Queue/Definition

From Rosetta Code
< Queue(Redirected from FIFO)
Task
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.

You may see other such structures in the Data Structures category.
Illustration of FIFO behavior
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



ACL2[edit]

(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))

Ada[edit]

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.

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;

The FIFO implementation is described in the package body:

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;

A "main" procedure for this program is:

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;

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.

 
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;
 
 
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;
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;

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.

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;
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;
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;

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.

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;

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.

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;
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;

ALGOL 68[edit]

Works with: ALGOL 68 version Revision 1 - one extension to language used - PRAGMA READ - a non standard feature similar to C's #include directive.
Works with: ALGOL 68G version Any - tested with release algol68g-2.7.
File: prelude/queue_base.a68
# -*- 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
See also: Queue/Usage

ALGOL W[edit]

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.
Output:
fred
whilma
betty
barney

AutoHotkey[edit]

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
}

AWK[edit]

#!/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
}
 
Output:
 empty? 1
 push a
 push b
 empty? 0
 pop b
 pop a
 empty? 1
 Popping from empty queue.

Batch File[edit]

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.

 
@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
 

BBC BASIC[edit]

      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
Output:
Push 3
Push 4
Push 5
Pop 3
Push 6
Pop 4
Pop 5
Pop 6
Pop
Error: queue empty


Bracmat[edit]

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

  ( queue
= (list=)
(enqueue=.(.!arg) !(its.list):?(its.list))
( dequeue
= x
.  !(its.list):?(its.list) (.?x)
& !x
)
(empty=.!(its.list):)
)

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[edit]

Dynamic array[edit]

Dynamic array working as a circular buffer.

#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;
}

Doubly linked list[edit]

#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;
}
 

Test code This main function works with both implementions above.

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;
}

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[edit]

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)

#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;
}

C++[edit]

Works with: g++ version 4.1.2 20061115 (prerelease) (Debian 4.1.1-21)

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.

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;
}
}

C#[edit]

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.

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;
}
}

Clojure[edit]

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:

(defn make-queue []
(atom []))
 
(defn enqueue [q x]
(swap! q conj x))
 
(defn dequeue [q]
(if-let [[f & r] (seq @q)]
(do (reset! q r) f)
(throw (IllegalStateException. "Can't pop an empty queue."))))
 
(defn queue-empty? [q]
(empty? @q))

The implementation is thread-safe if there is at most one writer thread, i.e. only one thread ever calls dequeue on a given queue.

CoffeeScript[edit]

 
# 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
 
Output:
> time coffee fifo.coffee 
{ number: 1 }
{ number: 5000000 }

real    0m2.394s
user    0m2.089s
sys 0m0.265s

Common Lisp[edit]

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.

(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))))

Component Pascal[edit]

BlackBox Component Builder

 
MODULE Queue;
IMPORT StdLog,Boxes;
 
TYPE
Queue* = POINTER TO RECORD
first,last: LONGINT;
queue: POINTER TO ARRAY OF Boxes.Object;
END;
 
PROCEDURE NewQueue*(cap: LONGINT): Queue;
VAR
q: Queue;
BEGIN
NEW(q);q.first := 0; q.last := 0;
NEW(q.queue,cap);
RETURN q
END NewQueue;
 
PROCEDURE (q: Queue) IsEmpty*(): BOOLEAN, NEW;
VAR
BEGIN
RETURN (q.first = q.last)
END IsEmpty;
 
PROCEDURE (q: Queue) Push*(o: Boxes.Object),NEW;
VAR
i,j,oldSize,newSize: LONGINT;
queue: POINTER TO ARRAY OF Boxes.Object;
BEGIN
IF q.IsEmpty() THEN
q.queue[q.last] := o;
q.last := (q.last + 1) MOD LEN(q.queue)
ELSE
q.queue[q.last] := o;
q.last := (q.last + 1) MOD LEN(q.queue);
IF q.first = q.last THEN
(* grow queue *)
newSize := LEN(q.queue) + (LEN(q.queue) DIV 2);
(* new queue*)
NEW(queue,newSize);
(* move data from old queue to new queue *)
i := q.first; j := 0;oldSize := LEN(q.queue) - q.first + q.last;
WHILE (j < oldSize ) & (j < LEN(queue) - 1) DO
queue[j] := q.queue[i];
i := (i + 1) MOD LEN(q.queue);INC(j)
END;
q.queue := queue;q.first := 0;q.last := j
END
END;
END Push;
 
