Queue/Definition

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< Queue(Redirected from Queue)
Task
Queue/Definition
You are encouraged to solve this task according to the task description, using any language you may know.

Data Structure
This illustrates a data structure, a means of storing data within a program.

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

Implement a FIFO queue.

Elements are added at one side and popped from the other in the order of insertion.


Operations:

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


Errors:

  •   handle the error of trying to pop from an empty queue (behavior depends on the language and platform)


See
  •   Queue/Usage   for the built-in FIFO or queue of your language or standard library.


See also



11l[edit]

Translation of: Python
T FIFO
   [Int] contents

   F push(item)
      .contents.append(item)
   F pop()
      R .contents.pop(0)
   F empty()
      R .contents.empty

V f = FIFO()
f.push(3)
f.push(2)
f.push(1)
L !f.empty()
   print(f.pop())
Output:
3
2
1

AArch64 Assembly[edit]

Works with: as version Raspberry Pi 3B version Buster 64 bits
/* ARM assembly AARCH64 Raspberry PI 3B */
/*  program defqueue64.s   */
 
/*******************************************/
/* Constantes file                         */
/*******************************************/
/* for this file see task include a file in language AArch64 assembly*/
.include "../includeConstantesARM64.inc"
 
.equ  NBMAXIELEMENTS,    100
 
/*******************************************/
/* Structures                               */
/********************************************/
/* example structure  for value of item  */
    .struct  0
value_ident:                     // ident
    .struct  value_ident + 8 
value_value1:                    // value 1 
    .struct  value_value1 + 8 
value_value2:                    // value 2
    .struct  value_value2 + 8 
value_fin:
/* example structure  for queue  */
    .struct  0
queue_ptdeb:                     // begin pointer of item
    .struct  queue_ptdeb + 8 
queue_ptfin:                     // end pointer of item
    .struct  queue_ptfin + 8 
queue_stvalue:                   // structure of value item
    .struct  queue_stvalue + (value_fin * NBMAXIELEMENTS)
queue_fin:
 
 
/*********************************/
/* Initialized data              */
/*********************************/
.data
szMessEmpty:       .asciz "Empty queue. \n"
szMessNotEmpty:    .asciz "Not empty queue. \n"
szMessError:       .asciz "Error detected !!!!. \n"
szMessResult:      .asciz "Ident : @ value 1 : @ value 2 : @ \n"  // message result
 
szCarriageReturn:  .asciz "\n"
/*********************************/
/* UnInitialized data            */
/*********************************/
.bss 
.align 4
Queue1:                .skip queue_fin      // queue memory place 
stItem:                .skip value_fin      // value item memory place
sZoneConv:             .skip 100
/*********************************/
/*  code section                 */
/*********************************/
.text
.global main 
main:                                       // entry of program 
    ldr x0,qAdrQueue1                       // queue structure address
    bl isEmpty
    cmp x0,#0
    beq 1f
    ldr x0,qAdrszMessEmpty
    bl affichageMess                        // display message empty
    b 2f
1:
    ldr x0,qAdrszMessNotEmpty
    bl affichageMess                        // display message not empty
2:
    // init item 1
    ldr x0,qAdrstItem
    mov x1,#1
    str x1,[x0,#value_ident]
    mov x1,#11
    str x1,[x0,#value_value1]
    mov x1,#12
    str x1,[x0,#value_value2]
 
    ldr x0,qAdrQueue1                       // queue structure address
    ldr x1,qAdrstItem
    bl pushQueue                            // add item in queue
    cmp x0,#-1                              // error ?
    beq 99f
    // init item 2
    ldr x0,qAdrstItem
    mov x1,#2
    str x1,[x0,#value_ident]
    mov x1,#21
    str x1,[x0,#value_value1]
    mov x1,#22
    str x1,[x0,#value_value2]
 
    ldr x0,qAdrQueue1                       // queue structure address
    ldr x1,qAdrstItem
    bl pushQueue                            // add item in queue
    cmp x0,#-1
    beq 99f
    ldr x0,qAdrQueue1                       // queue structure address
    bl isEmpty
    cmp x0,#0                               // not empty
    beq 3f
    ldr x0,qAdrszMessEmpty
    bl affichageMess                        // display message empty
    b 4f
3:
    ldr x0,qAdrszMessNotEmpty
    bl affichageMess                        // display message not empty
 
4:
    ldr x0,qAdrQueue1                       // queue structure address
    bl popQueue                             // return address item
    cmp x0,#-1                              // error ?
    beq 99f
    mov x2,x0                               // save item pointer 
    ldr x0,[x2,#value_ident]
    ldr x1,qAdrsZoneConv                    // conversion ident
    bl conversion10S                        // decimal conversion
    ldr x0,qAdrszMessResult
    ldr x1,qAdrsZoneConv
    bl strInsertAtCharInc                   // insert result at first @ character
    mov x5,x0
    ldr x0,[x2,#value_value1]
    ldr x1,qAdrsZoneConv                    // conversion value 1
    bl conversion10S                        // decimal conversion
    mov x0,x5
    ldr x1,qAdrsZoneConv
    bl strInsertAtCharInc                   // insert result at Second @ character
    mov x5,x0
    ldr x0,[x2,#value_value2]
    ldr x1,qAdrsZoneConv                    // conversion value 2
    bl conversion10S                        // decimal conversion
    mov x0,x5
    ldr x1,qAdrsZoneConv
    bl strInsertAtCharInc                   // insert result at third @ character
    bl affichageMess                        // display message final
    b 4b                                    // loop
 
99:                                         // error
    ldr x0,qAdrszMessError
    bl affichageMess       
100:                                        // standard end of the program 
    mov x0,0                                // return code
    mov x8,EXIT                             // request to exit program
    svc 0                                   // perform the system call
 
qAdrQueue1:               .quad Queue1
qAdrstItem:               .quad stItem
qAdrszMessError:          .quad szMessError
qAdrszMessEmpty:          .quad szMessEmpty
qAdrszMessNotEmpty:       .quad szMessNotEmpty
qAdrszMessResult:         .quad szMessResult
qAdrszCarriageReturn:     .quad szCarriageReturn
qAdrsZoneConv:            .quad sZoneConv

/******************************************************************/
/*     test if queue empty                                        */ 
/******************************************************************/
/* x0 contains the address of queue structure */
/* x0 returns 0 if not empty, 1 if empty    */
isEmpty:
    stp x1,lr,[sp,-16]!            // save  registers
    stp x2,x3,[sp,-16]!            // save  registers
    ldr x1,[x0,#queue_ptdeb]       // begin pointer 
    ldr x2,[x0,#queue_ptfin]       // begin pointer 
    cmp x1,x2
    bne 1f
    mov x0,#1                      // empty queue
    b 2f
1:
    mov x0,#0                      // not empty
2:
    ldp x2,x3,[sp],16              // restaur  2 registers
    ldp x1,lr,[sp],16              // restaur  2 registers
    ret                            // return to address lr x30

/******************************************************************/
/*     add item  in queue                                         */ 
/******************************************************************/
/* x0 contains the address of queue structure */
/* x1 contains the address of item            */
pushQueue:
    stp x1,lr,[sp,-16]!            // save  registers
    stp x2,x3,[sp,-16]!            // save  registers
    add x2,x0,#queue_stvalue       // address of values structure
    ldr x3,[x0,#queue_ptfin]       // end pointer
    add x2,x2,x3                      // free address of queue
    ldr x4,[x1,#value_ident]       // load ident item
    str x4,[x2,#value_ident]       // and store in queue
    ldr x4,[x1,#value_value1]      // idem
    str x4,[x2,#value_value1]
    ldr x4,[x1,#value_value2]
    str x4,[x2,#value_value2]
    add x3,x3,#value_fin
    cmp x3,#value_fin * NBMAXIELEMENTS
    beq 99f
    str x3,[x0,#queue_ptfin]       // store new end pointer
    b 100f
99:
    mov x0,#-1                     // error
100:
    ldp x2,x3,[sp],16              // restaur  2 registers
    ldp x1,lr,[sp],16              // restaur  2 registers
    ret                            // return to address lr x30

/******************************************************************/
/*     pop queue                                                  */ 
/******************************************************************/
/* x0 contains the address of queue structure */
popQueue:
    stp x1,lr,[sp,-16]!            // save  registers
    stp x2,x3,[sp,-16]!            // save  registers
    mov x1,x0                      // control if empty queue
    bl isEmpty
    cmp x0,#1                      // yes -> error
    beq 99f
    mov x0,x1
    ldr x1,[x0,#queue_ptdeb]       // begin pointer 
    add x2,x0,#queue_stvalue       // address of begin values item
    add x2,x2,x1                   // address of item
    add x1,x1,#value_fin
    str x1,[x0,#queue_ptdeb]       // store nex begin pointer
    mov x0,x2                      // return pointer item
    b 100f
99:
    mov x0,#-1                     // error
100:
    ldp x2,x3,[sp],16              // restaur  2 registers
    ldp x1,lr,[sp],16              // restaur  2 registers
    ret                            // return to address lr x30
 
/********************************************************/
/*        File Include fonctions                        */
/********************************************************/
/* for this file see task include a file in language AArch64 assembly */
.include "../includeARM64.inc"
Output:
Empty queue.
Not empty queue.
Ident : +1 value 1 : +11 value 2 : +12
Ident : +2 value 1 : +21 value 2 : +22
Error detected !!!!.

