue
do n=1 while queued()\==0
parse pull xxx
say "queue entry" n': ' xxx
end /*n*/
call viewQueue exit /*stick a fork in it, we're done.*/ /*──────────────────────────────────viewQueue subroutine────────────────*/ viewQueue: if queued()==0 then say 'Queue is empty'
else say 'There are' queued() 'elements in the queue'
return</lang> output
Queue is empty There are 2 elements in the queue There are 4 elements in the queue queue entry 1: Fred 2 queue entry 2: queue entry 3: Toft 2 queue entry 4: Queue is empty
Ring
<lang ring>
- Project : Queue/Definition
load "stdlib.ring" oQueue = new Queue for n = 5 to 7
see "Push: " + n + nl
oQueue.add(n)
next see "Pop: " + oQueue.remove() + nl see "Push: 8" + nl oQueue.add(8) see "Pop: " + oQueue.remove() + nl see "Pop: " + oQueue.remove() + nl see "Pop: " + oQueue.remove() + nl if len(oQueue) != 0
oQueue.print()
else
see "Error: queue is empty" + nl
ok </lang> Output:
Push: 5 Push: 6 Push: 7 Pop: 5 Push: 8 Pop: 6 Pop: 7 Pop: 8 Error: queue is empty
Ruby
The core class Array already implements all queue operations, so this class FIFO delegates everything to methods of Array.
<lang ruby>require 'forwardable'
- A FIFO queue contains elements in first-in, first-out order.
- FIFO#push adds new elements to the end of the queue;
- FIFO#pop or FIFO#shift removes elements from the front.
class FIFO
extend Forwardable
# Creates a FIFO containing _objects_. def self.[](*objects) new.push(*objects) end
# Creates an empty FIFO. def initialize; @ary = []; end
# Appends _objects_ to the end of this FIFO. Returns self. def push(*objects) @ary.push(*objects) self end alias << push alias enqueue push
## # :method: pop # :call-seq: # pop -> obj or nil # pop(n) -> ary # # Removes an element from the front of this FIFO, and returns it. # Returns nil if the FIFO is empty. # # If passing a number _n_, removes the first _n_ elements, and returns # an Array of them. If this FIFO contains fewer than _n_ elements, # returns them all. If this FIFO is empty, returns an empty Array. def_delegator :@ary, :shift, :pop alias shift pop alias dequeue shift
## # :method: empty? # Returns true if this FIFO contains no elements. def_delegator :@ary, :empty?
## # :method: size # Returns the number of elements in this FIFO. def_delegator :@ary, :size alias length size
# Converts this FIFO to a String.
def to_s
"FIFO#{@ary.inspect}"
end
alias inspect to_s
end</lang>
<lang ruby>f = FIFO.new f.empty? # => true f.pop # => nil f.pop(2) # => [] f.push(14) # => FIFO[14] f << "foo" << [1,2,3] # => FIFO[14, "foo", [1, 2, 3]] f.enqueue("bar", Hash.new, "baz")
- => FIFO[14, "foo", [1, 2, 3], "bar", {}, "baz"]
f.size # => 6 f.pop(3) # => [14, "foo", [1, 2, 3]] f.dequeue # => "bar" f.empty? # => false g = FIFO[:a, :b, :c] g.pop(2) # => [:a, :b] g.pop(2) # => [:c] g.pop(2) # => []</lang>
Rust
Using the standard library
The standard library has a double-ended queue implementation (VecDeque<T>) which will work here.
<lang rust>use std::collections::VecDeque;
fn main() {
let mut stack = VecDeque::new();
stack.push_back("Element1");
stack.push_back("Element2");
stack.push_back("Element3");
assert_eq!(Some(&"Element1"), stack.front());
assert_eq!(Some("Element1"), stack.pop_front());
assert_eq!(Some("Element2"), stack.pop_front());
assert_eq!(Some("Element3"), stack.pop_front());
assert_eq!(None, stack.pop_front());
}</lang>
A simple implementation
This shows the implementation of a singly-linked queue with dequeue and enqueue. There are two peek implementations, one returns an immutable reference, the other returns a mutable one. This implementation also shows iteration over the Queue by value (consumes queue), immutable reference, and mutable reference.
