Queue/Definition: Difference between revisions
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Revision as of 20:56, 23 June 2012
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.
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 FIFO (usage) for the built-in FIFO or queue of your language or standard library.
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>
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>
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>
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; if (q->tail == q->head) { /* 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++
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>
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>
Clojure
The "pop" function implies mutating the input, but since Clojure data structures are immutable we use a mutable reference to an immutable data structure; in this case an atom holding a vector:
<lang lisp>(defn make-queue []
(atom []))
(defn enqueue [q x]
(swap! q conj x))
(defn dequeue [q]
(if (seq @q)
(let [x (first @q)]
(swap! q subvec 1)
x)
(throw (IllegalStateException. "Can't pop an empty queue."))))
(defn queue-empty? [q]
(empty? @q))</lang>
The implementation is thread-safe if there is at most one reader thread, i.e. only one thread ever calls dequeue on a given queue.
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 <lang> > time coffee fifo.coffee { number: 1 } { number: 5000000 }
real 0m2.394s user 0m2.089s sys 0m0.265s </lang>
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>
D
Implemented a queue class, by reusing previous stack class definition. See Stack#D. <lang d>module stack ; class Stack(T){ ...
void push(T top) { ... }
T pop() { ... }
bool empty() { ... }
}</lang> <lang d>module fifo ; import stack ; class FIFO(T) : Stack!(T){
override T pop() {
if (empty)
throw new Exception("FIFO Empty") ;
T top = content[0] ;
content = content[1..$] ;
return top ;
}
alias push enqueue ;
alias pop dequeue ;
}</lang>
Statement content = content[1..$] is efficient enough, because no array content is moved/copyed, but pointer modified.
Using the Singly-Linked List (element): <lang d>module fifolink ; class FIFOLinked(T) {
alias Node!(T) Node;
private Node head = null;
private Node tail = null;
void push(T last) {
head = new Node(last, head);
if (tail is null)
tail = head;
}
T pop() {
if(empty)
throw new Exception("FIFO Empty") ;
T first = head.data;
if (head is tail) // is last one?
tail = null; // release tail reference so that GC can collect afterward
head = head.next;
return first;
}
bool empty() { return head is null; }
alias push enqueue ;
alias pop dequeue ;
}</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>
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>
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
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>
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>
GAP
<lang gap>Enqueue := function(v, x)
Add(v[1], x);
end;
Dequeue := function(v)
local n, x;
n := Size(v[2]);
if n = 0 then
v[2] := Reversed(v[1]);
v[1] := [ ];
n := Size(v[2]);
if n = 0 then
return fail;
fi;
fi;
return Remove(v[2], n);
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>
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.
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>
Mathematica
<lang Mathematica>Empty[a_] := If[Length[a] == 0, True, False] 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>
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
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.
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>
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;
}
}</lang>
Example usage:
<lang perl6>my @queue does FIFO;
say @queue.is-empty; # -> Bool::True say @queue.enqueue: <A B C>; # -> 3 say @queue.enqueue: Any; # -> 1 say @queue.enqueue: 7, 8; # -> 2 say @queue.is-empty; # -> Bool::False say @queue.dequeue; # -> A say @queue.elems; # -> 5 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 \n 8 \n OHAI! say @queue.is-empty; # -> Bool::True say @queue.dequeue; # -></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 PL/I> /* 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>
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>
Outputs
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>
REBOL
<lang REBOL>rebol [
Title: "FIFO" Author: oofoe Date: 2009-12-11 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 "PUSH" (LIFO), "QUEUE" (FIFO), "PULL" (and/or "PARSE PULL ..."; "PULL" is a synonym for "PARSE UPPER PULL ...") (dequeue) keywords in conjunction with the QUEUED() built–in function deliver this capability.
version 1
<lang Rexx> /* Rexx */
Do
Call viewQueue
a = "Fred" Push /* Puts a null line onto the queue */ Push a 2 /* Puts "Fred 2" onto the queue */ Call viewQueue
a = "Toft" Queue a 2 /* Enqueues "Toft 2" */ Queue /* Enqueues a null line behind the last */ Call viewQueue
Do q_ = 1 while queued() > 0 Parse pull line Say right(q_, 3)':' line End q_ Call viewQueue
Return
End Exit
viewQueue: Procedure Do
If queued() = 0 then Do Say 'Queue is empty' End else Do Say 'There are' queued() 'elements in the queue' End
Return
End Exit </lang>
Output:
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
version 2
This REXX program performs like version 1 but with superflous statements removed. <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 n':' xxx
end
call viewQueue exit
/*─────────────────────────────────────viewQueue subroutine─────────────*/ viewQueue: procedure if queued()==0 then say 'Queue is empty'
else say 'There are' queued() 'elements in the queue'
return</lang>
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>
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>
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>
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>
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</lang>
=true
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