ACL2[edit]

(defun enqueue (x xs)
   (cons x xs))

(defun dequeue (xs)
   (declare (xargs :guard (and (consp xs)
                               (true-listp xs))))
   (if (or (endp xs) (endp (rest xs)))
       (mv (first xs) nil)
       (mv-let (x ys)
               (dequeue (rest xs))
          (mv x (cons (first xs) ys)))))

(defun empty (xs)
   (endp xs))

Action![edit]

Static memory[edit]

Following solution uses fixed array as a buffer for the queue.

DEFINE MAXSIZE="200"
BYTE ARRAY queue(MAXSIZE)
BYTE queueFront=[0],queueRear=[0]

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

PROC Push(BYTE v)
  BYTE rear

  rear=queueRear+1
  IF rear=MAXSIZE THEN
    rear=0
  FI
  IF rear=queueFront THEN
    PrintE("Error: queue is full!")
    Break()
  FI
  queue(queueRear)=v
  queueRear=rear
RETURN

BYTE FUNC Pop()
  BYTE v

  IF IsEmpty() THEN
    PrintE("Error: queue is empty!")
    Break()
  FI
  v=queue(queueFront)
  queueFront==+1
  IF queueFront=MAXSIZE THEN
    queueFront=0
  FI
RETURN (v)

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

PROC TestPush(BYTE v)
  PrintF("Push: %B%E",v)
  Push(v)
RETURN

PROC TestPop()
  BYTE v

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

PROC Main()
  TestIsEmpty()
  TestPush(10)
  TestIsEmpty()
  TestPush(31)
  TestPop()
  TestIsEmpty()
  TestPush(5)
  TestPop()
  TestPop()
  TestPop()
RETURN

Dynamic memory[edit]

Following solution uses module for dynamic memory allocation. The user must type in the monitor the following command after compilation and before running the program!
SET EndProg=*
CARD EndProg ;required for ALLOCATE.ACT

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

DEFINE PTR="CARD"
DEFINE NODE_SIZE="3"
TYPE QueueNode=[BYTE data PTR nxt]

QueueNode POINTER queueFront,queueRear

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

PROC Push(BYTE 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

BYTE FUNC Pop()
  QueueNode POINTER node
  BYTE 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(BYTE v)
  PrintF("Push: %B%E",v)
  Push(v)
RETURN

PROC TestPop()
  BYTE v

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

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

  Put(125) PutE() ;clear screen

  TestIsEmpty()
  TestPush(10)
  TestIsEmpty()
  TestPush(31)
  TestPop()
  TestIsEmpty()
  TestPush(5)
  TestPop()
  TestPop()
  TestPop()
RETURN
Output:

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

Queue is empty
Push: 10
Queue is not empty
Push: 31
Pop: 10
Queue is not empty
Push: 5
Pop: 31
Pop: 5
Pop: Error: queue is empty!

RETURN
Error: 128

Ada[edit]

The first example below demonstrates a FIFO created for single-threaded computing. This version has the advantage of using a minimum of memory per FIFO element, and being very fast.

The interface specification for a FIFO is described in the package specification.

generic
   type Element_Type is private;
package Fifo is
   type Fifo_Type is private;
   procedure Push(List : in out Fifo_Type; Item : in Element_Type);
   procedure Pop(List : in out Fifo_Type; Item : out Element_Type);
   function Is_Empty(List : Fifo_Type) return Boolean;
   Empty_Error : exception;
private
   type Fifo_Element;
   type Fifo_Ptr is access Fifo_Element;
   type Fifo_Type is record
      Head : Fifo_Ptr := null;
      Tail : Fifo_Ptr := null;
   end record;
   type Fifo_Element is record
      Value : Element_Type;
      Next  : Fifo_Ptr := null;
   end record;
end Fifo;

The FIFO implementation is described in the package body:

with Ada.Unchecked_Deallocation;

package body Fifo is 

   ----------
   -- Push --
   ----------

   procedure Push (List : in out Fifo_Type; Item : in Element_Type) is
      Temp : Fifo_Ptr := new Fifo_Element'(Item, null);
   begin
      if List.Tail = null then
         List.Tail := Temp;
      end if;
      if List.Head /= null then
        List.Head.Next := Temp;
      end if;
      List.Head := Temp;
   end Push;

   ---------
   -- Pop --
   ---------

   procedure Pop (List : in out Fifo_Type; Item : out Element_Type) is
      procedure Free is new Ada.Unchecked_Deallocation(Fifo_Element, Fifo_Ptr);
      Temp : Fifo_Ptr := List.Tail;
   begin
      if List.Head = null then
         raise Empty_Error;
      end if;
      Item := List.Tail.Value;
      List.Tail := List.Tail.Next;
      if List.Tail = null then
         List.Head := null;
      end if;
      Free(Temp);
   end Pop;

   --------------
   -- Is_Empty --
   --------------

   function Is_Empty (List : Fifo_Type) return Boolean is
   begin
      return List.Head = null;
   end Is_Empty; 

end Fifo;

A "main" procedure for this program is:

with Fifo;
with Ada.Text_Io; use Ada.Text_Io;

procedure Fifo_Test is
   package Int_Fifo is new Fifo(Integer);
   use Int_Fifo;
   My_Fifo : Fifo_Type;
   Val : Integer;
begin
   for I in 1..10 loop
      Push(My_Fifo, I);
   end loop;
   while not Is_Empty(My_Fifo) loop
      Pop(My_Fifo, Val);
      Put_Line(Integer'Image(Val));
   end loop;
end Fifo_Test;

The following implementation produces equivalent functionality by deriving from the standard Ada Container type Doubly_Linked_Lists.

This example needs fewer lines of code on the part of the application programmer, but the implementation is less efficient than the previous example. Each element has all the data members needed for a doubly linked list. It also links in all the functionality of a doubly linked list. Most of that functionality is unneeded in a FIFO.

 
 with Ada.Containers.Doubly_Linked_Lists;
 generic
    type Element_Type is private;
 package Generic_Fifo is
    type Fifo_Type is tagged private;
    procedure Push(The_Fifo : in out Fifo_Type; Item : in Element_Type);
    procedure Pop(The_Fifo : in out Fifo_Type; Item : out Element_Type);
    Empty_Error : Exception;
 private
    package List_Pkg is new Ada.Containers.Doubly_Linked_Lists(Element_Type);
    use List_Pkg;
    Type Fifo_Type is new List with null record;
 end Generic_Fifo;
 package body Generic_Fifo is
 
    ----------
    -- Push --
    ---------- 
 
    procedure Push (The_Fifo : in out Fifo_Type; Item : in Element_Type) is
    begin
       The_Fifo.Prepend(Item);
    end Push;
 
    ---------
    -- Pop --
    ---------
 
    procedure Pop (The_Fifo : in out Fifo_Type; Item : out Element_Type) is
    begin
       if Is_Empty(The_Fifo) then
          raise Empty_Error;
       end if;
       Item := The_Fifo.Last_Element;
       The_Fifo.Delete_Last;
    end Pop;
 
 end Generic_Fifo;
with Generic_Fifo;
with Ada.Text_Io; use Ada.Text_Io;

procedure Generic_Fifo_Test is
   package Int_Fifo is new Generic_Fifo(Integer);
   use Int_Fifo;
   My_Fifo : Fifo_Type;
   Val : Integer;
begin
   for I in 1..10 loop
      My_Fifo.Push(I);
   end loop;
   while not My_Fifo.Is_Empty loop
      My_Fifo.Pop(Val);
      Put_Line(Integer'Image(Val));
   end loop;
end Generic_Fifo_Test;

The function Is_Empty is inherited from the Lists type.

The next two examples provide simple FIFO functionality for concurrent tasks. The buffer in each example holds a single value. When running concurrent tasks, one writing to the buffer, and one reading from the buffer, either the writer will be faster than the reader, or the reader will be faster than the writer. If the writer is faster a dynamic FIFO will grow to consume all available memory on the computer. If the reader is faster the FIFO will either contain a single value or it will be empty. In either case, no implementation is more efficient than a single element buffer.

