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.
- 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
- See also
- Array
- Associative array: Creation, Iteration
- Collections
- Compound data type
- Doubly-linked list: Definition, Element definition, Element insertion, List Traversal, Element Removal
- Linked list
- Queue: Definition, Usage
- Set
- Singly-linked list: Element definition, Element insertion, List Traversal, Element Removal
- Stack
11l
<lang 11l>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())</lang>
- 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.
<lang 6502asm>
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</lang>
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.
<lang 6502asm>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</lang>
Output of Example 1:
Queue Pointer = $FA
Hexdump of $00fa: 00 c0 80 40
Address of each: (FA FB FC FD)
POP
<lang 6502asm>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</lang>
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. <lang 6502asm>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</lang>
Output of Example 3:
Queue Pointer = $FA Hexdump of $00FA: 00 ff c0 80 Address of each: (FA FB FC FD)
8th
<lang forth> 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 </lang>
Action!
The user must type in the monitor the following command after compilation and before running the program!
SET EndProg=*
<lang Action!>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</lang>
- 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
Ada
<lang 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;</lang> Sample output:
Is_Empty TRUE
ALGOL 68
File: prelude/link.a68 c.f. Queue/Definition
File: prelude/queue_base.a68 c.f. Queue/Definition
File: test/data_stigler_diet.a68<lang algol68># -*- 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)
)</lang>File: test/queue.a68<lang algol68>#!/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</lang>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
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
<lang 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</lang>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*)
Astro
<lang python>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'</lang>
AutoHotkey
<lang autohotkey>push("qu", 2), push("qu", 44), push("qu", "xyz") ; TEST
MsgBox % "Len = " len("qu") ; Number of entries While !empty("qu") ; Repeat until queue is not empty
MsgBox % pop("qu") ; Print popped values (2, 44, xyz)
MsgBox Error = %ErrorLevel% ; ErrorLevel = 0: OK MsgBox % pop("qu") ; Empty MsgBox Error = %ErrorLevel% ; ErrorLevel = -1: popped too much MsgBox % "Len = " len("qu") ; Number of entries
push(queue,_) { ; push _ onto queue named "queue" (!=_), _ string not containing |
Global %queue% .= %queue% = "" ? _ : "|" _
}
pop(queue) { ; pop value from queue named "queue" (!=_,_1,_2)
Global RegExMatch(%queue%, "([^\|]*)\|?(.*)", _) Return _1, ErrorLevel := -(%queue%=""), %queue% := _2
}
empty(queue) { ; check if queue named "queue" is empty
Global Return %queue% = ""
}
len(queue) { ; number of entries in "queue"
Global StringReplace %queue%, %queue%, |, |, UseErrorLevel Return %queue% = "" ? 0 : ErrorLevel+1
}</lang>
AWK
<lang 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)
}</lang>
BBC BASIC
<lang bbcbasic> FIFOSIZE = 1000
FOR n = 3 TO 5
PRINT "Push ";n : PROCenqueue(n)
NEXT
PRINT "Pop " ; FNdequeue
PRINT "Push 6" : PROCenqueue(6)
REPEAT
PRINT "Pop " ; FNdequeue
UNTIL FNisempty
PRINT "Pop " ; FNdequeue
END
DEF PROCenqueue(n) : LOCAL f%
DEF FNdequeue : LOCAL f% : f% = 1
DEF FNisempty : LOCAL f% : f% = 2
PRIVATE fifo(), rptr%, wptr%
DIM fifo(FIFOSIZE-1)
CASE f% OF
WHEN 0:
wptr% = (wptr% + 1) MOD FIFOSIZE
IF rptr% = wptr% ERROR 100, "Error: queue overflowed"
fifo(wptr%) = n
WHEN 1:
IF rptr% = wptr% ERROR 101, "Error: queue empty"
rptr% = (rptr% + 1) MOD FIFOSIZE
= fifo(rptr%)
WHEN 2:
= (rptr% = wptr%)
ENDCASE
ENDPROC</lang>
Output:
Push 3 Push 4 Push 5 Pop 3 Push 6 Pop 4 Pop 5 Pop 6 Pop Error: queue empty
Bracmat
Below, queue is the name of a class with a data member list and three methods enqueue, dequeue and empty.