<lang rust>use std::ptr;
pub struct Queue<T> {
head: Link<T>, tail: *mut Item<T>, // Raw, C-like pointer. Cannot be guaranteed safe
}
type Link<T> = Option<Box<Item<T>>>;
struct Item<T> {
elem: T, next: Link<T>,
}
pub struct IntoIter<T>(Queue<T>);
pub struct Iter<'a, T:'a> {
next: Option<&'a Item<T>>,
}
pub struct IterMut<'a, T: 'a> {
next: Option<&'a mut Item<T>>,
}
impl<T> Queue<T> {
pub fn new() -> Self {
Queue { head: None, tail: ptr::null_mut() }
}
pub fn enqueue(&mut self, elem: T) {
let mut new_tail = Box::new(Item {
elem: elem,
next: None,
});
let raw_tail: *mut _ = &mut *new_tail;
if !self.tail.is_null() {
unsafe {
(*self.tail).next = Some(new_tail);
}
} else {
self.head = Some(new_tail);
}
self.tail = raw_tail; }
pub fn dequeue(&mut self) -> Option<T> {
self.head.take().map(|head| {
let head = *head;
self.head = head.next;
if self.head.is_none() {
self.tail = ptr::null_mut();
}
head.elem
})
}
pub fn peek(&self) -> Option<&T> {
self.head.as_ref().map(|item| {
&item.elem
})
}
pub fn peek_mut(&mut self) -> Option<&mut T> {
self.head.as_mut().map(|item| {
&mut item.elem
})
}
pub fn into_iter(self) -> IntoIter<T> {
IntoIter(self)
}
pub fn iter(&self) -> Iter<T> {
Iter { next: self.head.as_ref().map(|item| &**item) }
}
pub fn iter_mut(&mut self) -> IterMut<T> {
IterMut { next: self.head.as_mut().map(|item| &mut **item) }
}
}
impl<T> Drop for Queue<T> {
fn drop(&mut self) {
let mut cur_link = self.head.take();
while let Some(mut boxed_item) = cur_link {
cur_link = boxed_item.next.take();
}
}
}
impl<T> Iterator for IntoIter<T> {
type Item = T;
fn next(&mut self) -> Option<Self::Item> {
self.0.dequeue()
}
}
impl<'a, T> Iterator for Iter<'a, T> {
type Item = &'a T;
fn next(&mut self) -> Option<Self::Item> {
self.next.map(|item| {
self.next = item.next.as_ref().map(|item| &**item);
&item.elem
})
}
}
impl<'a, T> Iterator for IterMut<'a, T> {
type Item = &'a mut T;
fn next(&mut self) -> Option<Self::Item> {
self.next.take().map(|item| {
self.next = item.next.as_mut().map(|item| &mut **item);
&mut item.elem
})
}
}</lang>
Scala
<lang scala>class Queue[T] {
private[this] class Node[T](val value:T) {
var next:Option[Node[T]]=None
def append(n:Node[T])=next=Some(n)
}
private[this] var head:Option[Node[T]]=None
private[this] var tail:Option[Node[T]]=None
def isEmpty=head.isEmpty
def enqueue(item:T)={
val n=new Node(item)
if(isEmpty) head=Some(n) else tail.get.append(n)
tail=Some(n)
}
def dequeue:T=head match {
case Some(item) => head=item.next; item.value
case None => throw new java.util.NoSuchElementException()
}
def front:T=head match {
case Some(item) => item.value
case None => throw new java.util.NoSuchElementException()
}
def iterator: Iterator[T]=new Iterator[T]{
private[this] var it=head;
override def hasNext=it.isDefined
override def next:T={val n=it.get; it=n.next; n.value}
}
override def toString()=iterator.mkString("Queue(", ", ", ")")
}</lang> Usage: <lang scala>val q=new Queue[Int]() println("isEmpty = " + q.isEmpty) try{q dequeue} catch{case _:java.util.NoSuchElementException => println("dequeue(empty) failed.")} q enqueue 1 q enqueue 2 q enqueue 3 println("queue = " + q) println("front = " + q.front) println("dequeue = " + q.dequeue) println("dequeue = " + q.dequeue) println("isEmpty = " + q.isEmpty)</lang>
- Output:
isEmpty = true dequeue(empty) failed. queue = Queue(1, 2, 3) front = 1 dequeue = 1 dequeue = 2 isEmpty = false
Scheme
Using a vector for mutable data. Can be optimized by using an extra slot in the vector to hold tail pointer to avoid the append call.