If we wish for the reader to read every value written by the writer we must synchronize the tasks. The writer can only write a new value when the buffer contains a stale value. The reader can only read a value when the value is fresh. This synchronization forces the two tasks to run at the same speed.

generic
   type Element_Type is private;
package Synchronous_Fifo is
   protected type Fifo is
      entry Push(Item : Element_Type);
      entry Pop(Item : out Element_Type);
   private
      Value : Element_Type;
      Is_New : Boolean := False;
   end Fifo;
end Synchronous_Fifo;
package body Synchronous_Fifo is

   ----------
   -- Fifo --
   ----------

   protected body Fifo is 

      ---------
      -- Push --
      ---------

      entry Push (Item : Element_Type) when not Is_New is
      begin
         Value := Item;
         Is_New := True;
      end Push; 

      ---------
      -- Pop --
      ---------

      entry Pop (Item : out Element_Type) when Is_New is
      begin
         Item := Value;
         Is_New := False;
      end Pop; 

   end Fifo;

end Synchronous_Fifo;
with Synchronous_Fifo;
with Ada.Text_Io; use Ada.Text_Io;

 procedure Synchronous_Fifo_Test is
    package Int_Fifo is new Synchronous_Fifo(Integer);
    use Int_Fifo;
    Buffer : Fifo;
    
    task Writer is
       entry Stop;
    end Writer;
    
    task body Writer is
       Val : Positive := 1;
    begin
       loop
          select
             accept Stop;
             exit;
          else
             select
                Buffer.Push(Val);
                Val := Val + 1;
             or
                delay 1.0;
             end select;
          end select;
       end loop;
    end Writer;
    
    task Reader is
       entry Stop;
    end Reader;
    
    task body Reader is
       Val : Positive;
    begin
       loop
          select
             accept Stop;
             exit;
          else
             select
                Buffer.Pop(Val);
                Put_Line(Integer'Image(Val));
             or
                 delay 1.0;
            end select;
          end select;
       end loop;
    end Reader;
 begin
    delay 0.1;
    Writer.Stop;
    Reader.Stop;
 end Synchronous_Fifo_Test;

Another choice is to cause the two tasks to run independently. The writer can write whenever it is scheduled. The reader reads whenever it is scheduled, after the writer writes the first valid value.

In this example the writer writes several values before the reader reads a value. The reader will then read that same value several times before the writer is scheduled to write more values.

In a fully asynchronous system the reader only samples the values written by the writer. There is no control over the number of values not sampled by the reader, or over the number of times the reader reads the same value.

generic
   type Element_Type is private;
package Asynchronous_Fifo is
   protected type Fifo is
      procedure Push(Item : Element_Type);
      entry Pop(Item : out Element_Type);
   private
      Value : Element_Type;
      Valid : Boolean := False;
   end Fifo;
end Asynchronous_Fifo;

You may notice that the protected type specification is remarkably similar to the synchronous example above. The only important difference is that Push is declared to be an Entry in the synchronous example while it is a procedure in the asynchronous example. Entries only execute when their boundary condition evaluates to TRUE. Procedures execute unconditionally.

package body Asynchronous_Fifo is

   ----------
   -- Fifo --
   ----------

   protected body Fifo is 

      ----------
      -- Push --
      ----------

      procedure Push (Item : Element_Type) is
      begin
          Value := Item;
         Valid := True;
      end Push;

      ---------
      -- Pop --
      ---------

      entry Pop (Item : out Element_Type) when Valid is
      begin
         Item := Value;
      end Pop;

   end Fifo; 

end Asynchronous_Fifo;
with Asynchronous_Fifo;
with Ada.Text_Io; use Ada.Text_Io; 

 procedure Asynchronous_Fifo_Test is
    package Int_Fifo is new Asynchronous_Fifo(Integer);
    use Int_Fifo;
    Buffer : Fifo;
    
    task Writer is
       entry Stop;
    end Writer;
    
    task body Writer is
       Val : Positive := 1;
    begin
       loop
          select
             accept Stop;
             exit;
          else
             Buffer.Push(Val);
             Val := Val + 1;
          end select;
       end loop;
    end Writer;
    
    task Reader is
       entry Stop;
    end Reader;
    
    task body Reader is
       Val : Positive;
    begin
       loop
          select 
             accept Stop;
             exit;
          else
             Buffer.Pop(Val);
             Put_Line(Integer'Image(Val));
          end select;
       end loop;<syntaxhighlight lang="ada">
    end Reader;
 begin
    delay 0.1;
    Writer.Stop;
    Reader.Stop;
 end Asynchronous_Fifo_Test;

ALGOL 68[edit]

Works with: ALGOL 68 version Revision 1 - one extension to language used - PRAGMA READ - a non standard feature similar to C's #include directive.
Works with: ALGOL 68G version Any - tested with release algol68g-2.7.
File: prelude/queue_base.a68
# -*- coding: utf-8 -*- #
CO REQUIRES:
  MODE OBJLINK = STRUCT(
    REF OBJLINK next,
    REF OBJLINK prev,
    OBJVALUE value # ... etc. required #
  );
  PROC obj link new = REF OBJLINK: ~;
  PROC obj link free = (REF OBJLINK free)VOID: ~
END CO

# actually a pointer to the last LINK, there ITEMs are ADDED/get #
MODE OBJQUEUE = REF OBJLINK; 

OBJQUEUE obj queue empty = NIL;

BOOL obj queue par = FALSE; # make code thread safe #
SEMA obj queue sema = LEVEL ABS obj queue par;
# Warning: 1 SEMA for all queues of type obj, i.e. not 1 SEMA per queue #

PROC obj queue init = (REF OBJQUEUE self)REF OBJQUEUE:
  self := obj queue empty;

PROC obj queue put = (REF OBJQUEUE self, OBJVALUE obj)REF OBJQUEUE: (
  REF OBJLINK out = obj link new;
  IF obj queue par THEN DOWN obj queue sema FI;
  IF self IS obj queue empty THEN
    out := (out, out, obj) # self referal #
  ELSE # join into list #
    out := (self, prev OF self, obj);
    next OF prev OF out := prev OF next OF out := out
  FI;
  IF obj queue par THEN UP obj queue sema FI;
  self := out
);

# define a useful prepend/put/plusto (+=:) operator... #
PROC obj queue plusto = (OBJVALUE obj, REF OBJQUEUE self)OBJQUEUE: obj queue put(self,obj);
OP +=: = (OBJVALUE obj, REF OBJQUEUE self)REF OBJQUEUE: obj queue put(self,obj);
# a potential append/plusab (+:=) operator...
OP (REF OBJQUEUE, OBJVALUE)OBJQUEUE +:= = obj queue plusab;
#

# see if the program/coder wants the OBJ problem mended... #
PROC (REF OBJQUEUE #self#)BOOL obj queue index error mended 
  := (REF OBJQUEUE self)BOOL: (abend("obj queue index error"); stop);

PROC on obj queue index error = (REF OBJQUEUE self, PROC(REF OBJQUEUE #self#)BOOL mended)VOID:
  obj queue index error mended := mended;

PROC obj queue get = (REF OBJQUEUE self)OBJVALUE: (
# DOWN obj queue sema; #
  IF self IS obj queue empty THEN
    IF NOT obj queue index error mended(self) THEN abend("obj stack index error") FI FI;
  OBJQUEUE old tail = prev OF self;
  IF old tail IS self THEN # free solo member #
    self := obj queue empty
  ELSE # free self/tail member #
    OBJQUEUE new tail = prev OF old tail;
    next OF new tail := self;
    prev OF self := new tail
  FI;
#;UP obj queue sema #
  OBJVALUE out = value OF old tail;
# give a recovery hint to the garbage collector #
  obj link free(old tail);
  out
);

PROC obj queue is empty = (REF OBJQUEUE self)BOOL:
  self IS obj queue empty;