No special provision is implemented to "throw and exception" in case you try to dequeue from and empty queue, because, in Bracmat, evaluation of an expression, besides resulting in an evaluated expression, always also either "succeeds" or "fails". (There is, in fact, a third possibility, "ignore", telling Bracmat to close an eye even though an evaluation didn't succeed.) So in the example below, the last dequeue operation fails and the program continues on the right hand side of the bar (|) operator
<lang bracmat> ( queue
= (list=)
(enqueue=.(.!arg) !(its.list):?(its.list))
( dequeue
= x
. !(its.list):?(its.list) (.?x)
& !x
)
(empty=.!(its.list):)
)
& 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"
)
- </lang>
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. <lang c>#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);
}</lang>
C#
In C# we can use the Queue<T> class in the .NET 2.0 framework. <lang csharp>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."
}
}
}
}</lang>
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(). <lang cpp>#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() );
}</lang>
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 <lang cpp> std::queue<int, std::list<int> ></lang>
(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: <lang lisp>(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</lang> Or use a java implementation: <lang lisp>(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</lang>
CoffeeScript
<lang 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}"
</lang> output <lang> > coffee queue.coffee 1 100000 Error: queue is empty </lang>
Common Lisp
Using the implementation from FIFO.
<lang 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)))</lang>
Component Pascal
BlackBox Component Builder <lang oberon2> 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.
</lang>
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.
<lang cowgol>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);</lang>
- 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
<lang 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());
}</lang>
Faster Version
This versions creates a circular queue able to grow. Define "queue_usage2_main" to run the main and its tests. <lang d>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;
}
}
}</lang>
Delphi
Generics were added in Delphi2009.
<lang 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.</lang>
Output:
First Second Third Queue is empty.
Déjà Vu
This uses the definition from Queue/Definition#Déjà Vu <lang dejavu>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</lang>
- 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:
<lang diego>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
add_var({int},b)_value(8);
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[];</lang>
queue is a derivative of array, so arrays can also be used as queues.
E
Using the implementation from FIFO.
<lang e>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 })</lang>
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 : <lang elena>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." }
}</lang>
Elisa
A generic component for Queues and its usage are described in Queue/Definition
Elixir
Here a list is used as Queue. <lang Elixir> 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 </lang> Example: <lang> 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 </lang>
Erlang
All functions, from the shell: <lang 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</lang>
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.
<lang factor>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</lang> Alternatively, batch operations can be used. <lang factor>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</lang>
Fantom
Using definition of Queue in: Queue/Definition task.
<lang fantom> 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) }
}
} </lang>
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
<lang>: 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
- ->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</lang>
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
<lang forth> 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? . </lang>
Fortran
<lang fortran>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</lang>
FreeBASIC
As FreeBASIC does not have a built-in Queue type, I am reusing the type I wrote for the Queue/Definition task: <lang freebasic>' 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</lang>
- 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: <lang go>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")
}
}
}</lang> 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.) <lang go>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")
}
}
}</lang>
With linked lists
<lang go>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")
}
}
}</lang>
Groovy
Solution: <lang groovy>def q = new LinkedList()</lang>
Test: <lang groovy>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</lang>
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.
<lang Haskell> 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 </lang>
Icon and Unicon
Icon and Unicon provide built-in queue and stack functions. <lang Icon>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</lang>
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:
<lang j> 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</lang>
Using function-level FIFO queue implementation from FIFO
This is an interactive J session: <lang j> 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</lang>
Java
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+. <lang java>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</lang>
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.
