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{{task|Data Structures}}{{Data structure}} [[File:Fifo.gif|frame|right|Illustration of FIFO behavior]]
;Task: 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
{{Template:See also lists}}
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
Ada
with FIFO;
with Ada.Text_Io; use Ada.Text_Io;
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|Revision 1 - one extension to language used - PRAGMA READ - a non standard feature similar to C's #include directive.}} {{works with|ALGOL 68G|Any - tested with release [http://sourceforge.net/projects/algol68/files/algol68g/algol68g-2.7 algol68g-2.7].}} {{wont work with|ELLA ALGOL 68|Any (with appropriate job cards) - tested with release [http://sourceforge.net/projects/algol68/files/algol68toc/algol68toc-1.8.8d/algol68toc-1.8-8d.fc9.i386.rpm/download 1.8-8d] - due to extensive use of '''format'''[ted] ''transput''.}}
'''File: prelude/link.a68''' c.f. [[Queue/Definition#ALGOL 68|Queue/Definition]]
'''File: prelude/queue_base.a68''' c.f. [[Queue/Definition#ALGOL 68|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/link.a68" PR;# c.f. [[rc:Queue/Definition]] #
PR read "prelude/queue_base.a68" PR; # c.f. [[rc:Queue/Definition]] #
PR read "test/data_stigler_diet.a68" PR;
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
'''See also:''' [[Stack#ALGOL_68|Stack]]
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*)
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. :)
[https://lh6.googleusercontent.com/-dTvs9totvDE/Uu3ZiFeE90I/AAAAAAAAJ-w/lJBVHOd-p0g/s1600/Untitled.png CLICK HERE TO VIEW THE CODE BLOCKS AND ANDROID APP SCREEN]
END
Arturo
Queue #{
list #()
push {
list list+&
}
pop {
if $(empty) {
panic "trying to pop from an empty queue!"
}
first_item $(first list)
list $(deleteBy list 0)
return first_item
}
empty {
$(size list)=0
}
inspect {
log this
}
}
q $(new ~Queue)
q.push "one"
q.push "two"
q.push "three"
q.inspect
print "popped = " + $(q.pop)
print "is it empty? = " + $(q.empty)
{{out}}
#{
empty <function: 0x1093917A0>
inspect <function: 0x109391800>
list #(
"one"
"two"
"three"
)
pop <function: 0x109391740>
push <function: 0x1093916E0>
}
popped = one
is it empty? = false
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
}
BBC BASIC
{{works with|BBC BASIC for Windows}}
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;
FIFOList head;
TAILQ_INIT(&head);
/* insert 20 integer values */
for(i=0; i < 20; i++) {
m_enqueue(i, &head);
}
/* dequeue and print */
while( m_dequeue(&i, &head) )
printf("%d\n", i);
fprintf(stderr, "FIFO list %s\n",
( m_dequeue(&i, &head) ) ?
"had still an element" :
"is void!");
exit(0);
}
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
q.push(1); // add an element
assert( !q.empty() ); // now the queue isn't empty any more
assert( q.front() == 1 ); // the first element is, of course, 1
q.push(2); // add another element
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 , 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.
C#
In C# we can use the Queue
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."
}
}
}
}
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.))
(.add q 1)
(.add q 2)
(.add q 3)
(.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]].
```lisp
(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
D
class LinkedQueue(T) {
private static struct Node {
T data;
Node* next;
}
private Node* head, tail;
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())
throw new Exception("Empty LinkedQueue.");
auto item = head.data;
head = head.next;
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() {
auto q = new LinkedQueue!int();
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
Generics were added in Delphi2009.
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.
=={{header|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
{{out}}
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
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)
require(reader.dequeue(throw) == 1)
writer.enqueue(3)
require(reader.dequeue(throw) == 2)
require(reader.dequeue(throw) == 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.
