# Queue/Usage

(Redirected from FIFO (usage))
Queue/Usage
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.

Create a queue data structure and demonstrate its operations.

(For implementations of queues, see the FIFO task.)

Operations:

•   push       (aka enqueue) - add element
•   pop         (aka dequeue) - pop first element
•   empty     - return truth value when empty

## 11l

```Deque[String] my_queue

my_queue.append(‘foo’)
my_queue.append(‘bar’)
my_queue.append(‘baz’)

print(my_queue.pop_left())
print(my_queue.pop_left())
print(my_queue.pop_left())```
Output:
```foo
bar
baz
```

## 6502 Assembly

Implementing a queue is very similar to a software stack, except the `POP` command is a litte more involved. The basic principles are the same: Before the queue can be used, a "queue pointer" must first be loaded into X, which points to the first empty slot in the queue. The queue grows down in memory as new elements join the queue. This software queue uses the zero page as the storage area.

```queuePointerStart equ #\$FD
queuePointerMinus1 equ #\$FC     ;make this equal whatever "queuePointerStart" is, minus 1.
pushQueue:
STA 0,x
DEX
RTS

popQueue:
STX temp
LDX #queuePointerStart
LDA 0,x ;get the item that's first in line
PHA
LDX #queuePointerMinus1
loop_popQueue:
LDA 0,X
STA 1,X
DEX
CPX temp
BNE loop_popQueue
LDX temp
INX
PLA ;return the item that just left the queue
RTS

isQueueEmpty:
LDA #1
CPX #queuePointerStart
BEQ yes  ;return 1

SEC
SBC #1   ;return 0

yes:
RTS```

### PUSH

This example uses Easy6502 to test the various modes. The first test pushes three values into the queue. For all examples, the subroutines above are defined below the `BRK`.

```define temp \$00
define queueEmpty \$FD
define queueAlmostEmpty \$FC

LDX #queueEmpty  ;set up software queue

LDA #\$40
jsr pushQueue

LDA #\$80
jsr pushQueue

LDA #\$C0
jsr pushQueue

brk```

Output of Example 1:

```Queue Pointer = \$FA

Hexdump of \$00fa: 00 c0 80 40
Address of each: (FA FB FC FD)
```

### POP

```define temp \$00
define queueEmpty \$FD
define queueAlmostEmpty \$FC

LDX #queueEmpty  ;set up software queue

LDA #\$40
jsr pushQueue

LDA #\$80
jsr pushQueue

LDA #\$C0
jsr pushQueue

jsr popQueue

brk```

Output of Example 2:

```Queue Pointer = \$FB
Hexdump of \$00FB: c0 c0 80
Address of each: (FB FC FD)

Note that c0 still exists in FB, but its slot is "empty" so it will get overwritten in the 3rd example.
```

### PUSH,POP,PUSH

This example shows that once an item leaves the queue, the "ghost" of the last item in line gets overwritten with the next item to join.

```define temp \$00
define queueEmpty \$FD
define queueAlmostEmpty \$FC

LDX #queueEmpty  ;set up software queue

LDA #\$40
jsr pushQueue

LDA #\$80
jsr pushQueue

LDA #\$C0
jsr pushQueue

jsr popQueue

lda #\$ff
jsr pushQueue

brk```

Output of Example 3:

```Queue Pointer = \$FA
Hexdump of \$00FA: 00 ff c0 80
Address of each: (FA FB FC FD)
```

## 8th

```10 q:new  \ create a new queue 10 deep
123 q:push
341 q:push  \ push 123, 341 onto the queue
q:pop . cr  \ displays 123
q:len . cr  \ displays 1
q:pop . cr  \ displays 341
q:len . cr  \ displays 0
```

## Action!

The user must type in the monitor the following command after compilation and before running the program!

`SET EndProg=*`
```CARD EndProg ;required for ALLOCATE.ACT

INCLUDE "D2:ALLOCATE.ACT" ;from the Action! Tool Kit. You must type 'SET EndProg=*' from the monitor after compiling, but before running this program!

DEFINE PTR="CARD"
DEFINE NODE_SIZE="4"
TYPE QueueNode=[PTR data,nxt]

QueueNode POINTER queueFront,queueRear

BYTE FUNC IsEmpty()
IF queueFront=0 THEN
RETURN (1)
FI
RETURN (0)

PROC Push(CHAR ARRAY v)
QueueNode POINTER node

node=Alloc(NODE_SIZE)
node.data=v
node.nxt=0
IF IsEmpty() THEN
queueFront=node
ELSE
queueRear.nxt=node
FI
queueRear=node
RETURN

PTR FUNC Pop()
QueueNode POINTER node
CHAR ARRAY v

IF IsEmpty() THEN
PrintE("Error: queue is empty!")
Break()
FI

node=queueFront
v=node.data
queueFront=node.nxt
Free(node,NODE_SIZE)
RETURN (v)

PROC TestIsEmpty()
IF IsEmpty() THEN
PrintE("Queue is empty")
ELSE
PrintE("Queue is not empty")
FI
RETURN

PROC TestPush(CHAR ARRAY v)
PrintF("Push: %S%E",v)
Push(v)
RETURN

PROC TestPop()
CHAR ARRAY v

Print("Pop: ")
v=Pop()
PrintE(v)
RETURN

PROC Main()
AllocInit(0)
queueFront=0
queueRear=0

Put(125) PutE() ;clear screen

TestIsEmpty()
TestPush("foo")
TestIsEmpty()
TestPush("bar")
TestPop()
TestIsEmpty()
TestPush("baz")
TestPop()
TestIsEmpty()
TestPop()
TestIsEmpty()
TestPop()
RETURN```
Output:

Error at the end of the program is intentional. Screenshot from Atari 8-bit computer

```Queue is empty
Push: foo
Queue is not empty
Push: bar
Pop: foo
Queue is not empty
Push: baz
Pop: bar
Queue is not empty
Pop: baz
Queue is empty
Pop: Error: queue is empty!

RETURN
Error: 128
```

```with FIFO;

procedure Queue_Test is
package Int_FIFO is new FIFO (Integer);
use Int_FIFO;
Queue : FIFO_Type;
Value : Integer;
begin
Push (Queue, 1);
Push (Queue, 2);
Push (Queue, 3);
Pop (Queue, Value);
Pop (Queue, Value);
Push (Queue, 4);
Pop (Queue, Value);
Pop (Queue, Value);
Push (Queue, 5);
Pop (Queue, Value);
Put_Line ("Is_Empty " & Boolean'Image (Is_Empty (Queue)));
end Queue_Test;
```

Sample output:

`Is_Empty TRUE`

## ALGOL 68

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 c.f. Queue/Definition

File: test/data_stigler_diet.a68

```# -*- coding: utf-8 -*- #
MODE DIETITEM = STRUCT(
STRING food, annual quantity, units, REAL cost
);

# Stigler's 1939 Diet ... #
FORMAT diet item fmt = \$g": "g" "g" = \$"zd.dd\$;
[]DIETITEM stigler diet = (
("Cabbage",           "111","lb.",  4.11),
("Dried Navy Beans",  "285","lb.", 16.80),
("Evaporated Milk",    "57","cans", 3.84),
("Spinach",            "23","lb.",  1.85),
("Wheat Flour",       "370","lb.", 13.33),
("Total Annual Cost",    "","",    39.93)
)```

File: test/queue.a68

```#!/usr/bin/a68g --script #
# -*- coding: utf-8 -*- #

MODE OBJVALUE = DIETITEM;
PR read "prelude/queue_base.a68" PR; # c.f. [[rc:Queue/Definition]] #

OBJQUEUE example queue; obj queue init(example queue);

FOR i TO UPB stigler diet DO
# obj queue put(example queue, stigler diet[i]) or ... #
stigler diet[i] +=: example queue
OD;

printf(\$"Get remaining values from queue:"l\$);
WHILE NOT obj queue is empty(example queue) DO
# OR example queue ISNT obj queue empty #
printf((diet item fmt, obj queue get(example queue), \$l\$))
OD```

Output:

```Get remaining values from queue:
Cabbage: 111 lb. = \$ 4.11
Dried Navy Beans: 285 lb. = \$16.80
Evaporated Milk: 57 cans = \$ 3.84
Spinach: 23 lb. = \$ 1.85
Wheat Flour: 370 lb. = \$13.33
Total Annual Cost:   = \$39.93
```

## App Inventor

This Rosetta Code Task requires that the queue operations of push (enqueue), pop (dequeue) and empty be demonstrated with App Inventor.
This is easy to do as those operations are basically available in a slightly different form as list operations.
In addition for this example, I added a top function to view the first item in the queue.

The solution is a complete (although greatly simplified) hamburger restaurant where the customers and orders are the queues.