PROCEDURE (q: Queue) Pop*(): Boxes.Object, NEW;
VAR
o: Boxes.Object;
BEGIN
ASSERT(~q.IsEmpty());
o := q.queue[q.first];
q.queue[q.first] := NIL;
q.first := (q.first + 1) MOD LEN(q.queue);
RETURN o;
END Pop;
 
END Queue.
 

Interface extracted from implementation

 
DEFINITION Queue;
 
IMPORT Boxes;
 
TYPE
Queue = POINTER TO RECORD
(q: Queue) IsEmpty (): BOOLEAN, NEW;
(q: Queue) Pop (): Boxes.Object, NEW;
(q: Queue) Push (o: Boxes.Object), NEW
END;
 
PROCEDURE NewQueue (cap: LONGINT): Queue;
 
END Queue.
 

D[edit]

See code here: http://rosettacode.org/wiki/Queue/Usage#D

Déjà Vu[edit]

This uses a dictionary to have a sort of circular buffer of infinite size.

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

E[edit]

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).

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]
}

EchoLisp[edit]

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.

 
;; 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)
 

Elisa[edit]

This is a generic Queue component based on bi-directional lists. See how in Elisa these lists are defined.

 
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;
 

In the following tests we will also show how the internal structure of the queue can be made visible to support debugging.

 
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
 

Elixir[edit]

Translation of: Erlang
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

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[edit]

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.

-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.

Note that there exists a 'queue' module in the standard library handling this for you in the first place

ERRE[edit]

With ERRE 3.0 you can use a class to define the task (in C-64 version you can simply use procedures):

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
Output:
Push 1
Push 2
Push 3
Push 4
Pop 4
Pop 3
Pop 2
Pop 1
Queue is empty!
* End *

Fantom[edit]

 
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
}
}
 

Forth[edit]

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.

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 +! ;

Linked list version[edit]

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:

 
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 ;
 

Fortran[edit]

Works with: Fortran version 90 and later

See FIFO (usage) for an example of fifo_nodes

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

FreeBASIC[edit]

We first use a macro to define a generic Queue type :

' 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

We now use this type to create a Queue of Cat instances :

' 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
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[edit]

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

Go[edit]

Hard coded to be a queue of strings. Implementation is a circular buffer which grows as needed.

 
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
}
 

Groovy[edit]

Solution:

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}" }
}

Test:

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 }
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[edit]

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.

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
 

Icon and Unicon[edit]

Icon[edit]

The following works in both Icon and Unicon:

 
# 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
 
Output:
Popped value: 1
Popped value: 2
Popped value: 3
Popped value: 4
Popped value: 5
empty queue
empty queue

Unicon[edit]

Unicon also provides classes:

 
# 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
 

Produces the same output as above.

J[edit]

Object oriented technique, using mutable state:

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
)

Function-level technique, with no reliance on mutable state:

pop        =: ( {.^:notnull  ;  }. )@: > @: ]  /
push =: ( ''  ; ,~ )& > /
tell_atom =: >& {.
tell_queue =: >& {:
is_empty =: '' -: 1 tell_queue
 
make_empty =: a: , a: [ ]
onto =: [ ; }.@]
 
notnull =: 0 ~: #

See also FIFO (usage)#J

Java[edit]

Works with: Java version 1.5+

This task could be done using a LinkedList from java.util, but here is a user-defined version with generics:

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;
}
}

JavaScript[edit]

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[edit]

var fifo = [];
fifo.push(42); // Enqueue.
fifo.push(43);
var x = fifo.shift(); // Dequeue.
alert(x); // 42

Custom constructor function[edit]

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;
}

jq[edit]

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.