<lang java>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</lang>
JavaScript
JavaScript arrays can be used as FIFOs. <lang javascript>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());</lang>
outputs:
0 1 3 true undefined
Julia
<lang julia>using DataStructures
queue = Queue(String) @show enqueue!(queue, "foo") @show enqueue!(queue, "bar") @show dequeue!(queue) # -> foo @show dequeue!(queue) # -> bar</lang>
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: <lang scala>// 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 ${if(q.isEmpty()) "empty" else "not empty"}")
try {
q.remove()
}
catch (e: NoSuchElementException) {
println("Can't remove elements from an empty queue")
}
}</lang>
- Output:
[1, 2, 3, 4, 5] Size of queue = 5 Removing: 1 2 3 Remaining in queue: [4, 5] Head element is now 4 After clearing, queue is empty Can't remove elements from an empty queue
Lasso
Lasso has a queue type that uses the following for the operators:
push: queue->insert pop: queue->get empty: queue->size == 0
Example: <lang lasso> local(queue) = queue
- queue->size
// => 0
- queue->insert('a')
- queue->insert('b')
- queue->insert('c')
- queue->size
// => 3
loop(#queue->size) => {
stdoutnl(#queue->get)
} // => // a // b // c
- queue->size == 0
// => true </lang>
Logo
UCB Logo comes with a protocol for treating lists as queues.
<lang logo>make "fifo [] print empty? :fifo ; true queue "fifo 1 queue "fifo 2 queue "fifo 3 show :fifo ; [1 2 3] print dequeue "fifo ; 1 show :fifo ; [2 3] print empty? :fifo ; false</lang>
Lua
Uses the queue-definition given at Queue/Definition#Lua <lang lua>q = Queue.new() Queue.push( q, 5 ) Queue.push( q, "abc" )
while not Queue.empty( q ) do
print( Queue.pop( q ) )
end</lang>
One can also just use a regular Lua table (shown here in interactive mode):
<lang lua>> -- create queue: > q = {} > -- push: > q[#q+1] = "first" > q[#q+1] = "second" > q[#q+1] = "third" > -- pop: > =table.remove(q, 1) first > =table.remove(q, 1) second > =table.remove(q, 1) third > -- empty? > =#q == 0 true</lang>
M2000 Interpreter
M2000 has always a current stack object. We can define a new one using a pointer to a stack object (here the variable a). We can swap the currernt one with that on a, so Push, number, letter$ and Empty can be used on that object. Also we can use functions using the stack object as first parameter like stackitem(), stackitem$() and stacktype$().
<lang M2000 Interpreter>
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
</lang>
Maple
There are more builtin operations like reverse(), length(),etc. <lang Maple>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</lang>
Mathematica/Wolfram Language
<lang Mathematica>Empty[a_] := If[Length[a] == 0, True, False] SetAttributes[Push, HoldAll]; Push[a_, elem_] := AppendTo[a, elem] SetAttributes[Pop, HoldAllComplete]; Pop[a_] := If[EmptyQ[a], False, b = First[a]; Set[a, Most[a]]; b]
Queue = {} -> {} Empty[Queue] -> True Push[Queue, "1"] -> {"1"} EmptyQ[Queue] ->False Pop[Queue] ->1 Pop[Queue] ->False</lang>
Nemerle
The Nemerle.Collections namespace contains an implementation of a Queue. <lang Nemerle>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()</lang>
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.
<lang NetRexx>/* 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
</lang>
- 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.
<lang nim>import deques
var queue = initDeque[int]()
queue.addLast(26) queue.addLast(99) queue.addLast(2) echo "Queue size: ", queue.len() echo "Popping: ", queue.popFirst() echo "Popping: ", queue.popFirst() echo "Popping: ", queue.popFirst() echo "Popping: ", queue.popFirst()</lang>
- 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
<lang 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(); }
} </lang>
OCaml
<lang 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</lang>
Oforth
Using FIFO implementation :
<lang oforth>: testQueue | q i |
Queue new ->q 20 loop: i [ i q push ] while ( q empty not ) [ q pop . ] ;</lang>
ooRexx
ooRexx includes a built-in queue class. <lang ooRexx> 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 </lang> Output:
1 3 2 1
Oz
<lang 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</lang>
Perl
Perl has built-in support to these operations: <lang perl>@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 ?</lang> Output: <lang>1 2345 array is empty</lang>
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
<lang php><?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 $queue->isEmpty() ? 'true' : 'false', "\n"; // returns false ?></lang>