<lang scheme>(define (make-queue)
(make-vector 1 '()))
(define (push a queue)
(vector-set! queue 0 (append (vector-ref queue 0) (list a))))
(define (empty? queue)
(null? (vector-ref queue 0)))
(define (pop queue)
(if (empty? queue)
(error "can not pop an empty queue")
(let ((ret (car (vector-ref queue 0))))
(vector-set! queue 0 (cdr (vector-ref queue 0)))
ret)))
</lang>
Message passing
<lang scheme>(define (make-queue) (let ((q (cons '() '()))) (lambda (cmd . arg) (case cmd
((empty?) (null? (car q)))
((put) (let ((a (cons (car arg) '())))
(if (null? (car q))
(begin (set-car! q a) (set-cdr! q a))
(begin (set-cdr! (cdr q) a) (set-cdr! q a)))))
((get) (if (null? (car q)) 'empty
(let ((x (caar q)))
(set-car! q (cdar q))
(if (null? (car q)) (set-cdr! q '()))
x)))
))))
(define q (make-queue)) (q 'put 1) (q 'put 6) (q 'get)
- 1
(q 'get)
- 6
(q 'get)
- empty</lang>
SenseTalk
A queue in SenseTalk is implemented using push and pull operations on a list. <lang sensetalk> set myFoods to be an empty list
push "grapes" into myFoods push "orange" into myFoods push "apricot" into myFoods
put "The foods in my queue are: " & myFoods
pull from myFoods into firstThingToEat
put "The first thing to eat is: " & firstThingToEat
if myFoods is empty then
put "The foods list is empty!"
else
put "The remaining foods are: " & myFoods
end if </lang> Output: <lang sensetalk> The foods in my queue are: (grapes,orange,apricot) The first thing to eat is: grapes The remaining foods are: (orange,apricot) </lang>
Sidef
Implemented as a class: <lang ruby>class FIFO(*array) {
method pop {
array.is_empty && die "underflow";
array.shift;
}
method push(*items) {
array += items;
self;
}
method empty {
array.len == 0;
}
}</lang>
Slate
Toy code based on Slate's Queue standard library (which is optimized for FIFO access): <lang slate>collections define: #Queue &parents: {ExtensibleArray}.
q@(Queue traits) isEmpty [resend]. q@(Queue traits) push: obj [q addLast: obj]. q@(Queue traits) pop [q removeFirst]. q@(Queue traits) pushAll: c [q addAllLast: c]. q@(Queue traits) pop: n [q removeFirst: n].</lang>
Smalltalk
An OrderedCollection can be easily used as a FIFO queue.
<lang smalltalk>OrderedCollection extend [
push: obj [ ^(self add: obj) ]
pop [
(self isEmpty) ifTrue: [
SystemExceptions.NotFound signalOn: self
reason: 'queue empty'
] ifFalse: [
^(self removeFirst)
]
]
]
|f| f := OrderedCollection new. f push: 'example'; push: 'another'; push: 'last'. f pop printNl. f pop printNl. f pop printNl. f isEmpty printNl. f pop. "queue empty error"</lang>
Standard ML
Here is the signature for a basic queue: <lang Standard ML> signature QUEUE = sig
type 'a queue
val empty_queue: 'a queue
exception Empty
val enq: 'a queue -> 'a -> 'a queue
val deq: 'a queue -> ('a * 'a queue)
val empty: 'a queue -> bool
end; </lang> A very basic implementation of this signature backed by a list is as follows: <lang Standard ML> structure Queue:> QUEUE = struct
type 'a queue = 'a list val empty_queue = nil exception Empty fun enq q x = q @ [x] fun deq nil = raise Empty | deq (x::q) = (x, q) fun empty nil = true | empty _ = false
end; </lang>
Stata
See Singly-linked list/Element definition#Stata.