Customers enter the restaurant at random intervals between 2 and 10 seconds (Customers Clock Timer)
Each customer will request a random item from the menu.
If the item is not available, the customer leaves.
If that item is available (there are only 30 of each item) then the order is placed and payment is accepted (push|enqueue Customer, push|enqueue Order).
Once an order is placed, the customer must wait for the meal to be prepared -- each menu item takes a different number of seconds to prepare (Orders Clock Timer.)
Once the item is prepared, their customer name and the ordered item are removed from the queues (pop|dequeue Customer, pop|dequeue Order).
If there are no pending orders, (empty Orders queue) the cook just waits for one to be placed (the orders clock continues to run to poll for new orders by testing if the Orders queue is not empty.)
Eventually, all items will have been sold, and the store manager will empty the cash register and fly to Tahiti with the waitress.
The eager -- but destined to be frustrated customers -- will continue to request their random items, forever. :)
CLICK HERE TO VIEW THE CODE BLOCKS AND ANDROID APP SCREEN --- END

## AppleScript

```on push(StackRef, value)
set StackRef's contents to {value} & StackRef's contents
return StackRef
end push

on pop(StackRef)
set R to missing value
if StackRef's contents ≠ {} then
set R to StackRef's contents's item 1
set StackRef's contents to {} & rest of StackRef's contents
end if
return R
end pop

on isStackEmpty(StackRef)
if StackRef's contents = {} then return true
return false
end isStackEmpty

set theStack to {}
repeat with i from 1 to 5
push(a reference to theStack, i)
log result
end repeat
repeat until isStackEmpty(theStack) = true
pop(a reference to theStack)
log result
end repeat
```

Output (in Script Editor Event Log):

```  (*1*)
(*2, 1*)
(*3, 2, 1*)
(*4, 3, 2, 1*)
(*5, 4, 3, 2, 1*)
(*5*)
(*4*)
(*3*)
(*2*)
(*1*)
```

## Arturo

```define :queue [][
init: [
this\items: []
]
]

empty?: function [this :queue][
zero? this\items
]

push: function [this :queue, item][
this\items: this\items ++ item
]

pop: function [this :queue][
ensure -> not? empty? this

result: this\items\0
this\items: remove.index this\items 0
return result
]

Q: to :queue []

push Q 1
push Q 2
push Q 3

print ["queue is empty?" empty? Q]

print ["popping:" pop Q]
print ["popping:" pop Q]
print ["popping:" pop Q]

print ["queue is empty?" empty? Q]
```
Output:
```queue is empty? false
popping: 1
popping: 2
popping: 3
queue is empty? true```

## Astro

```let my_queue = Queue()

my_queue.push!('foo')
my_queue.push!('bar')
my_queue.push!('baz')

print my_queue.pop!() # 'foo'
print my_queue.pop!() # 'bar'
print my_queue.pop!() # 'baz'
```

## AutoHotkey

```push("qu", 2), push("qu", 44), push("qu", "xyz") ; TEST

MsgBox % "Len = " len("qu") ; Number of entries
While !empty("qu")          ; Repeat until queue is not empty
MsgBox % pop("qu")      ; Print popped values (2, 44, xyz)
MsgBox Error = %ErrorLevel% ; ErrorLevel =  0: OK
MsgBox % pop("qu")          ; Empty
MsgBox Error = %ErrorLevel% ; ErrorLevel = -1: popped too much
MsgBox % "Len = " len("qu") ; Number of entries

push(queue,_) {             ; push _ onto queue named "queue" (!=_), _ string not containing |
Global
%queue% .= %queue% = "" ? _ : "|" _
}

pop(queue) {                ; pop value from queue named "queue" (!=_,_1,_2)
Global
RegExMatch(%queue%, "([^\|]*)\|?(.*)", _)
Return _1, ErrorLevel := -(%queue%=""), %queue% := _2
}

empty(queue) {              ; check if queue named "queue" is empty
Global
Return %queue% = ""
}

len(queue) {                ; number of entries in "queue"
Global
StringReplace %queue%, %queue%, |, |, UseErrorLevel
Return %queue% = "" ? 0 : ErrorLevel+1
}
```

## AWK

```function deque(arr) {
arr["start"] = 0
arr["end"] = 0
}

function dequelen(arr) {
return arr["end"] - arr["start"]
}

function empty(arr) {
return dequelen(arr) == 0
}

function push(arr, elem) {
arr[++arr["end"]] = elem
}

function pop(arr) {
if (empty(arr)) {
return
}
return arr[arr["end"]--]
}

function unshift(arr, elem) {
arr[arr["start"]--] = elem
}

function shift(arr) {
if (empty(arr)) {
return
}
return arr[++arr["start"]]
}

function printdeque(arr,    i, sep) {
printf("[")
for (i = arr["start"] + 1; i <= arr["end"]; i++) {
printf("%s%s", sep, arr[i])
sep = ", "
}
printf("]\n")
}

BEGIN {
deque(q)
for (i = 1; i <= 10; i++) {
push(q, i)
}
printdeque(q)
for (i = 1; i <= 10; i++) {
print shift(q)
}
printdeque(q)
}
```

## BASIC

### BBC BASIC

```      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

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):)
)
& new\$queue:?Q
& (   (Q..enqueue)\$1
& (Q..enqueue)\$2
& (Q..enqueue)\$3
& out\$((Q..dequeue)\$)
& (Q..enqueue)\$4
& out\$((Q..dequeue)\$)
& out\$((Q..dequeue)\$)
&   out
\$ ( The
queue
is
((Q..empty)\$&|not)
empty
)
& out\$((Q..dequeue)\$)
&   out
\$ ( The
queue
is
((Q..empty)\$&|not)
empty
)
& out\$((Q..dequeue)\$)
& out\$Success!
| out\$"Attempt to dequeue failed"
)
;```

Output:

```1
2
3
The queue is not empty
4
The queue is empty
Attempt to dequeue failed```

## C

See FIFO for the needed code.

```#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>

#include <sys/queue.h>

/* #include "fifolist.h" */

int main()
{
int i;

/* insert 20 integer values */
for(i=0; i < 20; i++) {
}

/* dequeue and print */
printf("%d\n", i);

fprintf(stderr, "FIFO list %s\n",
"is void!");

exit(0);
}
```

## C#

In C# we can use the Queue<T> class in the .NET 2.0 framework.

```using System;
using System.Collections.Generic;

namespace RosettaCode
{
class Program
{
static void Main()
{
// Create a queue and "push" items into it
Queue<int> queue  = new Queue<int>();
queue.Enqueue(1);
queue.Enqueue(3);
queue.Enqueue(5);

// "Pop" items from the queue in FIFO order
Console.WriteLine(queue.Dequeue()); // 1
Console.WriteLine(queue.Dequeue()); // 3
Console.WriteLine(queue.Dequeue()); // 5

// To tell if the queue is empty, we check the count
bool empty = queue.Count == 0;
Console.WriteLine(empty); // "True"

// If we try to pop from an empty queue, an exception
// is thrown.
try
{
queue.Dequeue();
}
catch (InvalidOperationException exception)
{
Console.WriteLine(exception.Message); // "Queue empty."
}
}
}
}
```

## C++

Note that with C++'s standard queue, accessing the first element of the queue and removing it are two separate operations, front() and pop().

```#include <queue>
#include <cassert> // for run time assertions

int main()
{
std::queue<int> q;
assert( q.empty() );        // initially the queue is empty

assert( !q.empty() );       // now the queue isn't empty any more
assert( q.front() == 1 );   // the first element is, of course, 1

assert( !q.empty() );       // it's of course not empty again
assert( q.front() == 1 );   // the first element didn't change

q.push(3);                  // add yet an other element
assert( !q.empty() );       // the queue is still not empty
assert( q.front() == 1 );   // and the first element is still 1

q.pop();                    // remove the first element
assert( !q.empty() );       // the queue is not yet empty
assert( q.front() == 2);    // the first element is now 2 (the 1 is gone)

q.pop();
assert( !q.empty() );
assert( q.front() == 3);

q.push(4);
assert( !q.empty() );
assert( q.front() == 3);

q.pop();
assert( !q.empty() );
assert( q.front() == 4);

q.pop();
assert( q.empty() );

q.push(5);
assert( !q.empty() );
assert( q.front() == 5);

q.pop();
assert( q.empty() );
}
```

Note that the container used to store the queue elements can be specified explicitly; to use a linked linst instead of a deque (the latter is the default), just replace the definition of q to

```  std::queue<int, std::list<int> >
```

(and add #include <list>, of course). Also note that the containers can be used directly; in that case push and pop have to be replaced by push_back and pop_front.

## Clojure

Using the implementation from FIFO:

```(def q (make-queue))

(enqueue q 1)
(enqueue q 2)
(enqueue q 3)

(dequeue q) ; 1
(dequeue q) ; 2
(dequeue q) ; 3

(queue-empty? q) ; true
```

Or use a java implementation:

```(def q (java.util.LinkedList.))

(.remove q) ; 1
(.remove q) ; 2
(.remove q) ; 3

(.isEmpty q) ; true
```

## CoffeeScript

```# We build a Queue on top of an ordinary JS array, which supports push
# and shift.  For simple queues, it might make sense to just use arrays
# directly, but this code shows how to encapsulate the array behind a restricted
# API.  For very large queues, you might want a more specialized data
# structure to implement the queue, in case arr.shift works in O(N) time, which
# is common for array implementations.  On my laptop I start noticing delay
# after about 100,000 elements, using node.js.
Queue = ->
arr = []
enqueue: (elem) ->
arr.push elem
dequeue: (elem) ->
throw Error("queue is empty") if arr.length == 0
arr.shift elem
is_empty: (elem) ->
arr.length == 0

# test
do ->
q = Queue()
for i in [1..100000]
q.enqueue i

console.log q.dequeue() # 1
while !q.is_empty()
v = q.dequeue()
console.log v # 1000

try
q.dequeue() # throws Error
catch e
console.log "#{e}"
```

output

```> coffee queue.coffee
1
100000
Error: queue is empty
```

## Common Lisp

Using the implementation from FIFO.