# 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;

Examples:

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]

Julia[edit]

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".

 
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
 
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

Kotlin[edit]

// 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)
}
}
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[edit]

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.
LabVIEW Queue Definition.png

Lasso[edit]

Definition:

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)
}

Usage:

local(q) = myqueue('a')
#q->isEmpty
// => false
 
#q->push('b')
#q->pop
// => a
#q->pop
// => b
 
#q->isEmpty
// => true
#q->pop
// => void


Lua[edit]

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

Mathematica[edit]

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]

MATLAB / Octave[edit]

Here is a simple implementation of a queue, that works in Matlab and Octave.

myfifo = {};
 
% push
myfifo{end+1} = x;
 
% pop
x = myfifo{1}; myfifo{1} = [];
 
% empty
isempty(myfifo)

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.

%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

Sample usage:

>> 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

Maxima[edit]

defstruct(queue(in=[], out=[]))$
 
enqueue(x, q) := ([email protected]: cons(x, [email protected]), done)$
 
dequeue(q) := (if not emptyp([email protected]) then first([first([email protected]), [email protected]: rest([email protected])])
elseif emptyp([email protected]) then 'fail
else ([email protected]: reverse([email protected]), [email protected]: [], first([first([email protected]), [email protected]: rest([email protected])])))$
 
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 */

NetRexx[edit]

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.

/* 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
 
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[edit]

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]
Output:
Fifo size: 3
Popping: 26
Popping: 99
Popping: 2

OCaml[edit]

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.

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

and a session in the top-level:

# 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".

The standard ocaml library also provides a FIFO module, but it is imperative, unlike the implementation above which is functional.

Oforth[edit]

If queue is empty, null is returned.

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 ;

OxygenBasic[edit]

This buffer pushes any primitive data (auto converted to strings), and pops strings. The buffer can expand or contract according to usage.

 
'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
 

Oz[edit]

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.

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}}

There is also a queue datatype in the Mozart standard library.

Pascal[edit]

Works with: Free Pascal version 2.2.0
Works with: GNU Pascal version 20060325, based on gcc-3.4.4

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).

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.


Perl[edit]

Lists are a central part of Perl. To implement a FIFO using OO will to many Perl programmers seem a bit awkward.

use Carp;
sub mypush (\@@) {my($list,@things)=@_; push @$list, @things}
sub mypop (\@) {my($list)=@_; @$list or croak "Empty"; shift @$list }
sub empty (@) {not @_}

Example:

my @fifo=qw(1 2 3 a b c);
 
mypush @fifo, 44, 55, 66;
mypop @fifo for 1 .. 6+3;
mypop @fifo; #empty now

Perl 6[edit]

Works with: rakudo version 2015-11-29

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.

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; # ->

Phix[edit]

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

PHP[edit]

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);
}
}

Example:

$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
 

PicoLisp[edit]

The built-in function 'fifo' maintains a queue in a circular list, with direct access to the first and the last cell

(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
Output:
->((a b c) 1 abc "abc" .)

PL/I[edit]

 
/* 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;
 

PostScript[edit]

Library: initlib
 
% our queue is just [] and empty? is already defined.
/push {exch tadd}.
/pop {uncons exch}.
 

PowerShell[edit]

Works with: PowerShell version 2

PowerShell can natively use the .Net Queue class.

 
$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' } }
Output:
1
2
False
3
True
Caught error

Prolog[edit]

Works with SWI-Prolog. One can push any data in queue.

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).
 

PureBasic[edit]

For FIFO function PureBasic normally uses linked lists. Usage as described above could look like;

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()
Output:
 3
 1
 4
 Pop(), out of range. Error at line 17
 0

Python[edit]

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:

   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,

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:

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()

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)

Works with: Python version 2.4+

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.

from collections import deque
fifo = deque()
fifo. appendleft(value) # push
value = fifo.pop()
not fifo # empty
fifo.pop() # raises IndexError when empty

R[edit]

Simple functional implementation[edit]

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.