Tcl
Here's a simple implementation using a list: <lang tcl>proc push {stackvar value} {
upvar 1 $stackvar stack lappend stack $value
} proc pop {stackvar} {
upvar 1 $stackvar stack set value [lindex $stack 0] set stack [lrange $stack 1 end] return $value
} proc size {stackvar} {
upvar 1 $stackvar stack llength $stack
} proc empty {stackvar} {
upvar 1 $stackvar stack
expr {[size stack] == 0}
} proc peek {stackvar} {
upvar 1 $stackvar stack lindex $stack 0
}
set Q [list] empty Q ;# ==> 1 (true) push Q foo empty Q ;# ==> 0 (false) push Q bar peek Q ;# ==> foo pop Q ;# ==> foo peek Q ;# ==> bar</lang>
<lang tcl>package require struct::queue struct::queue Q Q size ;# ==> 0 Q put a b c d e Q size ;# ==> 5 Q peek ;# ==> a Q get ;# ==> a Q peek ;# ==> b Q pop 4 ;# ==> b c d e Q size ;# ==> 0</lang>
UNIX Shell
<lang bash>queue_push() {
typeset -n q=$1
shift
q+=("$@")
}
queue_pop() {
if queue_empty $1; then
print -u2 "queue $1 is empty"
return 1
fi
typeset -n q=$1
print "${q[0]}" # emit the value of the popped element
q=( "${q[@]:1}" ) # and remove the first element from the queue
}
queue_empty() {
typeset -n q=$1
(( ${#q[@]} == 0 ))
}
queue_peek() {
typeset -n q=$1
print "${q[0]}"
}</lang>
Usage: <lang bash># any valid variable name can be used as a queue without initialization
queue_empty foo && echo foo is empty || echo foo is not empty
queue_push foo bar queue_push foo baz queue_push foo "element with spaces"
queue_empty foo && echo foo is empty || echo foo is not empty
print "peek: $(queue_peek foo)"; queue_pop foo print "peek: $(queue_peek foo)"; queue_pop foo print "peek: $(queue_peek foo)"; queue_pop foo print "peek: $(queue_peek foo)"; queue_pop foo</lang>
- Output:
foo is empty foo is not empty peek: bar peek: baz peek: element with spaces peek: queue foo is empty
UnixPipes
Uses moreutils <lang bash>init() {echo > fifo} push() {echo $1 >> fifo } pop() {head -1 fifo ; (cat fifo | tail -n +2)|sponge fifo} empty() {cat fifo | wc -l}</lang> Usage: <lang bash>push me; push you; push us; push them |pop;pop;pop;pop me you us them</lang>
V
V doesn't have mutable data. Below is an function interface for a fifo.
<lang v>[fifo_create []]. [fifo_push swap cons]. [fifo_pop [[*rest a] : [*rest] a] view]. [fifo_empty? dup empty?].</lang>
Using it <lang v>|fifo_create 3 fifo_push 4 fifo_push 5 fifo_push ?? =[5 4 3] |fifo_empty? puts =false |fifo_pop put fifo_pop put fifo_pop put =3 4 5 |fifo_empty? puts
=true</lang>
VBA
<lang vb>Public queue As New Collection
Private Sub push(what As Variant)
queue.Add what
End Sub
Private Function pop() As Variant
If queue.Count > 0 Then
what = queue(1)
queue.Remove 1
Else
what = CVErr(461)
End If
pop = what
End Function
Private Function empty_()
empty_ = queue.Count = 0
End Function</lang>
VBScript
Using an ArrayList. <lang vb>' Queue Definition - VBScript Option Explicit Dim queue, i, x Set queue = CreateObject("System.Collections.ArrayList") If Not empty_(queue) Then Wscript.Echo queue.Count push queue, "Banana" push queue, "Apple" push queue, "Pear" push queue, "Strawberry" Wscript.Echo "Count=" & queue.Count Wscript.Echo pull(queue) & " - Count=" & queue.Count ' Wscript.Echo "Head=" & queue.Item(0) Wscript.Echo "Tail=" & queue.Item(queue.Count-1) Wscript.Echo queue.IndexOf("Pear", 0) For i=1 To queue.Count Wscript.Echo join(queue.ToArray(), ", ") x = pull(queue) Next 'i
Sub push(q, what)
q.Add what
End Sub 'push
Function pull(q) Dim what
If q.Count > 0 Then
what = q(0)
q.RemoveAt 0
Else
what = ""
End If
pull = what
End Function 'pull
Function empty_(q)
empty_ = q.Count = 0
End Function 'empty_ </lang>
- Output:
Count=4 Banana - Count=3 Head=Apple Tail=Strawberry 1 Apple, Pear, Strawberry Pear, Strawberry Strawberry
Vlang
<lang vlang>const ( MaxDepth = 256 )
struct Queue { mut: data []f32=[f32(0)].repeat(MaxDepth) depth int=0 head int=0 }
fn (q mut Queue) enqueue(v f32) { if q.depth >= MaxDepth || q.depth < q.head { return } println('Enqueue: ${v : 3.2f}') q.data[q.depth] = v q.depth++ }
fn (q mut Queue) dequeue() ?f32 { if q.depth > 0 && q.head < q.depth { result := q.data[q.head] q.head++ println('Dequeue: top of Queue was ${result :3.2f}') return result } return error('Queue Underflow!!') }
fn (q Queue) peek() ?f32 { if q.depth > 0 && q.head < q.depth { result := q.data[q.head] println('Peek: top of Queue is ${result :3.2f}') return result } return error('Out of Bounds...') }
fn (q Queue) empty() bool { return q.depth == 0 }
fn main() { mut queue := Queue{} println('Queue is empty? ' + if queue.empty() { 'Yes' } else { 'No' }) queue.enqueue(5.0) queue.enqueue(4.2) println('Queue is empty? ' + if queue.empty() { 'Yes' } else { 'No' }) queue.peek() or { return } queue.dequeue() or { return } queue.dequeue() or { return } queue.enqueue(1.2) } </lang>
- Output:
Queue is empty? Yes Enqueue: 5.00 Enqueue: 4.20 Queue is empty? No Peek: top of Queue is 5.00 Dequeue: top of Queue was 5.00 Dequeue: top of Queue was 4.20 Enqueue: 1.20
Wart
Wart defines queues as lists with a pointer to the last element saved for constant-time enqueuing: <lang python>def (queue seq)
(tag queue (list seq lastcons.seq len.seq))
def (enq x q)
do1 x
let (l last len) rep.q
rep.q.2 <- (len + 1)
if no.l
rep.q.1 <- (rep.q.0 <- list.x)
rep.q.1 <- (cdr.last <- list.x)
def (deq q)
let (l last len) rep.q
ret ans car.l
unless zero?.len
rep.q.2 <- (len - 1)
rep.q.0 <- cdr.l
def (len q) :case (isa queue q)
rep.q.2</lang>
empty? relies on len by default, so there's no need to separately override it.