```(let ((queue (make-queue)))
(enqueue 38 queue)
(assert (not (queue-empty-p queue)))
(enqueue 23 queue)
(assert (eql 38 (dequeue queue)))
(assert (eql 23 (dequeue queue)))
(assert (queue-empty-p queue)))
```

## Component Pascal

BlackBox Component Builder

```MODULE UseQueue;
IMPORT
Queue,
Boxes,
StdLog;

PROCEDURE Do*;
VAR
q: Queue.Instance;
b: Boxes.Box;
BEGIN
q := Queue.New(10);
q.Push(Boxes.NewInteger(1));
q.Push(Boxes.NewInteger(2));
q.Push(Boxes.NewInteger(3));
b := q.Pop();
b := q.Pop();
q.Push(Boxes.NewInteger(4));
b := q.Pop();
b := q.Pop();
StdLog.String("Is empty:> ");StdLog.Bool(q.IsEmpty());StdLog.Ln
END Do;
END UseQueue.```

Execute: ^Q UseQueue.Do
Output:

```Is empty:  \$TRUE
```

## Cowgol

This code uses the queue code at Queue/Definition, which should be put in a file named `queue.coh`.

```include "cowgol.coh";

typedef QueueData is uint8; # the queue will contain bytes
include "queue.coh"; # from the Queue/Definition task

var queue := MakeQueue();

# enqueue bytes 0 to 20
print("Enqueueing: ");
var n: uint8 := 0;
while n < 20 loop
print_i8(n);
print_char(' ');
Enqueue(queue, n);
n := n + 1;
end loop;
print_nl();

# dequeue and print everything in the queue
print("Dequeueing: ");
while QueueEmpty(queue) == 0 loop
print_i8(Dequeue(queue));
print_char(' ');
end loop;
print_nl();

# free the queue
FreeQueue(queue);```
Output:
```Enqueueing: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
Dequeueing: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19```

## D

```class LinkedQueue(T) {
private static struct Node {
T data;
Node* next;
}

bool empty() { return head is null; }

void push(T item) {
if (empty())
head = tail = new Node(item);
else {
tail.next = new Node(item);
tail = tail.next;
}
}

T pop() {
if (empty())
if (head is tail) // Is last one?
// Release tail reference so that GC can collect.
tail = null;
return item;
}

alias push enqueue;
alias pop dequeue;
}

void main() {
q.push(10);
q.push(20);
q.push(30);
assert(q.pop() == 10);
assert(q.pop() == 20);
assert(q.pop() == 30);
assert(q.empty());
}
```

### Faster Version

This versions creates a circular queue able to grow. Define "queue_usage2_main" to run the main and its tests.

```module queue_usage2;

import std.traits: hasIndirections;

struct GrowableCircularQueue(T) {
public size_t length;
private size_t first, last;
private T[] A = [T.init];

this(T[] items...) pure nothrow @safe {
foreach (x; items)
push(x);
}

@property bool empty() const pure nothrow @safe @nogc {
return length == 0;
}

@property T front() pure nothrow @safe @nogc {
assert(length != 0);
return A[first];
}

T opIndex(in size_t i) pure nothrow @safe @nogc {
assert(i < length);
return A[(first + i) & (A.length - 1)];
}

void push(T item) pure nothrow @safe {
if (length >= A.length) { // Double the queue.
immutable oldALen = A.length;
A.length *= 2;
if (last < first) {
A[oldALen .. oldALen + last + 1] = A[0 .. last + 1];
static if (hasIndirections!T)
A[0 .. last + 1] = T.init; // Help for the GC.
last += oldALen;
}
}
last = (last + 1) & (A.length - 1);
A[last] = item;
length++;
}

@property T pop() pure nothrow @safe @nogc {
assert(length != 0);
auto saved = A[first];
static if (hasIndirections!T)
A[first] = T.init; // Help for the GC.
first = (first + 1) & (A.length - 1);
length--;
return saved;
}
}

version (queue_usage2_main) {
void main() {
GrowableCircularQueue!int q;
q.push(10);
q.push(20);
q.push(30);
assert(q.pop == 10);
assert(q.pop == 20);
assert(q.pop == 30);
assert(q.empty);

uint count = 0;
foreach (immutable i; 1 .. 1_000) {
foreach (immutable j; 0 .. i)
q.push(count++);
foreach (immutable j; 0 .. i)
q.pop;
}
}
}
```

## Delphi

```program QueueUsage;

{\$APPTYPE CONSOLE}

uses Generics.Collections;

var
lStringQueue: TQueue<string>;
begin
lStringQueue := TQueue<string>.Create;
try
lStringQueue.Enqueue('First');
lStringQueue.Enqueue('Second');
lStringQueue.Enqueue('Third');

Writeln(lStringQueue.Dequeue);
Writeln(lStringQueue.Dequeue);
Writeln(lStringQueue.Dequeue);

if lStringQueue.Count = 0 then
Writeln('Queue is empty.');
finally
lStringQueue.Free;
end
end.
```

Output:

```First
Second
Third
Queue is empty.```

## Déjà Vu

This uses the definition from Queue/Definition#Déjà Vu

```local :Q queue
!. empty Q
enqueue Q "HELLO"
enqueue Q 123
enqueue Q "It's a magical place"
!. empty Q
!. dequeue Q
!. dequeue Q
!. dequeue Q
!. empty Q
!. dequeue Q```
Output:
```true
false
"HELLO"
123
"It's a magical place"
true
Wrong value: popping from empty queue in Raise:
compiler.deja:857
queue.deja:28
queue.deja:10 in dequeue```

## Diego

Diego has a `queue` object and posit:

```set_ns(rosettacode)_me();

add_queue({int},q)_values(1..4);    // 1,2,3,4 (1 is first/bottom, 4 is last/top)
with_queue(q)_pop();                // 2,3,4
with_queue(q)_dequeue();            // 3,4
with_queue(q)_enqueue(5);           // 3,4,5

with_queue(q)_push()_v(6,7);        // 3,4,5,6,7

with_queue(q)_push[b];              // 3,4,5,6,7,8

with_queue(q)_pluck()_at(2);        // callee will return `with_queue(q)_err(pluck invalid with queue);`

me_msg()_queue(q)_top();            // "8"
me_msg()_queue(q)_last();           // "8"
me_msg()_queue(q)_peek();           // "8"

me_msg()_queue(q)_bottom();         // "3"
me_msg()_queue(q)_first();          // "3"
me_msg()_queue(q)_peer();           // "3"

me_msg()_queue(q)_isempty();            // "false"
with_queue(q)_empty();
with_queue(q)_msg()_isempty()_me();     // "true" (alternative syntax)

with_queue()_pop();                 // callee will return `with_queue(q)_err(pop invalid with empty queue);`

me_msg()_queue(q)_history()_all();      // returns the entire history of queue 'q' since its creation

reset_namespace[];```

`queue` is a derivative of `array`, so arrays can also be used as queues.

## E

Using the implementation from FIFO.

```def [reader, writer] := makeQueue()
require(escape empty { reader.dequeue(empty); false } catch _ { true })
writer.enqueue(1)
writer.enqueue(2)
writer.enqueue(3)
require(escape empty { reader.dequeue(empty); false } catch _ { true })```

E also has queues in the standard library such as `<import:org.erights.e.examples.concurrency.makeQueue>`, but they are designed for concurrency purposes and do not report emptiness but rather return a promise for the next element.

## EasyLang

Uses the queue-definition given at Queue/Definition#EasyLang

```#
# queue definition
#
##
qu_enq 2
qu_enq 5
qu_enq 7
while qu_empty = 0
print qu_deq
.```

## Elena

ELENA 6.x :

```import system'collections;
import extensions;

public program()
{
// Create a queue and "push" items into it
var queue := new Queue();

queue.push(1);
queue.push(3);
queue.push(5);

// "Pop" items from the queue in FIFO order
console.printLine(queue.pop()); // 1
console.printLine(queue.pop()); // 3
console.printLine(queue.pop()); // 5

// To tell if the queue is empty, we check the count
console.printLine("queue is ",(queue.Length == 0).iif("empty","nonempty"));

// If we try to pop from an empty queue, an exception
// is thrown.
queue.pop() \\ on::(e){ console.writeLine("Queue empty.") }
}```

## Elisa

A generic component for Queues and its usage are described in Queue/Definition

## Elixir

Here a list is used as Queue.

```defmodule Queue do
def empty?([]), do: true
def empty?(_), do: false

def pop([h|t]), do: {h,t}

def push(q,t), do: q ++ [t]

def front([h|_]), do: h
end
```

Example:

```iex(2)> q = [1,2,3,4,5]
[1, 2, 3, 4, 5]
iex(3)> Queue.push(q,10)
[1, 2, 3, 4, 5, 10]
iex(4)> front=Queue.front(q)
1
iex(5)> Queue.empty?(q)
false
iex(6)> Queue.pop(q)
{1, [2, 3, 4, 5]}
iex(7)> l=[]
[]
iex(8)> Queue.empty?(l)
true
```

## Erlang

All functions, from the shell:

```1> Q = fifo:new().
{fifo,[],[]}
2> fifo:empty(Q).
true
3> Q2 = fifo:push(Q,1).
{fifo,[1],[]}
4> Q3 = fifo:push(Q2,2).
{fifo,[2,1],[]}
5> fifo:empty(Q3).
false
6> fifo:pop(Q3).
{1,{fifo,[],[2]}}
7> {Popped, Q} = fifo:pop(Q2).
{1,{fifo,[],[]}}
8> fifo:pop(fifo:new()).
** exception error: 'empty fifo'
in function  fifo:pop/1
```

Crashing is the normal expected behavior in Erlang: let it crash, a supervisor will take responsibility of restarting processes, or the caller will take care of it. Only program for the successful cases.

## Factor

For this task, we'll use Factor's `deque` vocabulary (short for double-ended queue). The `deque` class is a mixin, one of whose instances is `dlist` (double-linked list). Hence, the deque protocol works with double-linked lists. When using a deque as a queue, the convention is to queue elements with `push-front` and deque them with `pop-back`.

```USING: combinators deques dlists kernel prettyprint ;
IN: rosetta-code.queue-usage

DL{ } clone {                ! make new queue
[ [ 1 ] dip push-front ] ! push 1
[ [ 2 ] dip push-front ] ! push 2
[ [ 3 ] dip push-front ] ! push 3
[ .                    ] ! DL{ 3 2 1 }
[ pop-back drop        ] ! pop 1 (and discard)
[ pop-back drop        ] ! pop 2 (and discard)
[ pop-back drop        ] ! pop 3 (and discard)
[ deque-empty? .       ] ! t
} cleave
```

Alternatively, batch operations can be used.