Elena
ELENA 4.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:
## Erlang
All functions, from the shell:
```Erlang>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:
http://rosettacode.org/wiki/Queue/Definition#Linked_list_version
\ calculate offsets into the queue data structure : ->head ( q -- adr ) ; \ syntactic sugar : ->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 r@ head++ r@ ->data r> ->head @ + c! ; \ data+head = adr, and store the char
Create 2 Queues and test the operators at the Forth console interactively
```txt
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 [[FIFO#Linked_list_version|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|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_head) :: thehead
type(fifo_node), dimension(5) :: ex, xe
integer :: i
call new_fifo(thehead)
do i = 1, 5
ex(i)%datum = i
call fifo_enqueue(thehead, ex(i))
end do
i = 1
do
call fifo_dequeue(thehead, xe(i))
print *, xe(i)%datum
i = i + 1
if ( fifo_isempty(thehead) ) exit
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
{{out}}
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 [[Queue/Definition#Go|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")
}
}
}
With linked lists
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
// "add" adds to end of "queue" list
q.add('Pete')
println q
assert !q.empty
// left shift operator ("<<") adds to end of "queue" list
q << 'John'
println q
assert !q.empty
// add assignment ("+=") adds the list elements
// 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
// implement java.util.Queue, like java.util.LinkedList
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]
[]
Haskell
Running the code from [[Queue/Definition#Haskell]] through GHC's interpreter.
Prelude> :l fifo.hs
[1 of 1] Compiling Main ( fifo.hs, interpreted )
Ok, modules loaded: Main.
*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
=={{header|Icon}} and {{header|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#J|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#J|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|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;
...
Queue<Integer> queue = new LinkedList<Integer>();
System.out.println(queue.isEmpty()); // empty test - true
// queue.remove(); // would throw NoSuchElementException
queue.add(1);
queue.add(2);
queue.add(3);
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|1.4}}
import java.util.LinkedList;
...
LinkedList queue = new LinkedList();
System.out.println(queue.isEmpty()); // empty test - true
queue.add(new Integer(1));
queue.add(new Integer(2));
queue.add(new Integer(3));
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|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>()
(1..5).forEach { q.add(it) }
println(q)
println("Size of queue = ${q.size}")
print("Removing: ")
(1..3).forEach { print("${q.remove()} ") }
println("\nRemaining in queue: $q")
println("Head element is now ${q.element()}")
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")
}
}
{{out}}
[1, 2, 3, 4, 5]
Size of queue = 5
Removing: 1 2 3
Remaining in queue: [4, 5]
Head element is now 4
After clearing, queue is empty
Can't remove elements from an empty queue
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
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
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
import queues
var deq: TQueue[int] = initQueue[int]()
deq.enqueue(26)
deq.add(99) # same as enqueue()
deq.enqueue(2)
echo("Dequeue size: ", deq.len())
echo("De-queue: ", deq.dequeue())
echo("De-queue: ", deq.dequeue())
echo("De-queue: ", deq.dequeue())
#echo("De-queue: ", deq.dequeue()) # dequeue an empty dequeue raises [EAssertionFailed]
{{out}}
Dequeue size: 3
De-queue: 26
De-queue: 99
De-queue: 2
Objeck
class Test {
function : Main(args : String[]) ~ Nil {
q := Struct.IntQueue->New();
q->Add(1);
q->Add(2);
q->Add(3);
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
# Queue.add 1 q;;
- : unit = ()
# Queue.is_empty q;;
- : bool = false
# Queue.add 2 q;;
- : unit = ()
# Queue.add 3 q;;
- : 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
# Queue.add 4 q;;
- : 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
[Queue] = {Link ['x-oz://system/adt/Queue.ozf']}
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
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:
## Perl 6
Perl 6 maintains the same list operators of Perl, for this task, the operations are:
<lang>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:
;
@queue.push('b', 'c'); # [ a, b, c ]
say @queue.shift; # a
say @queue.pop; # c
say @queue.perl; # [ b ]
say @queue.elems; # 1
@queue.unshift('A'); # [ A, b ]
@queue.push('C'); # [ A, b, C ]
Phix
Using the implementation from [[Queue/Definition#Phix|Queue/Definition]]
?empty() -- 1
push(5)
?empty() -- 0
push(6)
?pop() -- 5
?pop() -- 6
?empty() -- 1
PHP
{{works with|PHP|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 $queue->isEmpty() ? 'true' : 'false', "\n"; // returns false
?>
PicoLisp
Using the implementation from [[FIFO]]:
(println (fifo 'Queue)) # Retrieve the number '1'
(println (fifo 'Queue)) # Retrieve an internal symbol 'abc'
(println (fifo 'Queue)) # Retrieve a transient symbol "abc"
(println (fifo 'Queue)) # and a list (abc)
(println (fifo 'Queue)) # Queue is empty -> NIL
Output:
1
abc
"abc"
(a b c)
NIL
PL/I
test: proc options (main);
/* To implement a queue. */
define structure
1 node,
2 value fixed,
2 link handle(node);
declare (head, tail, t) handle (node);
declare null builtin;
declare i fixed binary;
head, tail = bind(:node, null:);
do i = 1 to 10; /* Add ten items to the tail of the queue. */
if head = bind(:node, null:) then
do;
head,tail = new(:node:);
get list (head => value);
put skip list (head => value);
head => link = bind(:node, null:); /* A NULL link */
end;
else
do;
t = new(:node:);
tail => link = t; /* Point the tail to the new node. */
tail = t;
tail => link = bind(:node, null:); /* Set the tail link to NULL */
get list (tail => value) copy;
put skip list (tail => value);
end;
end;
/* Pop all the items in the queue. */
put skip list ('The queue has:');
do while (head ^= bind(:node, null:));
put skip list (head => value);
head = head => link;
end;
end test;
The output:
## PostScript
{{libheader|initlib}}
```postscript
[1 2 3 4 5] 6 exch tadd
= [1 2 3 4 5 6]
uncons
= 1 [2 3 4 5 6]
[] empty?
=true
PowerShell
{{works with|PowerShell|4.0}}
[System.Collections.ArrayList]$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 $queue
{{Out}}
TypeName: System.Collections.Queue
Name MemberType Definition
---- ---------- ----------
Clear Method void Clear()
Clone Method System.Object Clone(), System.Object ICloneable.Clone()
Contains Method bool Contains(System.Object obj)
CopyTo Method void CopyTo(array array, int index), void ICollection.CopyTo(array array, int index)
Dequeue Method System.Object Dequeue()
Enqueue Method void Enqueue(System.Object obj)
Equals Method bool Equals(System.Object obj)
GetEnumerator Method System.Collections.IEnumerator GetEnumerator(), System.Collections.IEnumerator IEnumerable.GetEnumerator()
GetHashCode Method int GetHashCode()
GetType Method type GetType()
Peek Method System.Object Peek()
ToArray Method System.Object[] ToArray()
ToString Method string ToString()
TrimToSize Method void TrimToSize()
Count Property int Count {get;}
IsSynchronized Property bool IsSynchronized {get;}
SyncRoot Property System.Object SyncRoot {get;}
Put some stuff in the queue:
1,2,3 | ForEach-Object {$queue.Enqueue($_)}
Take a peek at the head of the queue:
$queue.Peek()
{{Out}}
1
Pop the head of the queue:
$queue.Dequeue()
{{Out}}
1
Clear the queue:
$queue.Clear()
Test if queue is empty:
if (-not $queue.Count) {"Queue is empty"}
{{Out}}
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())
AddElement(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
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
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. */
{{out|output|text=:}}
══════════════════════════════════ 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]]
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
Stata
See [[Singly-linked list/Element definition#Stata]].
Tcl
See [[FIFO#Tcl|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#UNIX_Shell|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
{{out}}
One Two Three True
Wart
See [[FIFO#Wart|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
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$()
{{out}}
Push 3
Push 4
Push 5
Pop 3
Push 6
Pop 4
Pop 5
Pop 6
Pop --Queue is empty--
zkl
See [[FIFO#zkl|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()
{{omit from|GUISS}}