Wren
The above module contains a suitable Queue class. <lang ecmascript>import "/queue" for Queue
var q = Queue.new() var item = q.pop() if (item == null) {
System.print("ERROR: attempted to pop from an empty queue")
} else {
System.print("'%(item)' was popped")
}</lang>
- Output:
ERROR: attempted to pop from an empty queue
XLISP
A queue is similar to a stack, except that values are pushed onto and popped from different "ends" of it (whereas in a stack it is the same end for both operations). This implementation is based on the XLISP implementation of a stack, therefore, but with a push method that appends a new value to the end rather than sticking it onto the front. Attempting to pop from an empty queue will return the empty list, equivalent to Boolean "false". <lang lisp>(define-class queue
(instance-variables vals))
(define-method (queue 'initialize)
(setq vals '()) self)
(define-method (queue 'push x)
(setq vals (nconc vals (cons x nil))))
(define-method (queue 'pop)
(define val (car vals)) (setq vals (cdr vals)) val)
(define-method (queue 'emptyp)
(null vals))</lang>
A sample REPL session: <lang lisp>[1] (define my-queue (queue 'new))
MY-QUEUE [2] (my-queue 'push 1)
(1) [3] (my-queue 'push 2)
(1 2) [4] (my-queue 'emptyp)
() [5] (my-queue 'pop)
1 [6] (my-queue 'pop)
2 [7] (my-queue 'emptyp)
- T
[8] (my-queue 'pop)
()</lang>
XPL0
<lang XPL0>include c:\cxpl\codes; def Size=8; int Fifo(Size); int In, Out; \fill and empty indexes into Fifo
proc Push(A); \Add integer A to queue int A; \(overflow not detected) [Fifo(In):= A; In:= In+1; if In >= Size then In:= 0; ];
func Pop; \Return first integer in queue int A; [if Out=In then \if popping empty queue
[Text(0, "Error"); exit 1]; \ then exit program with error code 1
A:= Fifo(Out); Out:= Out+1; if Out >= Size then Out:= 0; return A; ];
func Empty; \Return 'true' if queue is empty return In = Out;
[In:= 0; Out:= 0; Push(0); Text(0, if Empty then "true" else "false"); CrLf(0); IntOut(0, Pop); CrLf(0); Push(1); Push(2); Push(3); IntOut(0, Pop); CrLf(0); IntOut(0, Pop); CrLf(0); IntOut(0, Pop); CrLf(0); Text(0, if Empty then "true" else "false"); CrLf(0);
\A 256-byte queue is built in as device 8: OpenI(8); OpenO(8); ChOut(8, ^0); \push ChOut(0, ChIn(8)); CrLf(0); \pop ChOut(8, ^1); \push ChOut(8, ^2); \push ChOut(8, ^3); \push ChOut(0, ChIn(8)); CrLf(0); \pop ChOut(0, ChIn(8)); CrLf(0); \pop ChOut(0, ChIn(8)); CrLf(0); \pop ]</lang>
Output:
false 0 1 2 3 true 0 1 2 3
zkl
<lang zkl>class Queue{
var [const] q=List();
fcn push { q.append(vm.pasteArgs()) }
fcn pop { q.pop(0) }
fcn empty { q.len()==0 }
}</lang> <lang zkl>q:=Queue(); q.push(1,2,3); q.pop(); //-->1 q.empty(); //-->False q.pop();q.pop();q.pop() //-->IndexError thrown</lang>
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