SKIP
See also: Queue/Usage

ALGOL W[edit]

begin
    % define a Queue type that will hold StringQueueElements %
    record StringQueue ( reference(StringQueueElement) front, back );
    % define the StringQueueElement type %
    record StringQueueElement ( string(8)                     element
                              ; reference(StringQueueElement) next
                              );
    % we would need separate types for other element types   %
    % adds s to the end of the StringQueue q                 %
    procedure pushString ( reference(StringQueue) value q
                         ; string(8)              value e
                         ) ;
    begin
        reference(StringQueueElement) newElement;
        newElement := StringQueueElement( e, null );
        if front(q) = null then begin
            % adding to an empty queue %
            front(q) := newElement;
            back(q)  := newElement
            end
        else begin
            % the queue is not empty %
            next(back(q)) := newElement;
            back(q)       := newElement
        end
    end pushString ;
    % removes an element from the front of the StringQueue q %
    % asserts the queue is not empty, which will stop the    %
    % program if it is                                       %
    string(8) procedure popString ( reference(StringQueue) value q ) ;
    begin
        string(8) v;
        assert( not isEmptyStringQueue( q ) );
        v        := element(front(q));
        front(q) := next(front(q));
        if front(q) = null then % just popped the last element % back(q) := null;
        v
    end popStringQueue ;
    % returns true if the StringQueue q is empty, false otherwise %
    logical procedure isEmptyStringQueue ( reference(StringQueue) value q ) ; front(q) = null;

    begin % test the StringQueue operations %
        reference(StringQueue) q;
        q := StringQueue( null, null );
        pushString( q, "fred"   );
        pushString( q, "whilma" );
        pushString( q, "betty"  );
        pushString( q, "barney" );
        while not isEmptyStringQueue( q ) do write( popString( q ) )
    end
end.
Output:
fred
whilma
betty
barney

Applesoft BASIC[edit]

 0  DEF  FN E(MPTY) = SP = FIRST
 10  GOSUB 150EMPTY
 20  LET A$ = "A": GOSUB 100PUSH
 30  LET A$ = "B": GOSUB 100PUSH
 40  GOSUB 150EMPTY
 50  GOSUB 120PULL FIRST
 60  GOSUB 120PULL FIRST
 70  GOSUB 150EMPTY
 80  GOSUB 120PULL FIRST
 90  END
 100  PRINT "PUSH "A$
 110  LET S$(SP) = A$:SP = SP + 1: RETURN
 120  GOSUB 130: PRINT "POP "A$: RETURN
 130  IF  FN E(0) THEN  PRINT "POPPING FROM EMPTY QUEUE";: STOP
 140  A$ = S$(FI): FI = FI + 1 : RETURN
 150  PRINT "EMPTY? " MID$ ("YESNO",4 ^  FN E(0),3): RETURN

ARM Assembly[edit]

Works with: as version Raspberry Pi
/* ARM assembly Raspberry PI  */
/*  program defqueue.s   */

/* Constantes    */
.equ STDOUT, 1     @ Linux output console
.equ EXIT,   1     @ Linux syscall
.equ WRITE,  4     @ Linux syscall

.equ  NBMAXIELEMENTS,    100

/*******************************************/
/* Structures                               */
/********************************************/
/* example structure  for value of item  */
    .struct  0
value_ident:                     @ ident
    .struct  value_ident + 4 
value_value1:                    @ value 1 
    .struct  value_value1 + 4 
value_value2:                    @ value 2
    .struct  value_value2 + 4 
value_fin:
/* example structure  for queue  */
    .struct  0
queue_ptdeb:                     @ begin pointer of item
    .struct  queue_ptdeb + 4 
queue_ptfin:                     @ end pointer of item
    .struct  queue_ptfin + 4 
queue_stvalue:                   @ structure of value item
    .struct  queue_stvalue + (value_fin * NBMAXIELEMENTS)
queue_fin:


/*********************************/
/* Initialized data              */
/*********************************/
.data
szMessEmpty:       .asciz "Empty queue. \n"
szMessNotEmpty:    .asciz "Not empty queue. \n"
szMessError:       .asciz "Error detected !!!!. \n"
szMessResult:      .ascii "Ident :"                    @ message result
sMessIdent:        .fill 11, 1, ' '
                    .ascii " value 1 :"
sMessValue1:       .fill 11, 1, ' '
                    .ascii " value 2 :"
sMessValue2:       .fill 11, 1, ' '
                    .asciz "\n"

szCarriageReturn:  .asciz "\n"
/*********************************/
/* UnInitialized data            */
/*********************************/
.bss 
.align 4
Queue1:                .skip queue_fin      @ queue memory place 
stItem:                .skip value_fin      @ value item memory place
/*********************************/
/*  code section                 */
/*********************************/
.text
.global main 
main:                                       @ entry of program 
    ldr r0,iAdrQueue1                       @ queue structure address
    bl isEmpty
    cmp r0,#0
    beq 1f
    ldr r0,iAdrszMessEmpty
    bl affichageMess                        @ display message empty
    b 2f
1:
    ldr r0,iAdrszMessNotEmpty
    bl affichageMess                        @ display message not empty
2:
    @ init item 1
    ldr r0,iAdrstItem
    mov r1,#1
    str r1,[r0,#value_ident]
    mov r1,#11
    str r1,[r0,#value_value1]
    mov r1,#12
    str r1,[r0,#value_value2]

    ldr r0,iAdrQueue1                       @ queue structure address
    ldr r1,iAdrstItem
    bl pushQueue                            @ add item in queue
    cmp r0,#-1                              @ error ?
    beq 99f
    @ init item 2
    ldr r0,iAdrstItem
    mov r1,#2
    str r1,[r0,#value_ident]
    mov r1,#21
    str r1,[r0,#value_value1]
    mov r1,#22
    str r1,[r0,#value_value2]

    ldr r0,iAdrQueue1                       @ queue structure address
    ldr r1,iAdrstItem
    bl pushQueue                            @ add item in queue
    cmp r0,#-1
    beq 99f
    ldr r0,iAdrQueue1                       @ queue structure address
    bl isEmpty
    cmp r0,#0                               @ not empty
    beq 3f
    ldr r0,iAdrszMessEmpty
    bl affichageMess                        @ display message empty
    b 4f
3:
    ldr r0,iAdrszMessNotEmpty
    bl affichageMess                        @ display message not empty

4:
    ldr r0,iAdrQueue1                       @ queue structure address
    bl popQueue                             @ return address item
    cmp r0,#-1                              @ error ?
    beq 99f
    mov r2,r0                               @ save item pointer 
    ldr r0,[r2,#value_ident]
    ldr r1,iAdrsMessIdent                   @ display ident
    bl conversion10                         @ decimal conversion
    ldr r0,[r2,#value_value1]
    ldr r1,iAdrsMessValue1                  @ display value 1
    bl conversion10                         @ decimal conversion
    ldr r0,[r2,#value_value2]
    ldr r1,iAdrsMessValue2                  @ display value 2
    bl conversion10                         @ decimal conversion
    ldr r0,iAdrszMessResult
    bl affichageMess                        @ display message
    b 4b                                    @ loop

99:
    @ error
    ldr r0,iAdrszMessError
    bl affichageMess       
100:                                        @ standard end of the program 
    mov r0, #0                              @ return code
    mov r7, #EXIT                           @ request to exit program
    svc #0                                  @ perform the system call