```DL{ } clone {
[ [ { 1 2 3 } ] dip push-all-front ] ! push all from sequence
[ .                                ] ! DL{ 3 2 1 }
[ [ drop ] slurp-deque             ] ! pop and discard all
[ deque-empty? .                   ] ! t
} cleave
```

## Fantom

Using definition of Queue in: Queue/Definition task.

```class Main
{
public static Void main ()
{
q := Queue()
q.push (1)
q.push ("a")
echo ("Is empty? " + q.isEmpty)
echo ("Element: " + q.pop)
echo ("Element: " + q.pop)
echo ("Is empty? " + q.isEmpty)
try { q.pop } catch (Err e) { echo (e.msg) }
}
}```

Output:

```Is empty? false
Element: 1
Element: a
Is empty? true
queue is empty
```

## Forth

Forth is a low level language the runs on a virtual machine with 2 stacks. One stack for Parameters and the second is the call/return stack. Coding begins at an almost assembler like level but the work results in a higher level language.
In this demonstration code we show a feature of Forth that is one of the earliest examples of simple object creation using the word CREATE. With this mechanism we create a queue constructor that can build queue data structures of different sizes. Then we create two operators that enqueue a byte and dequeue a byte. The queue's address is passed to these operators on the data stack.
Implementations in other languages or libraries might use a linked list that could potentially consume all memory. Creating a static circular queue is more typical for Forth where it is commonly used in embedded high reliability systems. The code here makes use of the fact that if the queue size is a power of 2, the circular wrap around can be implemented without an IF statement, and uses logical AND with binary mask to wrap around.
NOTE: We also used a more Forth like naming convention QC@ (queue char fetch) and QC! (queue char store) rather than PUSH and POP which as stack users we felt were more appropriate for a Stack than a Queue.

A simpler implementation, where you only need 1 queue can be seen here: http://rosettacode.org/wiki/Queue/Definition#Forth
And a Forth version using some new features of Forth 2012, dynamic memory allocation and a linked list can be seen here:

```: cqueue: ( n -- <text>)
create                                                 \ compile time: build the data structure in memory
dup
dup 1- and abort" queue size must be power of 2"
0 ,                                                \ write pointer "HEAD"
0 ,                                                \ read  pointer "TAIL"
0 ,                                                \ byte counter
dup 1- ,                                           \ mask value used for wrap around
allot ;                                            \ run time: returns the address of this data structure

\ calculate offsets into the queue data structure
: ->tail ( q -- adr ) cell+   ;
: ->cnt  ( q -- adr ) 2 cells +   ;
: ->msk  ( q -- adr ) 3 cells +   ;
: ->data ( q -- adr ) 4 cells +   ;

: head++ ( q -- )                                         \ circular increment head pointer of a queue
dup >r ->head @ 1+  r@ ->msk @ and r> ->head ! ;

: tail++ ( q -- )                                         \ circular increment tail pointer of a queue
dup >r  ->tail @ 1+  r@ ->msk @ and r> ->tail ! ;

: qempty ( q -- flag)
dup ->head off   dup ->tail off  dup ->cnt  off    \ reset all fields to "off" (zero)
->cnt @ 0=  ;                                      \ per the spec qempty returns a flag

: cnt=msk?   ( q -- flag)  dup >r  ->cnt @ r> ->msk @ = ;
: ?empty     ( q -- )     ->cnt @ 0=  abort" queue is empty" ;
: ?full      ( q -- )     cnt=msk? abort" queue is full" ;
: 1+!   ( adr -- )  1 swap +! ;                            \ increment contents of adr
: 1-!   ( adr -- ) -1 swap +! ;          \ decrement contents of adr

: qc@    ( queue -- char )                                 \ fetch next char in queue
dup >r ?empty                                       \ abort if empty
r@ ->cnt 1-!                                        \ decr. the counter
r@ tail++
r@ ->data  r> ->tail @ + c@ ;                       \ calc. address and fetch the byte

: qc!    ( char queue -- )
dup >r ?full                                        \ abort if q full
r@ ->cnt 1+!                                        \ incr. the counter
```

Create 2 Queues and test the operators at the Forth console interactively

```64 cqueue: XQ ok
32 cqueue: YQ ok

char A XQ qc! ok
char B XQ qc! ok
char C XQ qc! ok

XQ qc@ emit A ok
XQ qc@ emit B ok
XQ qc@ emit C ok
XQ qc@ emit
^^^
Queue is empty

YQ qc@ emit
^^^
Queue is empty
```

### Version for the Linked List implementation

```make-queue constant q1
make-queue constant q2
q1 empty? .
5 q1 enqueue
q1 empty? .
7 q1 enqueue
9 q1 enqueue
q2 empty? .
3 q2 enqueue
q2 empty? .
q1 dequeue .
q1 dequeue .
q1 dequeue .
q1 empty? .
q2 dequeue .
q2 empty? .
```

## Fortran

Works with: Fortran version 90 and later
```module fifo_nodes
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
end module fifo_nodes

program FIFOTest
use fifo
implicit none

type(fifo_node), dimension(5) :: ex, xe
integer :: i

do i = 1, 5
ex(i)%datum = i
end do

i = 1
do
print *, xe(i)%datum
i = i + 1
end do

end program FIFOTest
```

## FreeBASIC

As FreeBASIC does not have a built-in Queue type, I am reusing the type I wrote for the Queue/Definition task:

```' FB 1.05.0 Win64

#Include "queue_rosetta.bi"  '' include macro-based generic Queue type used in earlier task

Declare_Queue(String) '' expand Queue type for Strings

Dim stringQueue As Queue(String)
With stringQueue  '' push some strings into the Queue
.push("first")
.push("second")
.push("third")
.push("fourth")
.push("fifth")
End With
Print "Number of Strings in the Queue :" ; stringQueue.count
Print "Capacity of string Queue       :" ; stringQueue.capacity
Print
' now pop them
While Not stringQueue.empty
Print stringQueue.pop(); " popped"
Wend
Print
Print "Number of Strings in the Queue :" ; stringQueue.count
Print "Capacity of string Queue       :" ; stringQueue.capacity   '' capacity should be unchanged
Print "Is Queue empty now             : "; stringQueue.empty
Print
Print "Press any key to quit"
Sleep```
Output:
```Number of Strings in the Queue : 5
Capacity of string Queue       : 8

first popped
second popped
third popped
fourth popped
fifth popped

Number of Strings in the Queue : 0
Capacity of string Queue       : 8
Is Queue empty now             : true
```

## Go

### With Queue/Definition code

Solution using package from the Queue/Definition task:

```package main

import (
"fmt"
"queue"
)

func main() {
q := new(queue.Queue)
fmt.Println("empty?", q.Empty())

x := "black"
fmt.Println("push", x)
q.Push(x)

fmt.Println("empty?", q.Empty())
r, ok := q.Pop()
if ok {
fmt.Println(r, "popped")
} else {
fmt.Println("pop failed")
}

var n int
for _, x := range []string{"blue", "red", "green"} {
fmt.Println("pushing", x)
q.Push(x)
n++
}

for i := 0; i < n; i++ {
r, ok := q.Pop()
if ok {
fmt.Println(r, "popped")
} else {
fmt.Println("pop failed")
}
}
}
```

Output:

```empty? true
push black
empty? false
black popped
pushing blue
pushing red
pushing green
blue popped
red popped
green popped
```

### With channels

Go buffered channels are FIFO, and better, are concurrency-safe (if you have an application for that.) Code below is same as code above only with Go channels rather than the home made queue implementation. Note that you don't have to start concurrent goroutines to use channels, they are useful all on their own. Other differences worth noting: Buffered channels are not dynamically resizable. This is a good thing, as queues that can grow without limit allow ugly bugs that consume memory and grind to a halt. Also blocking operations (as seen here with push) are probably a bad idea with a single goroutine. Much safer to use non-blocking operations that handle success and failure (the way pop is done here.)

```package main

import "fmt"

func main() {
q := make(chan string, 3)
fmt.Println("empty?", len(q) == 0)

x := "black"
fmt.Println("push", x)
q <- x

fmt.Println("empty?", len(q) == 0)
select {
case r := <-q:
fmt.Println(r, "popped")
default:
fmt.Println("pop failed")
}

var n int
for _, x := range []string{"blue", "red", "green"} {
fmt.Println("pushing", x)
q <- x
n++
}

for i := 0; i < n; i++ {
select {
case r := <-q:
fmt.Println(r, "popped")
default:
fmt.Println("pop failed")
}
}
}
```

```package main

import (
"fmt"
"container/list"
)

func main() {
q := list.New()
fmt.Println("empty?", q.Len() == 0)

x := "black"
fmt.Println("push", x)
q.PushBack(x)

fmt.Println("empty?", q.Len() == 0)
if e := q.Front(); e != nil {
r := q.Remove(e)
fmt.Println(r, "popped")
} else {
fmt.Println("pop failed")
}

var n int
for _, x := range []string{"blue", "red", "green"} {
fmt.Println("pushing", x)
q.PushBack(x)
n++
}

for i := 0; i < n; i++ {
if e := q.Front(); e != nil {
r := q.Remove(e)
fmt.Println(r, "popped")
} else {
fmt.Println("pop failed")
}
}
}
```

## Groovy

Solution:

```def q = new LinkedList()
```

Test:

```assert q.empty
println q
// "push" adds to end of "queue" list
q.push('Stuart')
println q
assert !q.empty
println q
assert !q.empty
// left shift operator ("<<") adds to end of "queue" list
q << 'John'
println q
assert !q.empty
// to the end of the "queue" list in list order
q += ['Paul', 'George']
println q
assert !q.empty
// "poll" removes and returns the first element in the
// "queue" list ("pop" exists for Groovy lists, but it
// removes and returns the LAST element for "Stack"
// semantics). "poll" only exists in objects that
assert q.poll() == 'Stuart'
println q
assert !q.empty
assert q.poll() == 'Pete'
println q
assert !q.empty
q << 'Ringo'
println q
assert !q.empty
assert q.poll() == 'John'
println q
assert !q.empty
assert q.poll() == 'Paul'
println q
assert !q.empty
assert q.poll() == 'George'
println q
assert !q.empty
assert q.poll() == 'Ringo'
println q
assert q.empty
assert q.poll() == null
```

Output:

```[]
[Stuart]
[Stuart, Pete]
[Stuart, Pete, John]
[Stuart, Pete, John, Paul, George]
[Pete, John, Paul, George]
[John, Paul, George]
[John, Paul, George, Ringo]
[Paul, George, Ringo]
[George, Ringo]
[Ringo]
[]```

Running the code from Queue/Definition#Haskell through GHC's interpreter.

```Prelude> :l fifo.hs
[1 of 1] Compiling Main             ( fifo.hs, interpreted )
*Main> let q = emptyFifo
*Main> isEmpty q
True
*Main> let q' = push q 1
*Main> isEmpty q'
False
*Main> let q'' = foldl push q' [2..4]
*Main> let (v,q''') = pop q''
*Main> v
Just 1
*Main> let (v',q'''') = pop q'''
*Main> v'
Just 2
*Main> let (v'',q''''') = pop q''''
*Main> v''
Just 3
*Main> let (v''',q'''''') = pop q'''''
*Main> v'''
Just 4
*Main> let (v'''',q''''''') = pop q''''''
*Main> v''''
Nothing
```

## Icon and Unicon

Icon and Unicon provide built-in queue and stack functions.

```procedure main(arglist)
queue := []
write("Usage:\nqueue x x x - x - - - - -\n\t- pops elements\n\teverything else pushes")
write("Queue is:")
every x := !arglist do {
case x of {
"-"     : pop(queue)  | write("pop(empty) failed.")    # pop if the next arglist[i] is a -
default : put(queue,x)                                 # push arglist[i]
}
if empty(queue) then writes("empty")
else every writes(!queue," ")
write()
}
end

procedure empty(X)        #: fail if X is not empty
if *X = 0 then return
end
```

Sample output:

```queue - 1 3 4 5 6 - - - - - - - -
Usage:
queue x x x - x - - - - -
- pops elements
everything else pushes
Queue is:
pop(empty) failed.
empty
1
1 3
1 3 4
1 3 4 5
1 3 4 5 6
3 4 5 6
4 5 6
5 6
6
empty
pop(empty) failed.
empty
pop(empty) failed.
empty
pop(empty) failed.
empty
```

## J

Using object-oriented FIFO queue implementation from FIFO

This is an interactive J session:

```   queue=: conew 'fifo'
isEmpty__queue ''
1
push__queue 9
9
push__queue 8
8
push__queue 7
7
isEmpty__queue ''
0
pop__queue ''
9
pop__queue ''
8
pop__queue ''
7
isEmpty__queue ''
1
```

Using function-level FIFO queue implementation from FIFO

This is an interactive J session:

```   is_empty make_empty _
1
first_named_state =: push 9 onto make_empty _
newer_state =: push 8 onto first_named_state
this_state =: push 7 onto newer_state
is_empty this_state
0
tell_queue this_state
9 8 7
tell_atom pop this_state
9
tell_atom pop pop this_state
8
tell_atom pop pop pop this_state
7
is_empty pop pop pop this_state
1
```

## Java

Works with: Java version 1.5+

LinkedList can always be used as a queue or stack, but not in conjunction with the Stack object provided by Java. To use a LinkedList as a stack, use the push and pop methods. A LinkedList can also be used as a double-ended queue (deque); LinkedList has implemented the Deque interface since Java 1.6+.

```import java.util.LinkedList;
import java.util.Queue;
...
System.out.println(queue.isEmpty());      // empty test - true
// queue.remove();       // would throw NoSuchElementException
System.out.println(queue);                // [1, 2, 3]
System.out.println(queue.remove());       // 1
System.out.println(queue);                // [2, 3]
System.out.println(queue.isEmpty());      // false
```

You can also use "offer" and "poll" methods instead of "add" and "remove", respectively. They indicate errors with the return value instead of throwing an exception.

Works with: Java version 1.4
```import java.util.LinkedList;
...
System.out.println(queue.isEmpty());      // empty test - true
System.out.println(queue);                // [1, 2, 3]
System.out.println(queue.removeFirst());  // 1
System.out.println(queue);                // [2, 3]
System.out.println(queue.isEmpty());      // false
```

## JavaScript

JavaScript arrays can be used as FIFOs.

```var f = new Array();
print(f.length);
f.push(1,2);         // can take multiple arguments
f.push(3);
f.shift();
f.shift();
print(f.length);
print(f.shift())
print(f.length == 0);
print(f.shift());
```

outputs:

```0
1
3
true
undefined```

## Julia

Works with: Julia version 0.6
```using DataStructures

queue = Queue(String)
@show enqueue!(queue, "foo")
@show enqueue!(queue, "bar")
@show dequeue!(queue) # -> foo
@show dequeue!(queue) # -> bar
```

## Kotlin

The related Queue/Definition task, where we wrote our own Queue class, intimated that we should use the language's built-in queue for this task so that's what I'm going to do here, using Java collection types as Kotlin doesn't have a Queue type in its standard library:

```// version 1.1.2

import java.util.*

fun main(args: Array<String>) {
val q: Queue<Int> = ArrayDeque<Int>()
println(q)
println("Size of queue = \${q.size}")
print("Removing: ")
(1..3).forEach { print("\${q.remove()} ") }
println("\nRemaining in queue: \$q")
q.clear()
println("After clearing, queue is \${if(q.isEmpty()) "empty" else "not empty"}")
try {
q.remove()
}
catch (e: NoSuchElementException) {
println("Can't remove elements from an empty queue")
}
}
```
Output:
```[1, 2, 3, 4, 5]
Size of queue = 5
Removing: 1 2 3
Remaining in queue: [4, 5]
After clearing, queue is empty
Can't remove elements from an empty queue
```

## Lambdatalk

The APIs of queues are built on lambdatalk array primitives, [A.new, A.disp, A.join, A.split, A.array?, A.null?, A.empty?, A.in?, A.equal?, A.length, A.get, A.first, A.last, A.rest, A.slice, A.duplicate, A.reverse, A.concat, A.map, A.set!, A.addlast!, A.sublast!, A.addfirst!, A.subfirst!, A.reverse!, A.sort!, A.swap!, A.lib]. Note that the [A.addlast!, A.sublast!, A.addfirst!, A.subfirst!] primitives are the standard [push!, shift!, pop!, unshift!] ones.

```{def queue.add
{lambda {:v :q}
{let { {_ {A.addlast! :v :q}}}
} ok}}

{def queue.get
{lambda {:q}
{let { {:v {A.first :q}}
{_ {A.subfirst! :q}}
} :v}}}
-> queue.get

{def queue.empty?
{lambda {:q}
{A.empty? :q}}}
-> queue.empty?

{def Q {A.new}}    -> Q      []
{queue.add 1 {Q}}  ->  ok    [1]
{queue.add 2 {Q}}  ->  ok    [1,2]
{queue.add 3 {Q}}  ->  ok    [1,2,3]
{queue.get {Q}}    -> 1      [2,3]
{queue.add 4 {Q}}  ->  ok    [2,3,4]
{queue.empty? {Q}} -> false
{queue.get {Q}}    -> 2      [3,4]
{queue.get {Q}}    -> 3      [4]
{queue.get {Q}}    -> 4      []
{queue.get {Q}}    -> undefined
{queue.empty? {Q}} -> true
```

## Lasso

Lasso has a queue type that uses the following for the operators:

``` push: queue->insert
pop: queue->get
empty: queue->size == 0
```

Example:

```local(queue) = queue
#queue->size
// => 0

#queue->insert('a')
#queue->insert('b')
#queue->insert('c')
#queue->size
// => 3

loop(#queue->size) => {
stdoutnl(#queue->get)
}
// =>
// a
// b
// c

#queue->size == 0
// => true
```

## Logo

Works with: UCB Logo

UCB Logo comes with a protocol for treating lists as queues.

```make "fifo []
print empty? :fifo    ; true
queue "fifo 1
queue "fifo 2
queue "fifo 3
show :fifo            ; [1 2 3]
print dequeue "fifo   ; 1
show :fifo            ; [2 3]
print empty? :fifo    ; false```

## Lua

Uses the queue-definition given at Queue/Definition#Lua

```q = Queue.new()
Queue.push( q, 5 )
Queue.push( q, "abc" )

while not Queue.empty( q ) do
print( Queue.pop( q ) )
end
```

One can also just use a regular Lua table (shown here in interactive mode):

```> -- create queue:
> q = {}
> -- push:
> q[#q+1] = "first"
> q[#q+1] = "second"
> q[#q+1] = "third"
> -- pop:
> =table.remove(q, 1)
first
> =table.remove(q, 1)
second
> =table.remove(q, 1)
third
> -- empty?
> =#q == 0
true
```

## M2000 Interpreter

M2000 has always a current stack object. We can define a new one using a pointer to a stack object (here the variable a). We can swap the currernt one with that on a, so Push, number, letter\$ and Empty can be used on that object. Also we can use functions using the stack object as first parameter like stackitem(), stackitem\$() and stacktype\$().

```Module CheckStackAsLIFO {
a=stack
Stack a {
Push 1, 2, 3
Print number=3
Print number=2
Print number=1
Print Empty=True
Push "A", "B", "C"
Print letter\$="C"
Print letter\$="B"
Print letter\$="A"
Print Empty=True
Push 1,"OK"
}
Print Len(a)=2, StackItem(a, 2)=1, StackItem\$(a, 1)="OK"
Print StackType\$(a, 1)="String", StackType\$(a,2)="Number"
}
CheckStackAsLIFO
Module CheckStackAsFIFO {
a=stack
Stack a {
Data 1, 2, 3
Print number=1
Print number=2
Print number=3
Print Empty=True
Data "A", "B", "C"
Print letter\$="A"
Print letter\$="B"
Print letter\$="C"
Print Empty=True
Push 1,"OK"
}
Print Len(a)=2, StackItem(a, 2)=1, StackItem\$(a, 1)="OK"
Print StackType\$(a, 1)="String", StackType\$(a,2)="Number"
}
CheckStackAsFIFO```

## Maple

There are more builtin operations like reverse(), length(),etc.