PicoLisp
Using the implementation from FIFO: <lang PicoLisp>(println (fifo 'Queue)) # Retrieve the number '1' (println (fifo 'Queue)) # Retrieve an internal symbol 'abc' (println (fifo 'Queue)) # Retrieve a transient symbol "abc" (println (fifo 'Queue)) # and a list (abc) (println (fifo 'Queue)) # Queue is empty -> NIL</lang> Output:
1 abc "abc" (a b c) NIL
PL/I
<lang PL/I> test: proc options (main);
/* To implement a queue. */
define structure
1 node,
2 value fixed,
2 link handle(node);
declare (head, tail, t) handle (node);
declare null builtin;
declare i fixed binary;
head, tail = bind(:node, null:);
do i = 1 to 10; /* Add ten items to the tail of the queue. */
if head = bind(:node, null:) then
do;
head,tail = new(:node:);
get list (head => value);
put skip list (head => value);
head => link = bind(:node, null:); /* A NULL link */
end;
else
do;
t = new(:node:);
tail => link = t; /* Point the tail to the new node. */
tail = t;
tail => link = bind(:node, null:); /* Set the tail link to NULL */
get list (tail => value) copy;
put skip list (tail => value);
end;
end;
/* Pop all the items in the queue. */
put skip list ('The queue has:');
do while (head ^= bind(:node, null:));
put skip list (head => value);
head = head => link;
end;
end test; </lang> The output: <lang>
1
3
5
7
9
11
13
15
17
19
The queue has:
1
3
5
7
9
11
13
15
17
19
</lang>
PostScript
<lang postscript>
[1 2 3 4 5] 6 exch tadd = [1 2 3 4 5 6] uncons = 1 [2 3 4 5 6] [] empty? =true
</lang>
PowerShell
<lang PowerShell> [System.Collections.ArrayList]$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" </lang> 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: <lang PowerShell> $queue = New-Object -TypeName System.Collections.Queue
- or
$queue = [System.Collections.Queue] @() </lang> Show the methods and properties of the queue object: <lang PowerShell> Get-Member -InputObject $queue </lang>
- Output:
TypeName: System.Collections.Queue
Name MemberType Definition
---- ---------- ----------
Clear Method void Clear()
Clone Method System.Object Clone(), System.Object ICloneable.Clone()
Contains Method bool Contains(System.Object obj)
CopyTo Method void CopyTo(array array, int index), void ICollection.CopyTo(array array, int index)
Dequeue Method System.Object Dequeue()
Enqueue Method void Enqueue(System.Object obj)
Equals Method bool Equals(System.Object obj)
GetEnumerator Method System.Collections.IEnumerator GetEnumerator(), System.Collections.IEnumerator IEnumerable.GetEnumerator()
GetHashCode Method int GetHashCode()
GetType Method type GetType()
Peek Method System.Object Peek()
ToArray Method System.Object[] ToArray()
ToString Method string ToString()
TrimToSize Method void TrimToSize()
Count Property int Count {get;}
IsSynchronized Property bool IsSynchronized {get;}
SyncRoot Property System.Object SyncRoot {get;}
Put some stuff in the queue: <lang PowerShell> 1,2,3 | ForEach-Object {$queue.Enqueue($_)} </lang> Take a peek at the head of the queue: <lang PowerShell> $queue.Peek() </lang>
- Output:
1
Pop the head of the queue: <lang PowerShell> $queue.Dequeue() </lang>
- Output:
1
Clear the queue: <lang PowerShell> $queue.Clear() </lang> Test if queue is empty: <lang PowerShell> if (-not $queue.Count) {"Queue is empty"} </lang>
- Output:
Queue is empty
Prolog
Works with SWI-Prolog.
<lang 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).
</lang> 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
<lang PureBasic>NewList MyStack()
Procedure Push(n)
Shared MyStack() LastElement(MyStack()) AddElement(MyStack()) MyStack()=n
EndProcedure
Procedure Pop()
Shared MyStack() Protected n If FirstElement(MyStack()) ; e.g. Stack not empty n=MyStack() DeleteElement(MyStack(),1) 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 </lang>
Outputs
3 1 4 1 5
Python
<lang 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</lang>
Quackery
<lang Quackery>[ [] ] is queue ( --> q )
[ nested join ] is push ( q x --> q )
[ behead ] is pop ( q --> q x )
[ [] = ] is empty? ( q --> b )</lang> 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>#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</lang>
Raku
(formerly Perl 6)
Raku maintains the same list operators of Perl 5, for this task, the operations are:
<lang>push (aka enqueue) -- @list.push
pop (aka dequeue) -- @list.shift
empty -- !@list.elems</lang>
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:
<lang perl6>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 ]</lang>
REBOL
See FIFO#REBOL for implementation. Example repeated here for completeness.