iAdrQueue1:               .int Queue1
iAdrstItem:               .int stItem
iAdrszMessError:          .int szMessError
iAdrszMessEmpty:          .int szMessEmpty
iAdrszMessNotEmpty:       .int szMessNotEmpty
iAdrszMessResult:         .int szMessResult
iAdrszCarriageReturn:     .int szCarriageReturn
iAdrsMessIdent:           .int sMessIdent
iAdrsMessValue1:          .int sMessValue1
iAdrsMessValue2:          .int sMessValue2
/******************************************************************/
/*     test if queue empty                                        */ 
/******************************************************************/
/* r0 contains the address of queue structure */
isEmpty:
    push {r1,r2,lr}                         @ save  registres
    ldr r1,[r0,#queue_ptdeb]                @ begin pointer 
    ldr r2,[r0,#queue_ptfin]                @ begin pointer 
    cmp r1,r2
    moveq r0,#1                             @ empty queue
    movne r0,#0                             @ not empty
    pop {r1,r2,lr}                          @ restaur registers 
    bx lr                                   @ return  
/******************************************************************/
/*     add item  in queue                                         */ 
/******************************************************************/
/* r0 contains the address of queue structure */
/* r1 contains the address of item            */
pushQueue:
    push {r1-r4,lr}                         @ save  registres
    add r2,r0,#queue_stvalue                @ address of values structure
    ldr r3,[r0,#queue_ptfin]                @ end pointer
    add r2,r3                               @ free address of queue
    ldr r4,[r1,#value_ident]                @ load ident item
    str r4,[r2,#value_ident]                @ and store in queue
    ldr r4,[r1,#value_value1]               @ idem
    str r4,[r2,#value_value1]
    ldr r4,[r1,#value_value2]
    str r4,[r2,#value_value2]
    add r3,#value_fin
    cmp r3,#value_fin * NBMAXIELEMENTS
    moveq r0,#-1                            @ error
    beq 100f
    str r3,[r0,#queue_ptfin]                @ store new end pointer
100:
    pop {r1-r4,lr}                          @ restaur registers 
    bx lr                                   @ return 
/******************************************************************/
/*     pop queue                                                  */ 
/******************************************************************/
/* r0 contains the address of queue structure */
popQueue:
    push {r1,r2,lr}                         @ save  registres
    mov r1,r0                               @ control if empty queue
    bl isEmpty
    cmp r0,#1                               @ yes -> error
    moveq r0,#-1
    beq 100f
    mov r0,r1
    ldr r1,[r0,#queue_ptdeb]                @ begin pointer 
    add r2,r0,#queue_stvalue                @ address of begin values item
    add r2,r1                               @ address of item
    add r1,#value_fin
    str r1,[r0,#queue_ptdeb]                @ store nex begin pointer
    mov r0,r2                               @ return pointer item
100:
    pop {r1,r2,lr}                          @ restaur registers 
    bx lr                                   @ return  
/******************************************************************/
/*     display text with size calculation                         */ 
/******************************************************************/
/* r0 contains the address of the message */
affichageMess:
    push {r0,r1,r2,r7,lr}                   @ save  registres
    mov r2,#0                               @ counter length 
1:                                          @ loop length calculation 
    ldrb r1,[r0,r2]                         @ read octet start position + index 
    cmp r1,#0                               @ if 0 its over 
    addne r2,r2,#1                          @ else add 1 in the length 
    bne 1b                                  @ and loop 
                                            @ so here r2 contains the length of the message 
    mov r1,r0                               @ address message in r1 
    mov r0,#STDOUT                          @ code to write to the standard output Linux 
    mov r7, #WRITE                          @ code call system "write" 
    svc #0                                  @ call systeme 
    pop {r0,r1,r2,r7,lr}                    @ restaur registers */ 
    bx lr                                   @ return  
/******************************************************************/
/*     Converting a register to a decimal                                 */ 
/******************************************************************/
/* r0 contains value and r1 address area   */
.equ LGZONECAL,   10
conversion10:
    push {r1-r4,lr}                         @ save registers 
    mov r3,r1
    mov r2,#LGZONECAL
1:                                          @ start loop
    bl divisionpar10                        @ r0 <- dividende. quotient ->r0 reste -> r1
    add r1,#48                              @ digit
    strb r1,[r3,r2]                         @ store digit on area
    cmp r0,#0                               @ stop if quotient = 0 
    subne r2,#1                               @ previous position    
    bne 1b                                  @ else loop
                                            @ end replaces digit in front of area
    mov r4,#0
2:
    ldrb r1,[r3,r2] 
    strb r1,[r3,r4]                         @ store in area begin
    add r4,#1
    add r2,#1                               @ previous position
    cmp r2,#LGZONECAL                       @ end
    ble 2b                                  @ loop
    mov r1,#' '
3:
    strb r1,[r3,r4]
    add r4,#1
    cmp r4,#LGZONECAL                       @ end
    ble 3b
100:
    pop {r1-r4,lr}                          @ restaur registres 
    bx lr                                   @return
/***************************************************/
/*   division par 10   signé                       */
/* Thanks to http://thinkingeek.com/arm-assembler-raspberry-pi/*  
/* and   http://www.hackersdelight.org/            */
/***************************************************/
/* r0 dividende   */
/* r0 quotient */	
/* r1 remainder  */
divisionpar10:	
  /* r0 contains the argument to be divided by 10 */
    push {r2-r4}                           @ save registers  */
    mov r4,r0  
    mov r3,#0x6667                         @ r3 <- magic_number  lower
    movt r3,#0x6666                        @ r3 <- magic_number  upper
    smull r1, r2, r3, r0                   @ r1 <- Lower32Bits(r1*r0). r2 <- Upper32Bits(r1*r0) 
    mov r2, r2, ASR #2                     @ r2 <- r2 >> 2
    mov r1, r0, LSR #31                    @ r1 <- r0 >> 31
    add r0, r2, r1                         @ r0 <- r2 + r1 
    add r2,r0,r0, lsl #2                   @ r2 <- r0 * 5 
    sub r1,r4,r2, lsl #1                   @ r1 <- r4 - (r2 * 2)  = r4 - (r0 * 10)
    pop {r2-r4}
    bx lr                                  @ return
Output:

Empty queue.

Not empty queue.

Ident :1 value 1 :11 value 2 :12

Ident :2 value 1 :21 value 2 :22

Error detected !!!!.

ATS[edit]

A common theme in these examples is that there is no runtime error for popping from an empty queue. Instead, you simply cannot compile a program that tries to pop from an empty queue. The type of a queue depends on its size, and you will get a type error if that size is not proven to be nonzero.

One way to get such a proof is with an assertion that calls the is_empty predicate. But the compiler does not insert that check for you, and the example programs do not need it.

A linear linked list as a queue[edit]

(*------------------------------------------------------------------*)

#define ATS_DYNLOADFLAG 0

#include "share/atspre_staload.hats"

staload UN = "prelude/SATS/unsafe.sats"

(*------------------------------------------------------------------*)

vtypedef queue_vt (vt : vt@ype+, n : int) =
  (* A list that forms the queue, and a pointer to its last node. *)
  @(list_vt (vt, n), ptr)

#define NIL list_vt_nil ()
#define :: list_vt_cons

fn {}
queue_vt_nil {vt : vt@ype}
            () :
    queue_vt (vt, 0) =
  @(NIL, the_null_ptr)

fn {}
queue_vt_is_empty
          {n  : int}
          {vt : vt@ype}
          (q  : !queue_vt (vt, n)) :
    [is_empty : bool | is_empty == (n == 0)]
    bool is_empty =
  case+ q.0 of
  | NIL => true
  | _ :: _ => false

fn {vt : vt@ype}
queue_vt_enqueue
          {n : int}
          (q : queue_vt (vt, n),
           x : vt) :
    (* Returns the new queue. *)
    [m : int | 1 <= m; m == n + 1]
    queue_vt (vt, m) =
  let
    val @(lst, tail_ptr) = q
    prval _ = lemma_list_vt_param lst
  in
    case+ lst of
    | ~ NIL =>
      let
        val lst = x :: NIL
        val tail_ptr = $UN.castvwtp1{ptr} lst
      in
        @(lst, tail_ptr)
      end
    | _ :: _ =>
      let
        val old_tail = $UN.castvwtp0{list_vt (vt, 1)} tail_ptr
        val+ @ (hd :: tl) = old_tail

        (* Extend the list by one node, at its tail end. *)
        val new_tail : list_vt (vt, 1) = x :: NIL
        val tail_ptr = $UN.castvwtp1{ptr} new_tail
        prval _ = $UN.castvwtp0{void} tl
        val _ = tl := new_tail

        prval _ = fold@ old_tail
        prval _ = $UN.castvwtp0{void} old_tail

        (* Let us cheat and simply *assert* (rather than prove) that
           the list has grown by one node. *)
        val lst = $UN.castvwtp0{list_vt (vt, n + 1)} lst
      in
        @(lst, tail_ptr)
      end
  end

(* The dequeue routine simply CANNOT BE CALLED with an empty queue.
   It requires a queue of type queue_vt (vt, n) where n is positive. *)
fn {vt : vt@ype}
queue_vt_dequeue
          {n : int | 1 <= n}
          (q : queue_vt (vt, n)) :
    (* Returns a tuple: the dequeued element and the new queue. *)
    [m : int | 0 <= m; m == n - 1]
    @(vt, queue_vt (vt, m)) =
  case+ q.0 of
  | ~ (x :: lst) => @(x, @(lst, q.1))

(* queue_vt is a linear type that must be freed. *)
extern fun {vt : vt@ype}
queue_vt$element_free : vt -> void
fn {vt : vt@ype}
queue_vt_free {n : int}
              (q : queue_vt (vt, n)) :
    void =
  let
    fun
    loop {n : nat} .<n>. (lst : list_vt (vt, n)) : void =
      case+ lst of
      | ~ NIL => begin end
      | ~ (hd :: tl) =>
        begin
          queue_vt$element_free<vt> hd;
          loop tl
        end
    prval _ = lemma_list_vt_param (q.0)
  in
    loop (q.0)
  end