```q := queue[new]();
queue[enqueue](q,1);
queue[enqueue](q,2);
queue[enqueue](q,3);
queue[empty](q);
>>>false
queue[dequeue](q);
>>>1
queue[dequeue](q);
>>>2
queue[dequeue](q);
>>>3
queue[empty](q);
>>>true```

## Mathematica/Wolfram Language

```Empty[a_] := If[Length[a] == 0, True, False]
SetAttributes[Push, HoldAll]; Push[a_, elem_] := AppendTo[a, elem]
SetAttributes[Pop, HoldAllComplete]; Pop[a_] := If[EmptyQ[a], False, b = First[a]; Set[a, Most[a]]; b]

Queue = {}
-> {}
Empty[Queue]
-> True
Push[Queue, "1"]
-> {"1"}
EmptyQ[Queue]
->False
Pop[Queue]
->1
Pop[Queue]
->False
```

## Nemerle

The Nemerle.Collections namespace contains an implementation of a Queue.

```mutable q = Queue(); // or use immutable version as per Haskell example
def empty = q.IsEmpty(); // true at this point
q.Push(empty); // or Enqueue(), or Add()
def a = q.Pop(); // or Dequeue() or Take()
```

## NetRexx

This example demonstrates the `push`, `pop` and `empty` operations from an implementation of a queue as specified for the task.

The demonstration employs an in-line deployment of a queue object having as it's underlying implementation a `java.util.Deque` interface instanciated as a `java.util.ArrayDeque`. Typically this queue implementation would reside outside of the demonstration program and be imported at run-time rather than within the body of this source.

```/* NetRexx */
options replace format comments java crossref savelog symbols nobinary

-- Queue Usage Demonstration Program -------------------------------------------
method main(args = String[]) public constant
kew = RCQueueImpl()
do
say kew.pop()
catch ex = IndexOutOfBoundsException
say ex.getMessage
say
end

melancholyDane = ''
melancholyDane[0] = 4
melancholyDane[1] = 'To be'
melancholyDane[2] = 'or'
melancholyDane[3] = 'not to be?'
melancholyDane[4] = 'That is the question.'

loop p_ = melancholyDane[0] to 1 by -1
kew.push(melancholyDane[p_])
end p_

loop while \kew.empty
popped = kew.pop
say popped '\-'
end
say; say

-- demonstrate stowing something other than a text string in the queue
kew.push(melancholyDane)
md = kew.pop
loop l_ = 1 to md[0]
say md[l_] '\-'
end l_
say

return

-- Queue implementation --------------------------------------------------------
class RCQueueImpl
properties private
qqq = Deque

method RCQueueImpl() public
qqq = ArrayDeque()
return

method push(stuff) public
qqq.push(stuff)
return

method pop() public returns Rexx signals IndexOutOfBoundsException
if qqq.isEmpty then signal IndexOutOfBoundsException('The queue is empty')
return Rexx qqq.pop()

method empty() public binary returns boolean
return qqq.isEmpty

method isTrue public constant binary returns boolean
return 1 == 1

method isFalse public constant binary returns boolean
return \isTrue```
Output
```The queue is empty

To be or not to be? That is the question.

To be or not to be? That is the question.
```

## Nim

Nim standard library no longer provides a “queues” module, but it provides the more powerful module “deques” which allows to manage FIFO and stacks. Internally, this module uses a sequence and, thus, is more efficient than a linked list implementation.

When popping from an empty list, the module raises an IndexDefect which, as defect, is considered to be non catchable. In fact, by default, with version 1.4 of Nim the defects are still catchable but this may (will) change in some next version. The option `--panics:on|off` allows to control this behavior. Here, we have chosen to not try to catch the exception and the program terminates in error when trying to pop a fourth element from the queue.

```import deques

var queue = initDeque[int]()

echo "Queue size: ", queue.len()
echo "Popping: ", queue.popFirst()
echo "Popping: ", queue.popFirst()
echo "Popping: ", queue.popFirst()
echo "Popping: ", queue.popFirst()
```
Output:
```Queue size: 3
Popping: 26
Popping: 99
Popping: 2
/home/lse/Documents/nim/Rosetta/queue_usage.nim(13) queue_usage
/home/lse/.choosenim/toolchains/nim-1.4.4/lib/pure/collections/deques.nim(113) popFirst
Error: unhandled exception: Empty deque. [IndexDefect]```

## Objeck

```class Test {
function : Main(args : String[]) ~ Nil {
q := Struct.IntQueue->New();

q->Remove()->PrintLine();
q->Remove()->PrintLine();
q->Remove()->PrintLine();

q->IsEmpty()->PrintLine();
}
}```

## OCaml

```# let q = Queue.create ();;
val q : '_a Queue.t = <abstr>
# Queue.is_empty q;;
- : bool = true
- : unit = ()
# Queue.is_empty q;;
- : bool = false
- : unit = ()
- : unit = ()
# Queue.peek q;;
- : int = 1
# Queue.length q;;
- : int = 3
# Queue.iter (Printf.printf "%d, ") q; print_newline ();;
1, 2, 3,
- : unit = ()
# Queue.take q;;
- : int = 1
# Queue.take q;;
- : int = 2
# Queue.peek q;;
- : int = 3
# Queue.length q;;
- : int = 1
- : unit = ()
# Queue.take q;;
- : int = 3
# Queue.peek q;;
- : int = 4
# Queue.take q;;
- : int = 4
# Queue.is_empty q;;
- : bool = true
```

## Oforth

Using FIFO implementation :

```: testQueue
| q i |
Queue new ->q
20 loop: i [ i q push ]
while ( q empty not ) [ q pop . ] ;```

## ooRexx

ooRexx includes a built-in queue class.

```q = .queue~new      -- create an instance
q~queue(3)          -- adds to the end, but this is at the front
q~push(1)           -- push on the front
q~queue(2)          -- add to the end
say q~pull q~pull q~pull q~isempty  -- should display all and be empty```

Output:

```1 3 2 1
```

## Oz

```declare
MyQueue = {Queue.new}
in
{MyQueue.isEmpty} = true
{MyQueue.put foo}
{MyQueue.put bar}
{MyQueue.put baz}
{MyQueue.isEmpty} = false
{Show {MyQueue.get}}  %% foo
{Show {MyQueue.get}}  %% bar
{Show {MyQueue.get}}  %% baz```

## PascalABC.NET

```begin
var q := new Queue<integer>;
for var i:=1 to 5 do
q.Enqueue(i);
while q.Count > 0 do
Print(q.Dequeue);
end.
```
Output:
```1 2 3 4 5
```

## Perl

Perl has built-in support to these operations:

```@queue = (); # we will simulate a queue in a array

push @queue, (1..5); # enqueue numbers from 1 to 5

print shift @queue,"\n"; # dequeue

print "array is empty\n" unless @queue; # is empty ?

print \$n while(\$n = shift @queue); # dequeue all
print "\n";
print "array is empty\n" unless @queue; # is empty ?
```

Output:

```1
2345
array is empty
```

## Phix

Using the implementation from Queue/Definition

```with javascript_semantics
printf(1,"empty:%t\n",empty())          -- true
push_item(5)
printf(1,"empty:%t\n",empty())          -- false
push_item(6)
printf(1,"pop_item:%v\n",pop_item())    -- 5
printf(1,"pop_item:%v\n",pop_item())    -- 6
printf(1,"empty:%t\n",empty())          -- true
```

Using the builtins (same output):

```with javascript_semantics
constant queue = new_queue()
printf(1,"empty:%t\n",queue_empty(queue))
push(queue,5)
printf(1,"empty:%t\n",queue_empty(queue))
push(queue,6)
printf(1,"pop:%v\n",pop(queue))
printf(1,"pop:%v\n",pop(queue))
printf(1,"empty:%t\n",queue_empty(queue))
```

## PHP

Works with: PHP version 5.3+
```<?php
\$queue = new SplQueue;
echo \$queue->isEmpty() ? 'true' : 'false', "\n";  // empty test - returns true
// \$queue->dequeue();                             // would raise RuntimeException
\$queue->enqueue(1);
\$queue->enqueue(2);
\$queue->enqueue(3);
echo \$queue->dequeue(), "\n";                     // returns 1
echo \$queue->isEmpty() ? 'true' : 'false', "\n";  // returns false
?>
```

## PicoLisp

Using the implementation from FIFO:

```(println (fifo 'Queue))    # Retrieve the number '1'
(println (fifo 'Queue))    # Retrieve an internal symbol 'abc'
(println (fifo 'Queue))    # Retrieve a transient symbol "abc"
(println (fifo 'Queue))    # and a list (abc)
(println (fifo 'Queue))    # Queue is empty -> NIL```

Output:

```1
abc
"abc"
(a b c)
NIL```

## PL/I

```test: proc options (main);

/* To implement a queue. */
define structure
1 node,
2 value fixed,
declare (head, tail, t) handle (node);
declare null builtin;
declare i fixed binary;

do i = 1 to 10; /* Add ten items to the tail of the queue. */
if head = bind(:node, null:) then
do;
put skip list (head => value);
end;
else
do;
t = new(:node:);
tail => link = t; /* Point the tail to the new node. */
tail = t;
tail => link = bind(:node, null:); /* Set the tail link to NULL */
get list (tail => value) copy;
put skip list (tail => value);
end;
end;

/* Pop all the items in the queue. */
put skip list ('The queue has:');
do while (head ^= bind(:node, null:));
put skip list (head => value);
end;
end test;```

The output:

```       1
3
5
7
9
11
13
15
17
19
The queue has:
1
3
5
7
9
11
13
15
17
19
```

## PostScript

Library: initlib
``` [1 2 3 4 5] 6 exch tadd
= [1 2 3 4 5 6]
uncons
= 1 [2 3 4 5 6]
[] empty?
=true
```

## PowerShell

Works with: PowerShell version 4.0
```[System.Collections.ArrayList]\$queue = @()
# isEmpty?
if (\$queue.Count -eq 0) {
"isEmpty? result : the queue is empty"
} else {
"isEmpty? result : the queue is not empty"
}
"the queue contains : \$queue"
\$queue += 1                    # push
"push result : \$queue"
\$queue += 2                    # push
\$queue += 3                    # push
"push result : \$queue"

\$queue.RemoveAt(0)             # pop
"pop result : \$queue"

\$queue.RemoveAt(0)             # pop
"pop result : \$queue"

if (\$queue.Count -eq 0) {
"isEmpty? result : the queue is empty"
} else {
"isEmpty? result : the queue is not empty"
}
"the queue contains : \$queue"
```

Output:

```isEmpty? result : the queue is empty
the queue contains :
push result : 1
push result : 1 2 3
pop result : 2 3
pop result : 3
isEmpty? result : the queue is not empty
the queue contains : 3
```

### PowerShell using the .NET Queue Class

Declare a new queue:

```\$queue = New-Object -TypeName System.Collections.Queue
#or
\$queue = [System.Collections.Queue] @()
```

Show the methods and properties of the queue object:

```Get-Member -InputObject \$queue
```
Output:
```   TypeName: System.Collections.Queue

Name           MemberType Definition
----           ---------- ----------
Clear          Method     void Clear()
Clone          Method     System.Object Clone(), System.Object ICloneable.Clone()
Contains       Method     bool Contains(System.Object obj)
CopyTo         Method     void CopyTo(array array, int index), void ICollection.CopyTo(array array, int index)
Dequeue        Method     System.Object Dequeue()
Enqueue        Method     void Enqueue(System.Object obj)
Equals         Method     bool Equals(System.Object obj)
GetEnumerator  Method     System.Collections.IEnumerator GetEnumerator(), System.Collections.IEnumerator IEnumerable.GetEnumerator()
GetHashCode    Method     int GetHashCode()
GetType        Method     type GetType()
Peek           Method     System.Object Peek()
ToArray        Method     System.Object[] ToArray()
ToString       Method     string ToString()
TrimToSize     Method     void TrimToSize()
Count          Property   int Count {get;}
IsSynchronized Property   bool IsSynchronized {get;}
SyncRoot       Property   System.Object SyncRoot {get;}
```

Put some stuff in the queue:

```1,2,3 | ForEach-Object {\$queue.Enqueue(\$_)}
```

Take a peek at the head of the queue:

```\$queue.Peek()
```
Output:
```1
```

Pop the head of the queue:

```\$queue.Dequeue()
```
Output:
```1
```

Clear the queue:

```\$queue.Clear()
```

Test if queue is empty:

```if (-not \$queue.Count) {"Queue is empty"}
```
Output:
```Queue is empty
```

## Prolog

Works with SWI-Prolog.

```%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%% definitions of queue
empty(U-V) :-
unify_with_occurs_check(U, V).

push(Queue, Value, NewQueue) :-
append_dl(Queue, [Value|X]-X, NewQueue).

pop([X|V]-U, X, V-U) :-
\+empty([X|V]-U).

append_dl(X-Y, Y-Z, X-Z).

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%% use of queue
queue :-
% create an empty queue
empty(Q),
format('Create queue ~w~n~n', [Q]),

% add numbers 1 and 2
write('Add numbers 1 and 2 : '),
push(Q, 1, Q1),
push(Q1, 2, Q2),

% display queue
format('~w~n~n', [Q2]),

% pop element
pop(Q2, V, Q3),

% display results
format('Pop : Value ~w Queue : ~w~n~n', [V, Q3]),

% test the queue
write('Test of the queue : '),
(   empty(Q3) -> writeln('Queue empy'); writeln('Queue not empty')), nl,

% pop the elements
write('Pop the queue : '),
pop(Q3, V1, Q4),
format('Value ~w Queue : ~w~n~n', [V1, Q4]),

write('Pop the queue : '),
pop(Q4, _V, _Q5).
```

Output :

```?- queue.
Create queue _G132-_G132

Add numbers 1 and 2 : [1,2|_G148]-_G148

Pop : Value 1 Queue : [2|_G148]-_G148

Test of the queue : Queue not empty

Pop the queue : Value 2 Queue : _G148-_G148

Pop the queue :
false.
```

## PureBasic

```NewList MyStack()

Procedure Push(n)
Shared MyStack()
LastElement(MyStack())
MyStack()=n
EndProcedure

Procedure Pop()
Shared MyStack()
Protected n
If FirstElement(MyStack())  ; e.g. Stack not empty
n=MyStack()
DeleteElement(MyStack(),1)
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)
Push(1)
Push(5)
While Not Empty()
Debug Pop()
Wend```

Outputs

```3
1
4
1
5
```

## Python

```import Queue
my_queue = Queue.Queue()
my_queue.put("foo")
my_queue.put("bar")
my_queue.put("baz")
print my_queue.get()  # foo
print my_queue.get()  # bar
print my_queue.get()  # baz
```

## Quackery

```[ [] ]          is queue  (     --> q   )

[ nested join ] is push   ( q x --> q   )

[ behead ]      is pop    (   q --> q x )

[ [] = ]        is empty? (   q --> b   )```

Demonstrating operations in Quackery shell:

```/O> queue
... 1 push
... \$ "two" push
... ' [ 1 2 + echo say "rd" ] push
... say "The queue is " dup empty? not if [ say "not " ] say "empty." cr
... pop echo cr
... pop echo\$ cr
... pop do cr
... say "The queue is " empty? not if [ say "not " ] say "empty." cr
...
The queue is not empty.
1
two
3rd
The queue is empty.

Stack empty.```

## Racket

```#lang racket

(require data/queue)

(define queue (make-queue))

(enqueue! queue 'black)
(queue-empty? queue) ; #f

(enqueue! queue 'red)
(enqueue! queue 'green)

(dequeue! queue) ; 'black
(dequeue! queue) ; 'red
(dequeue! queue) ; 'green

(queue-empty? queue) ; #t
```

## Raku

(formerly Perl 6)

Raku maintains the same list operators of Perl 5, for this task, the operations are:

```push (aka enqueue) -- @list.push
pop (aka dequeue)  -- @list.shift
empty              -- !@list.elems
```

but there's also @list.pop which removes a item from the end, and @list.unshift which add a item on the start of the list.
Example:

```my @queue = < a >;

@queue.push('b', 'c'); # [ a, b, c ]

say @queue.shift; # a
say @queue.pop; # c

say @queue; # [ b ]
say @queue.elems; # 1

@queue.unshift('A'); # [ A, b ]
@queue.push('C'); # [ A, b, C ]
```

## REBOL

See FIFO#REBOL for implementation. Example repeated here for completeness.

```; 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

The REXX language was developed under IBM VM/CMS operating system, and CMS had a stack mechanism built-into the
operating system, so REXX utilized that resource.

The   queue   instruction adds an entry to the bottom of the stack (FIFO),
the   push   instruction adds an entry to the top of the stack (LIFO).

The   queued   function returns the number of entries in the stack.

The   pull   or   parse pull   removes an entry from the top of the stack.

There are other instructions to manipulate the stack by "creating" multiple (named) stacks.

The entries in the stack may be anything, including "nulls".