<lang REBOL>; Create and populate a FIFO:
q: make fifo [] q/push 'a q/push 2 q/push USD$12.34 ; Did I mention that REBOL has 'money!' datatype? q/push [Athos Porthos Aramis] ; List elements pushed on one by one. q/push Huey Dewey Lewey ; This list is preserved as a list.
- Dump it out, with narrative
print rejoin ["Queue is " either q/empty [""]["not "] "empty."] while [not q/empty][print [" " q/pop]] print rejoin ["Queue is " either q/empty [""]["not "] "empty."] print ["Trying to pop an empty queue yields:" q/pop]</lang>
Output:
Queue is not empty. a 2 USD$12.34 Athos Porthos Aramis Huey Dewey Lewey Queue is empty. Trying to pop an empty queue yields: none
REXX
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". <lang rexx>/*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. */</lang>
- 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
Rust
<lang 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!");
}
} </lang>
Scala
<lang 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)</lang> 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 <lang ruby>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</lang>
Standard ML
- Functional interface
<lang sml>- 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</lang>
- Imperative interface
<lang sml>- 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</lang>
Stata
See Singly-linked list/Element definition#Stata.
Tcl
See FIFO for operation implementations: <lang tcl>set Q [list] empty Q ;# ==> 1 (true) push Q foo empty Q ;# ==> 0 (false) push Q bar peek Q ;# ==> foo pop Q ;# ==> foo peek Q ;# ==> bar</lang>
UNIX Shell
See Queue/Definition for implementation: <lang bash># any valid variable name can be used as a queue without initialization
queue_empty foo && echo foo is empty || echo foo is not empty
queue_push foo bar queue_push foo baz queue_push foo "element with spaces"
queue_empty foo && echo foo is empty || echo foo is not empty
print "peek: $(queue_peek foo)"; queue_pop foo print "peek: $(queue_peek foo)"; queue_pop foo print "peek: $(queue_peek foo)"; queue_pop foo print "peek: $(queue_peek foo)"; queue_pop foo</lang>
Output:
foo is empty foo is not empty peek: bar peek: baz peek: element with spaces peek: queue foo is empty
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. <lang vb>Public Sub fifo()
push "One" push "Two" push "Three" Debug.Print pop, pop, pop, empty_
End Sub</lang>
- 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
<lang ecmascript>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")")</lang>
- 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
<lang XPL0>include c:\cxpl\codes; def Size=8; int Fifo(Size); int In, Out; \fill and empty indexes into Fifo
proc Push(A); \Add integer A to queue int A; \(overflow not detected) [Fifo(In):= A; In:= In+1; if In >= Size then In:= 0; ];
func Pop; \Return first integer in queue int A; [if Out=In then \if popping empty queue
[Text(0, "Error"); exit 1]; \ then exit program with error code 1
A:= Fifo(Out); Out:= Out+1; if Out >= Size then Out:= 0; return A; ];
func Empty; \Return 'true' if queue is empty return In = Out;
[In:= 0; Out:= 0; Push(0); Text(0, if Empty then "true" else "false"); CrLf(0); IntOut(0, Pop); CrLf(0); Push(1); Push(2); Push(3); IntOut(0, Pop); CrLf(0); IntOut(0, Pop); CrLf(0); IntOut(0, Pop); CrLf(0); Text(0, if Empty then "true" else "false"); CrLf(0);
\A 256-byte queue is built in as device 8: OpenI(8); OpenO(8); ChOut(8, ^0); \push ChOut(0, ChIn(8)); CrLf(0); \pop ChOut(8, ^1); \push ChOut(8, ^2); \push ChOut(8, ^3); \push ChOut(0, ChIn(8)); CrLf(0); \pop ChOut(0, ChIn(8)); CrLf(0); \pop ChOut(0, ChIn(8)); CrLf(0); \pop ]</lang>
Output:
false 0 1 2 3 true 0 1 2 3
Yabasic
<lang 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$()</lang>
- 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()
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