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")
l[[1]] <<- NULL
print(l)
invisible()
}
l <- list()
empty()
# [1] TRUE
push(3)
# [[1]]
# [1] 3
push("abc")
# [[1]]
# [1] 3
# [[2]]
# [1] "abc"
push(matrix(1:6, nrow=2))
# [[1]]
# [1] 3
# [[2]]
# [1] "abc"
# [[3]]
# [,1] [,2] [,3]
# [1,] 1 3 5
# [2,] 2 4 6
empty()
# [1] FALSE
pop()
# [[1]]
# [1] 3
# [[2]]
# [1] "abc"
pop()
# [[1]]
# [1] 3
pop()
# list()
pop()
# Error in pop() : can't pop from an empty list

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[edit]

# 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 <- v[[1]]
v[[1]] <<- 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

Object oriented implementation[edit]

Library: proto

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.)

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(.$empty()) stop("can't pop from an empty list")
.$l[[1]] <- 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()

Racket[edit]

Racket comes with a queue implementation in the data/queue library. Here's an explicit implementation:

 
#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)
 

And this is an implementation of a functional queue.

 
#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
 

REBOL[edit]

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]
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[edit]

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.]
/*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

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

Ruby[edit]

The core class Array already implements all queue operations, so this class FIFO delegates everything to methods of Array.

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
"[email protected]}"
end
alias inspect to_s
end
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) # => []


Rust[edit]

Using the standard library[edit]

The standard library has a double-ended queue implementation (VecDeque<T>) which will work here.

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());
}

A simple implementation[edit]

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.

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
})
}
}

Scala[edit]

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(", ", ", ")")
}

Usage:

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)
Output:
isEmpty = true
dequeue(empty) failed.
queue   = Queue(1, 2, 3)
front   = 1
dequeue = 1
dequeue = 2
isEmpty = false

Scheme[edit]

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.

(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)))
 

Message passing[edit]

(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

Sidef[edit]

Implemented as a class:

class FIFO(*array) {
method pop {
array.is_empty && die "underflow";
array.shift;
}
method push(*items) {
array += items;
self;
}
method empty {
array.len == 0;
}
}

Slate[edit]

Toy code based on Slate's Queue standard library (which is optimized for FIFO access):

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].

Smalltalk[edit]

Works with: GNU Smalltalk

An OrderedCollection can be easily used as a FIFO queue.

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"

Standard ML[edit]

Here is the signature for a basic queue:

 
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;
 

A very basic implementation of this signature backed by a list is as follows:

 
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;
 


Tcl[edit]

Here's a simple implementation using a list:

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
Library: Tcllib (Package: struct::queue)
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

UnixPipes[edit]

Uses moreutils

init() {echo > fifo}
push() {echo $1 >> fifo }
pop() {head -1 fifo ; (cat fifo | tail -n +2)|sponge fifo}
empty() {cat fifo | wc -l}

Usage:

push me; push you; push us; push them
|pop;pop;pop;pop
me
you
us
them

UNIX Shell[edit]

Works with: ksh93
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]}"
}

Usage:

# 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
Output:
foo is empty
foo is not empty
peek: bar
peek: baz
peek: element with spaces
peek: 
queue foo is empty

V[edit]

V doesn't have mutable data. Below is an function interface for a fifo.

[fifo_create []].
[fifo_push swap cons].
[fifo_pop [[*rest a] : [*rest] a] view].
[fifo_empty? dup empty?].

Using it

|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

Wart[edit]

Here's how queues are defined in Wart:

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

empty? relies on len by default, so there's no need to separately override it.


XLISP[edit]

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".

(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))

A sample REPL session:

[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)
 
()

XPL0[edit]

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
]

Output:

false
0
1
2
3
true
0
1
2
3

zkl[edit]

class Queue{
var [const] q=List();
fcn push { q.append(vm.pasteArgs()) }
fcn pop { q.pop(0) }
fcn empty { q.len()==0 }
}
q:=Queue();
q.push(1,2,3);
q.pop(); //-->1
q.empty(); //-->False
q.pop();q.pop();q.pop() //-->IndexError thrown