(*------------------------------------------------------------------*)
(* An example: a queue of nonlinear strings. *)

vtypedef strq_vt (n : int) = queue_vt (string, n)

fn {}                  (* A parameterless template, for efficiency. *)
strq_vt_nil () : strq_vt 0 =
  queue_vt_nil ()

fn {}                  (* A parameterless template, for efficiency. *)
strq_vt_is_empty {n : int} (q : !strq_vt n) :
    [is_empty : bool | is_empty == (n == 0)] bool is_empty =
  queue_vt_is_empty<> q

fn
strq_vt_enqueue {n : int} (q : strq_vt n, x : string) :
    [m : int | 1 <= m; m == n + 1] strq_vt m =
  queue_vt_enqueue<string> (q, x)

fn (* Impossible to...     VVVVVV   ...call with an empty queue. *)
strq_vt_dequeue {n : int | 1 <= n} (q : strq_vt n) :
    [m : int | 0 <= m; m == n - 1] @(string, strq_vt m) =
  queue_vt_dequeue<string> q

fn
strq_vt_free {n : int} (q : strq_vt n) : void =
  let
    implement
    queue_vt$element_free<string> x =
      (* A nonlinear string will be allowed to leak. (It might be
         collected as garbage, however.) *)
      begin end
  in
    queue_vt_free<string> q
  end

macdef qnil = strq_vt_nil ()
overload iseqz with strq_vt_is_empty
overload << with strq_vt_enqueue
overload pop with strq_vt_dequeue
overload free with strq_vt_free

implement
main0 () =
  {
    val q = qnil
    val _ = println! ("val q = qnil")
    val _ = println! ("iseqz q = ", iseqz q)
    val _ = println! ("val q = q << \"one\" << \"two\" << \"three\"")
    val q = q << "one" << "two" << "three"
    val _ = println! ("iseqz q = ", iseqz q)
    val _ = println! ("val (x, q) = pop q")
    val (x, q) = pop q
    val _ = println! ("x = ", x)
    val _ = println! ("val (x, q) = pop q")
    val (x, q) = pop q
    val _ = println! ("x = ", x)
    val _ = println! ("val q = q << \"four\"")
    val q = q << "four"
    val _ = println! ("val (x, q) = pop q")
    val (x, q) = pop q
    val _ = println! ("x = ", x)
    val _ = println! ("val (x, q) = pop q")
    val (x, q) = pop q
    val _ = println! ("x = ", x)
    val _ = println! ("iseqz q = ", iseqz q)
    //val (x, q) = pop q  // If you uncomment this you cannot compile!
    val _ = free q
  }

(*------------------------------------------------------------------*)
Output:
$ patscc -O2 -DATS_MEMALLOC_GCBDW queues-postiats.dats -lgc && ./a.out
val q = qnil
iseqz q = true
val q = q << "one" << "two" << "three"
iseqz q = false
val (x, q) = pop q
x = one
val (x, q) = pop q
x = two
val q = q << "four"
val (x, q) = pop q
x = three
val (x, q) = pop q
x = four
iseqz q = true

A nonlinear circular queue with an automatically resizing buffer[edit]

(*------------------------------------------------------------------*)
(*

  The following implementation prevents us from trying to dequeue
  from an empty queue. A program that tries to do so cannot be
  compiled.

  However, it does not prove there are no buffer overruns.

  It contains much embedded C code, for which I used the quick and
  dirty "$extfcall" method.

*)
(*------------------------------------------------------------------*)

#define ATS_DYNLOADFLAG 0

#include "share/atspre_staload.hats"

(*------------------------------------------------------------------*)

(* For the demonstration, let us set BUFSIZE_INITIAL to the minimum
   possible. If you try setting it any lower, though, you cannot
   compile the program. *)
#define BUFSIZE_INITIAL 2

prval _ = prop_verify {2 <= BUFSIZE_INITIAL} ()

datatype queue_t (t : t@ype+,
                  n : int) =
| queue_t_empty (t, 0) of (size_t, ptr)
| {1 <= n}
  queue_t_nonempty (t, n) of
    (size_t, ptr, size_t n, size_t, size_t)

fn
queue_t_new {t : t@ype}
            () : queue_t (t, 0) =
  queue_t_empty (i2sz 0, the_null_ptr)

fn
queue_t_is_empty
          {n : int}
          {t : t@ype}
          (q : queue_t (t, n)) :
    [b : bool | b == (n == 0)]
    bool b =
  case+ q of
  | queue_t_empty _ => true
  | queue_t_nonempty _ => false

fn {t : t@ype}
queue_t_enqueue
          {n : int}
          (q : queue_t (t, n),
           x : t) :
    [m : int | 1 <= m; m == n + 1]
    queue_t (t, m) =
  let
    macdef tsz = sizeof<t>
    macdef zero = i2sz 0
    macdef one = i2sz 1
    var xvar = x
    val px = addr@ xvar
  in
    case+ q of
    | queue_t_empty (bufsize, pbuf) =>
      if bufsize = zero then
        let
          val bufsize = i2sz BUFSIZE_INITIAL
          val pbuf =
            $extfcall (ptr, "ATS_MALLOC", bufsize * tsz)
          val _ = $extfcall (ptr, "memcpy", pbuf, px, tsz)
        in
          queue_t_nonempty (bufsize, pbuf, one, zero, one)
        end
      else
        let
          val _ = $extfcall (ptr, "memcpy", pbuf, px, tsz)
        in
          queue_t_nonempty (bufsize, pbuf, one, zero, one)
        end
    | queue_t_nonempty (bufsize, pbuf, n, ihead, itail) =>
      if n = bufsize then
        let
          (* Resize the buffer. *)
          val bsize = i2sz 2 * bufsize
          val _ = assertloc (itail = ihead)   (* Sanity check. *)
          val _ = assertloc (bufsize < bsize) (* Overflow? *)
          val p = $extfcall (ptr, "ATS_MALLOC", bsize * tsz)
          val _ = $extfcall (ptr, "memcpy", p,
                             ptr_add<t> (pbuf, ihead),
                             (bufsize - ihead) * tsz)
          val _ = $extfcall (ptr, "memcpy",
                             ptr_add<t> (p, bufsize - ihead),
                             pbuf, ihead * tsz)
          val _ = $extfcall (ptr, "memcpy", ptr_add<t> (p, n),
                             px, tsz)
        in
          queue_t_nonempty (bsize, p, succ n, zero, succ n)
        end
      else
        let
          val _ = $extfcall (ptr, "memcpy", ptr_add<t> (pbuf, itail),
                             px, tsz)
          val itail = (succ itail) mod bufsize
        in
          queue_t_nonempty (bufsize, pbuf, succ n, ihead, itail)
        end
  end

fn {t : t@ype}
queue_t_dequeue
          {n : int | 1 <= n}
          (q : queue_t (t, n)) :
    [m : int | m == n - 1]
    @(t, queue_t (t, m)) =
  let
    macdef tsz = sizeof<t>
    macdef zero = i2sz 0
    macdef one = i2sz 1
    var xvar : t
    val px = addr@ xvar
    val queue_t_nonempty (bufsize, pbuf, n, ihead, itail) = q
    val _ = $extfcall (ptr, "memcpy", px, ptr_add<t> (pbuf, ihead),
                       tsz)
    val ihead = (succ ihead) mod bufsize
    val x = $UNSAFE.cast{t} xvar
  in
    if n = one then
      @(x, queue_t_empty (bufsize, pbuf))
    else
      @(x, queue_t_nonempty (bufsize, pbuf, pred n, ihead, itail))
  end

(*------------------------------------------------------------------*)
(* An example: a queue of strings. *)

vtypedef strq_t (n : int) = queue_t (string, n)

fn
strq_t_new () : strq_t 0 =
  queue_t_new ()

fn {}                  (* A parameterless template, for efficiency. *)
strq_t_is_empty {n : int} (q : strq_t n) :
    [is_empty : bool | is_empty == (n == 0)] bool is_empty =
  queue_t_is_empty q

fn
strq_t_enqueue {n : int} (q : strq_t n, x : string) :
    [m : int | 1 <= m; m == n + 1] strq_t m =
  queue_t_enqueue<string> (q, x)

fn (* Impossible to...    VVVVVV   ...call with an empty queue. *)
strq_t_dequeue {n : int | 1 <= n} (q : strq_t n) :
    [m : int | 0 <= m; m == n - 1] @(string, strq_t m) =
  queue_t_dequeue<string> q

overload strq with strq_t_new
overload iseqz with strq_t_is_empty
overload << with strq_t_enqueue
overload pop with strq_t_dequeue

implement
main0 () =
  {
    val q = strq ()
    val _ = println! ("val q = strq ()")
    val _ = println! ("iseqz q = ", iseqz q)
    val _ = println! ("val q = q << \"one\" << \"two\" << \"three\"")
    val q = q << "one" << "two" << "three"
    val _ = println! ("val q = q << \"ett\" << \"två\" << \"tre\"")
    val q = q << "ett" << "två" << "tre"
    val _ = println! ("iseqz q = ", iseqz q)
    val _ = println! ("val (x, q) = pop q")
    val (x, q) = pop q
    val _ = println! ("x = ", x)
    val _ = println! ("val (x, q) = pop q")
    val (x, q) = pop q
    val _ = println! ("x = ", x)
    val _ = println! ("val q = q << \"four\"")
    val q = q << "four"
    val _ = println! ("val (x, q) = pop q")
    val (x, q) = pop q
    val _ = println! ("x = ", x)
    val _ = println! ("val (x, q) = pop q")
    val (x, q) = pop q
    val _ = println! ("x = ", x)
    val _ = println! ("val q = q << \"fyra\"")
    val q = q << "fyra"
    val _ = println! ("val (x, q) = pop q")
    val (x, q) = pop q
    val _ = println! ("x = ", x)
    val _ = println! ("val (x, q) = pop q")
    val (x, q) = pop q
    val _ = println! ("x = ", x)
    val _ = println! ("val (x, q) = pop q")
    val (x, q) = pop q
    val _ = println! ("x = ", x)
    val _ = println! ("val (x, q) = pop q")
    val (x, q) = pop q
    val _ = println! ("x = ", x)
    val _ = println! ("iseqz q = ", iseqz q)
    //val (x, q) = pop q  // If you uncomment this you cannot compile!
  }