```/*REXX program demonstrates four  queueing  operations:   push,  pop,  empty,  query.   */
say '══════════════════════════════════ Pushing five values to the stack.'
do j=1  for 4                            /*a  DO  loop to  PUSH  four values.   */
call push  j * 10                        /*PUSH   (aka:   enqueue to the stack).*/
say 'pushed value:'    j * 10            /*echo the pushed value.               */
if j\==3  then iterate                   /*Not equal 3?   Then use a new number.*/
call push                                /*PUSH   (aka:   enqueue to the stack).*/
say 'pushed a "null" value.'             /*echo what was  pushed  to the stack. */
end   /*j*/
say '══════════════════════════════════ Quering the stack  (number of entries).'
say  queued()  ' entries in the stack.'
say '══════════════════════════════════ Popping all values from the stack.'
do k=1  while  \empty()                  /*EMPTY (returns  TRUE  [1]  if empty).*/
call pop                                 /*POP   (aka:  dequeue from the stack).*/
say k': popped value='  result           /*echo the popped value.               */
end   /*k*/
say '══════════════════════════════════ The stack is now empty.'
exit                                             /*stick a fork in it,  we're all done. */
/*──────────────────────────────────────────────────────────────────────────────────────*/
push:   queue arg(1);              return        /*(The  REXX  QUEUE   is FIFO.)        */
pop:    procedure;  parse pull x;  return x      /*REXX   PULL   removes a stack item.  */
empty:  return queued()==0                       /*returns the status of the stack.     */
```
output :
```══════════════════════════════════ Pushing five values to the stack.
pushed value: 10
pushed value: 20
pushed value: 30
pushed a "null" value.
pushed value: 40
══════════════════════════════════ Quering the stack  (number of entries).
5  entries in the stack.
══════════════════════════════════ Popping all values from the stack.
1: popped value= 10
2: popped value= 20
3: popped value= 30
4: popped value=
5: popped value= 40
══════════════════════════════════ The stack is now empty.
```

## Ruby

Sample usage at FIFO#Ruby.

Or use the built-in Queue class:

```q = Queue.new
q.push "Hello"  # .enq is an alias
q.push "world"
p q.pop         # .deq is an alias
p q.empty?      # => false
```

## Rust

```use std::collections::VecDeque;

fn main() {
let mut queue = VecDeque::new();
queue.push_back("Hello");
queue.push_back("World");
while let Some(item) = queue.pop_front() {
println!("{}", item);
}

if queue.is_empty() {
println!("Yes, it is empty!");
}
}
```

## Scala

```val q=scala.collection.mutable.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```

## Sidef

Using the class defined at FIFO#Sidef

```var f = FIFO();
say f.empty;        # true
f.push('foo');
f.push('bar', 'baz');
say f.pop;          # foo
say f.empty;        # false

var g = FIFO('xxx', 'yyy');
say g.pop;          # xxx
say f.pop;          # bar
```

## Standard ML

Works with: SML/NJ
Functional interface
```- open Fifo;
opening Fifo
datatype 'a fifo = ...
exception Dequeue
val empty : 'a fifo
val isEmpty : 'a fifo -> bool
val enqueue : 'a fifo * 'a -> 'a fifo
val dequeue : 'a fifo -> 'a fifo * 'a
val next : 'a fifo -> ('a * 'a fifo) option
val delete : 'a fifo * ('a -> bool) -> 'a fifo
val head : 'a fifo -> 'a
val peek : 'a fifo -> 'a option
val length : 'a fifo -> int
val contents : 'a fifo -> 'a list
val app : ('a -> unit) -> 'a fifo -> unit
val map : ('a -> 'b) -> 'a fifo -> 'b fifo
val foldl : ('a * 'b -> 'b) -> 'b -> 'a fifo -> 'b
val foldr : ('a * 'b -> 'b) -> 'b -> 'a fifo -> 'b
- val q = empty;
val q = Q {front=[],rear=[]} : 'a fifo
- isEmpty q;
val it = true : bool
- val q' = enqueue (q, 1);
val q' = Q {front=[],rear=[1]} : int fifo
- isEmpty q';
val it = false : bool
- val q'' = List.foldl (fn (x, q) => enqueue (q, x)) q' [2, 3, 4];
val q'' = Q {front=[],rear=[4,3,2,1]} : int fifo
- peek q'';
val it = SOME 1 : int option
- length q'';
val it = 4 : int
- contents q'';
val it = [1,2,3,4] : int list
- val (q''', v) = dequeue q'';
val q''' = Q {front=[2,3,4],rear=[]} : int fifo
val v = 1 : int
- val (q'''', v') = dequeue q''';
val q'''' = Q {front=[3,4],rear=[]} : int fifo
val v' = 2 : int
- val (q''''', v'') = dequeue q'''';
val q''''' = Q {front=[4],rear=[]} : int fifo
val v'' = 3 : int
- val (q'''''', v''') = dequeue q''''';
val q'''''' = Q {front=[],rear=[]} : int fifo
val v''' = 4 : int
- isEmpty q'''''';
val it = true : bool
```
Works with: SML/NJ
Imperative interface
```- open Queue;
opening Queue
type 'a queue
exception Dequeue
val mkQueue : unit -> 'a queue
val clear : 'a queue -> unit
val isEmpty : 'a queue -> bool
val enqueue : 'a queue * 'a -> unit
val dequeue : 'a queue -> 'a
val next : 'a queue -> 'a option
val delete : 'a queue * ('a -> bool) -> unit
val head : 'a queue -> 'a
val peek : 'a queue -> 'a option
val length : 'a queue -> int
val contents : 'a queue -> 'a list
val app : ('a -> unit) -> 'a queue -> unit
val map : ('a -> 'b) -> 'a queue -> 'b queue
val foldl : ('a * 'b -> 'b) -> 'b -> 'a queue -> 'b
val foldr : ('a * 'b -> 'b) -> 'b -> 'a queue -> 'b
- val q : int queue = mkQueue ();
val q = - : int queue
- isEmpty q;
val it = true : bool
- enqueue (q, 1);
val it = () : unit
- isEmpty q;
val it = false : bool
- enqueue (q, 2);
val it = () : unit
- enqueue (q, 3);
val it = () : unit
- peek q;
val it = SOME 1 : int option
- length q;
val it = 3 : int
- contents q;
val it = [1,2,3] : int list
- dequeue q;
val it = 1 : int
- dequeue q;
val it = 2 : int
- peek q;
val it = SOME 3 : int option
- length q;
val it = 1 : int
- enqueue (q, 4);
val it = () : unit
- dequeue q;
val it = 3 : int
- peek q;
val it = SOME 4 : int option
- dequeue q;
val it = 4 : int
- isEmpty q;
val it = true : bool
```

## Tcl

See FIFO for operation implementations:

```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
```

## UNIX Shell

Works with: ksh93

See Queue/Definition for implementation:

```# 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```

## VBA

See Queue/Definition#VBA for implementation. The FiFo queue has been implemented with Collection. queue.count will return number of items in the queue. queue(i) will return the i-th item in the queue.

```Public Sub fifo()
push "One"
push "Two"
push "Three"
Debug.Print pop, pop, pop, empty_
End Sub```
Output:
`One           Two           Three         True`

## Wart

See FIFO for implementation.

```q <- (queue)
empty? q
=> 1
enq 1 q
empty? q
=> nil
enq 2 q
len q
=> 2
deq q
len q
=> 1```

## Wren

Library: Wren-queue
```import "./queue" for Queue

var q = Queue.new()
q.push(1)
q.push(2)
System.print("Queue contains %(q)")
System.print("Number of elements in queue = %(q.count)")
var item = q.pop()
System.print("'%(item)' popped from the queue")
System.print("First element is now %(q.peek())")
q.clear()
System.print("Queue cleared")
System.print("Is queue now empty? %((q.isEmpty) ? "yes" : "no")")
```
Output:
```Queue contains [1, 2]
Number of elements in queue = 2
'1' popped from the queue
First element is now 2
Queue cleared
Is queue now empty? yes
```

## XPL0

```include c:\cxpl\codes;
def Size=8;
int Fifo(Size);
int In, Out;            \fill and empty indexes into Fifo

proc Push(A);           \Add integer A to queue
int  A;                 \(overflow not detected)
[Fifo(In):= A;
In:= In+1;
if In >= Size then In:= 0;
];

func Pop;               \Return first integer in queue
int  A;
[if Out=In then                   \if popping empty queue
[Text(0, "Error");  exit 1];  \ then exit program with error code 1
A:= Fifo(Out);
Out:= Out+1;
if Out >= Size then Out:= 0;
return A;
];

func Empty;             \Return 'true' if queue is empty
return In = Out;

[In:= 0;  Out:= 0;
Push(0);
Text(0, if Empty then "true" else "false");  CrLf(0);
IntOut(0, Pop);  CrLf(0);
Push(1);
Push(2);
Push(3);
IntOut(0, Pop);  CrLf(0);
IntOut(0, Pop);  CrLf(0);
IntOut(0, Pop);  CrLf(0);
Text(0, if Empty then "true" else "false");  CrLf(0);

\A 256-byte queue is built in as device 8:
OpenI(8);  OpenO(8);
ChOut(8, ^0);                   \push
ChOut(0, ChIn(8));  CrLf(0);    \pop
ChOut(8, ^1);                   \push
ChOut(8, ^2);                   \push
ChOut(8, ^3);                   \push
ChOut(0, ChIn(8));  CrLf(0);    \pop
ChOut(0, ChIn(8));  CrLf(0);    \pop
ChOut(0, ChIn(8));  CrLf(0);    \pop
]```

Output:

```false
0
1
2
3
true
0
1
2
3
```

## Yabasic

```sub push(x\$)
queue\$ = queue\$ + x\$ + "#"
end sub

sub pop\$()
local i, r\$

if queue\$ <> "" then
i = instr(queue\$, "#")
if i then
r\$ = left\$(queue\$, i-1)
stack\$ = right\$(queue\$, len(queue\$) - i)
else
r\$ = queue\$
queue\$ = ""
end if
return r\$
else
print "--Queue is empty--"
end if
end sub

sub empty()
return queue\$ = ""
end sub

// ======== test ========

for n = 3 to 5
print "Push ", n : push(str\$(n))
next

print "Pop ", pop\$()

print "Push ", 6 : push(str\$(6))

while(not empty())
print "Pop ", pop\$()
wend

print "Pop ", pop\$()```
Output:
```Push 3
Push 4
Push 5
Pop 3
Push 6
Pop 4
Pop 5
Pop 6
Pop --Queue is empty--```

## zkl

See FIFO for implementation.

```q:=Queue();
q.empty();   //-->True
q.push(1,2,3);
q.pop();     //-->1
q.empty();   //-->False
q.pop();q.pop();q.pop(); //-->IndexError thrown
```

Lists support these semantics, so if you don't want the overhead of a Queue class:

```q:=List();
q.len();    //-->0
q.append(1,2,3);
q.pop(0);   //-->1
q.len();    //-->2
q;          //-->L(2,3)
q.pop(0);q.pop(0);q.pop(0); //-->IndexError thrown
q;          //-->L()
```