(*------------------------------------------------------------------*)
Output:
$ patscc -O2 -DATS_MEMALLOC_GCBDW circular_queues-postiats.dats -lgc && ./a.out
val q = strq ()
iseqz q = true
val q = q << "one" << "two" << "three"
val q = q << "ett" << "två" << "tre"
iseqz q = false
val (x, q) = pop q
x = one
val (x, q) = pop q
x = two
val q = q << "four"
val (x, q) = pop q
x = three
val (x, q) = pop q
x = ett
val q = q << "fyra"
val (x, q) = pop q
x = två
val (x, q) = pop q
x = tre
val (x, q) = pop q
x = four
val (x, q) = pop q
x = fyra
iseqz q = true

AutoHotkey[edit]

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

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

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

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

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

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

AWK[edit]

#!/usr/bin/awk -f

BEGIN {
    delete q
    print "empty? " emptyP()
    print "push " push("a")
    print "push " push("b")
    print "empty? " emptyP()
    print "pop " pop()
    print "pop " pop()
    print "empty? " emptyP()
    print "pop " pop()
}

function push(n) {
    q[length(q)+1] = n
    return n
}

function pop() {
    if (emptyP()) {
        print "Popping from empty queue."
        exit
    }
    r = q[length(q)]
    delete q[length(q)]
    return r
}

function emptyP() {
    return length(q) == 0
}
Output:
 empty? 1
 push a
 push b
 empty? 0
 pop b
 pop a
 empty? 1
 Popping from empty queue.

Batch File[edit]

This solution uses an environment variable naming convention to implement a queue as a pseudo object containing a pseudo dynamic array and head and tail attributes, as well as an empty "method" that is a sort of macro. The implementation depends on delayed expansion being enabled at the time of each call to a queue function. More complex variations can be written that remove this limitation.

@echo off
setlocal enableDelayedExpansion

:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
:: FIFO queue usage

:: Define the queue
call :newQueue myQ

:: Populate the queue
for %%A in (value1 value2 value3) do call :enqueue myQ %%A

:: Test if queue is empty by examining the tail "attribute"
if myQ.tail==0 (echo myQ is empty) else (echo myQ is NOT empty)

:: Peek at the head of the queue
call:peekQueue myQ val && echo a peek at the head of myQueue shows !val!

:: Process the first queue value
call :dequeue myQ val && echo dequeued myQ value=!val!

:: Add some more values to the queue
for %%A in (value4 value5 value6) do call :enqueue myQ %%A

:: Process the remainder of the queue
:processQueue
call :dequeue myQ val || goto :queueEmpty
echo dequeued myQ value=!val!
goto :processQueue
:queueEmpty

:: Test if queue is empty using the empty "method"/"macro". Use of the
:: second IF statement serves to demonstrate the negation of the empty
:: "method". A single IF could have been used with an ELSE clause instead.
if %myQ.empty% echo myQ is empty
if not %myQ.empty% echo myQ is NOT empty
exit /b

:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
:: FIFO queue definition

:newQueue qName
set /a %~1.head=1, %~1.tail=0
:: Define an empty "method" for this queue as a sort of macro
set "%~1.empty=^!%~1.tail^! == 0"
exit /b

:enqueue qName value
set /a %~1.tail+=1
set %~1.!%~1.tail!=%2
exit /b

:dequeue qName returnVar
:: Sets errorlevel to 0 if success
:: Sets errorlevel to 1 if failure because queue was empty
if !%~1.tail! equ 0 exit /b 1
for %%N in (!%~1.head!) do (
  set %~2=!%~1.%%N!
  set %~1.%%N=
)
if !%~1.head! == !%~1.tail! (set /a "%~1.head=1, %~1.tail=0") else set /a %~1.head+=1
exit /b 0

:peekQueue qName returnVar
:: Sets errorlevel to 0 if success
:: Sets errorlevel to 1 if failure because queue was empty
if !%~1.tail! equ 0 exit /b 1
for %%N in (!%~1.head!) do set %~2=!%~1.%%N!
exit /b 0

BBC BASIC[edit]

      FIFOSIZE = 1000
      
      FOR n = 3 TO 5
        PRINT "Push ";n : PROCenqueue(n)
      NEXT
      PRINT "Pop " ; FNdequeue
      PRINT "Push 6" : PROCenqueue(6)
      REPEAT
        PRINT "Pop " ; FNdequeue
      UNTIL FNisempty
      PRINT "Pop " ; FNdequeue
      END
      
      DEF PROCenqueue(n) : LOCAL f%
      DEF FNdequeue : LOCAL f% : f% = 1
      DEF FNisempty : LOCAL f% : f% = 2
      PRIVATE fifo(), rptr%, wptr%
      DIM fifo(FIFOSIZE-1)
      CASE f% OF
        WHEN 0:
          wptr% = (wptr% + 1) MOD FIFOSIZE
          IF rptr% = wptr% ERROR 100, "Error: queue overflowed"
          fifo(wptr%) = n
        WHEN 1:
          IF rptr% = wptr% ERROR 101, "Error: queue empty"
          rptr% = (rptr% + 1) MOD FIFOSIZE
          = fifo(rptr%)
        WHEN 2:
          = (rptr% = wptr%)
      ENDCASE
      ENDPROC
Output:
Push 3
Push 4
Push 5
Pop 3
Push 6
Pop 4
Pop 5
Pop 6
Pop
Error: queue empty

BQN[edit]

Queues are already straightforward to make in BQN via its convenient builtins. This object is made for demonstration of BQN's object oriented features. It would generally be much simpler to apply the related functions to an array instead of creating a big object.

queue ← {
  data ← ⟨⟩
  Push ⇐ {data∾˜↩𝕩}
  Pop ⇐ {
    𝕊𝕩:
    0=≠data ? •Show "Cannot pop from empty queue";
    (data↓˜↩¯1)⊢⊑⌽data
  }
  Empty ⇐ {𝕊𝕩: 0=≠data}
  Display ⇐ {𝕊𝕩: •Show data}
}

q1 ← queue

•Show q1.Empty@
q1.Push 3
q1.Push 4
q1.Display@
•Show q1.Pop@
q1.Display@
1
⟨ 4 3 ⟩
3
⟨ 4 ⟩

It's also possible to build a queue out of linked node objects, an approach discussed in this section of the BQN documentation. While much slower to traverse, this approach opens up new possibilities, such as constant time deletion and insertion at an arbitrary node, that aren't available with plain arrays.

Bracmat[edit]

Below, queue is the name of a class with a data member list and three methods enqueue, dequeue and empty.

No special provision is implemented to "throw and exception" in case you try to dequeue from and empty queue, because, in Bracmat, evaluation of an expression, besides resulting in an evaluated expression, always also either "succeeds" or "fails". (There is, in fact, a third possibility, "ignore", telling Bracmat to close an eye even though an evaluation didn't succeed.) So in the example below, the last dequeue operation fails and the program continues on the right hand side of the bar (|) operator

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

Normally you would seldom use a class as depicted above, because the operations are so simple that you probably use them directly. Bracmat lists allow prepending as well as appending elements, and single elements can be removed from the beginning or from the end of a list.

Appending an element to a long list and removing an element from the end of a long list are quite expensive operations, with a cost O(n), where n is the number of elements in the queue.

C[edit]

Dynamic array[edit]

Dynamic array working as a circular buffer.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef int DATA; /* type of data to store in queue */
typedef struct {
    DATA *buf;
    size_t head, tail, alloc;
} queue_t, *queue;

queue q_new()
{
    queue q = malloc(sizeof(queue_t));
    q->buf = malloc(sizeof(DATA) * (q->alloc = 4));
    q->head = q->tail = 0;
    return q;
}

int empty(queue q)
{
    return q->tail == q->head;
}

void enqueue(queue q, DATA n)
{
    if (q->tail >= q->alloc) q->tail = 0;
    q->buf[q->tail++] = n;
   
    // Fixed bug where it failed to resizes
    if (q->tail == q->alloc) {  /* needs more room */
        q->buf = realloc(q->buf, sizeof(DATA) * q->alloc * 2);
        if (q->head) {
            memcpy(q->buf + q->head + q->alloc, q->buf + q->head,
                sizeof(DATA) * (q->alloc - q->head));
            q->head += q->alloc;
        } else
            q->tail = q->alloc;
        q->alloc *= 2;
    }
}

int dequeue(queue q, DATA *n)
{
    if (q->head == q->tail) return 0;
    *n = q->buf[q->head++];
    if (q->head >= q->alloc) { /* reduce allocated storage no longer needed */
        q->head = 0;
        if (q->alloc >= 512 && q->tail < q->alloc / 2)
            q->buf = realloc(q->buf, sizeof(DATA) * (q->alloc/=2));
    }
    return 1;
}

Doubly linked list[edit]

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

typedef struct node_t node_t, *node, *queue;
struct node_t { int val; node prev, next; };

#define HEAD(q) q->prev
#define TAIL(q) q->next
queue q_new()
{
    node q = malloc(sizeof(node_t));
    q->next = q->prev = 0;
    return q;
}

int empty(queue q)
{
    return !HEAD(q);
}

void enqueue(queue q, int n)
{
    node nd = malloc(sizeof(node_t));
    nd->val = n;
    if (!HEAD(q)) HEAD(q) = nd;
    nd->prev = TAIL(q);
    if (nd->prev) nd->prev->next = nd;
    TAIL(q) = nd;
    nd->next = 0;
}

int dequeue(queue q, int *val)
{
    node tmp = HEAD(q);
    if (!tmp) return 0;
    *val = tmp->val;

    HEAD(q) = tmp->next;
    if (TAIL(q) == tmp) TAIL(q) = 0;
    free(tmp);

    return 1;
}

Test code This main function works with both implementions above.

int main()
{
    int i, n;
    queue q = q_new();

    for (i = 0; i < 100000000; i++) {
        n = rand();
        if (n > RAND_MAX / 2) {
        //  printf("+ %d\n", n);
            enqueue(q, n);
        } else {
            if (!dequeue(q, &n)) {
            //  printf("empty\n");
                continue;
            }
        //  printf("- %d\n", n);
        }
    }
    while (dequeue(q, &n));// printf("- %d\n", n);

    return 0;
}

Of the above two programs, for int types the array method is about twice as fast for the test code given. The doubly linked list is marginally faster than the sys/queue.h below.

sys/queue.h[edit]

Using the sys/queue.h, which is not POSIX.1-2001 (but it is BSD). The example allows to push/pop int values, but instead of int one can use void * and push/pop any kind of "object" (of course changes to the commodity functions m_queue and m_dequeue are needed)

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

#include <sys/queue.h>

struct entry {
  int value;
  TAILQ_ENTRY(entry) entries;
};

typedef struct entry entry_t;

TAILQ_HEAD(FIFOList_s, entry);

typedef struct FIFOList_s FIFOList;


bool m_enqueue(int v, FIFOList *l)
{
  entry_t *val;
  val = malloc(sizeof(entry_t));
  if ( val != NULL ) {
    val->value = v;
    TAILQ_INSERT_TAIL(l, val, entries);
    return true;
  }
  return false;
}

bool m_dequeue(int *v, FIFOList *l)
{
  entry_t *e = l->tqh_first;
  if ( e != NULL ) {
    *v = e->value;
    TAILQ_REMOVE(l, e, entries);
    free(e);
    return true;
  }
  return false;
}

bool isQueueEmpty(FIFOList *l)
{
  if ( l->tqh_first == NULL ) return true;
  return false;
}

C#[edit]

Compatible with C# 3.0 specification, requires System library for exceptions (from either .Net or Mono). A FIFO class in C# using generics and nodes.

public class FIFO<T>
{
  class Node
  {
    public T Item { get; set; }
    public Node Next { get; set; }
  }
  Node first = null;
  Node last = null;
  public void push(T item)
  {
    if (empty())
    {
      //Uses object initializers to set fields of new node
      first = new Node() { Item = item, Next = null };
      last = first;
    }
    else
    {
      last.Next = new Node() { Item = item, Next = null };
      last = last.Next;
    }
  }
  public T pop()
  {
    if (first == null)
      throw new System.Exception("No elements"); 
    if (last == first)
      last = null;
    T temp = first.Item;
    first = first.Next;
    return temp;
  }
  public bool empty()
  {
    return first == null;
  }
}

C++[edit]

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

C++ already has a class queue in the standard library, however the following is a simple implementation based on a singly linkes list. Note that an empty queue is internally represented by head == 0, therefore it doesn't matter that the tail value is invalid in that case.

namespace rosettacode
{
  template<typename T> class queue
  {
  public:
    queue();
    ~queue();
    void push(T const& t);
    T pop();
    bool empty();
  private:
    void drop();
    struct node;
    node* head;
    node* tail;
  };

  template<typename T> struct queue<T>::node
  {
    T data;
    node* next;
    node(T const& t): data(t), next(0) {}
  };

  template<typename T>
   queue<T>::queue():
    head(0)
  {
  }

  template<typename T>
   inline void queue<T>::drop()
  {
    node* n = head;
    head = head->next;
    delete n;
  }

  template<typename T>
   queue<T>::~queue()
  {
    while (!empty())
      drop();
  }

  template<typename T>
   void queue<T>::push(T const& t)
  {
    node*& next = head? tail->next : head;
    next = new node(t);
    tail = next;
  }

  template<typename T>
   T queue<T>::pop()
  {
    T tmp = head->data;
    drop();
    return tmp;
  }

  template<typename T>
   bool queue<T>::empty()
  {
    return head == 0;
  }
}

Clojure[edit]

Clojure has a built-in persistent FIFO queue which can be accessed by referring to clojure.lang.PersistentQueue/EMPTY. Queues are manipulated similarly to Clojure's stacks using peek and pop.

user=> (def empty-queue clojure.lang.PersistentQueue/EMPTY)
#'user/empty-queue
user=> (def aqueue (atom empty-queue))
#'user/aqueue
; Check if queue is empty
user=> (empty? @aqueue)
true
; As with other Clojure data structures, you can add items using conj and into
user=> (swap! aqueue conj 1)
user=> (swap! aqueue into [2 3 4])
user=> (pprint @aqueue)
<-(1 2 3 4)-<
; You can read the head of the queue with peek
user=> (peek @aqueue)
1
; You can remove the head producing a new queue using pop
user=> (pprint (pop @aqueue))
<-(2 3 4)-<
; pop returns a new queue, the original is still intact
user=> (pprint @aqueue)
<-(1 2 3 4)-<
; you can treat a queue as a sequence
user=> (into [] @aqueue)
[1 2 3 4]
; but remember that using rest or next converts the queue to a seq. Compare:
user=> (-> @aqueue rest (conj 5) pprint)
(5 2 3 4)
; with:
user=> (-> @aqueue pop (conj 5) pprint)
<-(2 3 4 5)-<

Here's a link with further documentation Queues in Clojure

CoffeeScript[edit]

# Implement a fifo as an array of arrays, to
# greatly amortize dequeue costs, at some expense of
# memory overhead and insertion time.  The speedup
# depends on the underlying JS implementation, but
# it's significant on node.js.
Fifo = ->
  max_chunk = 512
  arr = [] # array of arrays
  count = 0

  self =
    enqueue: (elem) ->
      if count == 0 or arr[arr.length-1].length >= max_chunk
        arr.push []
      count += 1
      arr[arr.length-1].push elem
    dequeue: (elem) ->
      throw Error("queue is empty") if count == 0
      val = arr[0].shift()
      count -= 1
      if arr[0].length == 0
        arr.shift()
      val
    is_empty: (elem) ->
      count == 0

# test
do ->
  max = 5000000
  q = Fifo()
  for i in [1..max]
    q.enqueue
      number: i

  console.log q.dequeue()
  while !q.is_empty()
    v = q.dequeue()
  console.log v
Output:
> time coffee fifo.coffee 
{ number: 1 }
{ number: 5000000 }

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

Common Lisp[edit]

This defines a queue structure that stores its items in a list, and maintains a tail pointer (i.e., a pointer to the last cons in the list). Note that dequeuing the last item in the queue does not clear the tail pointer—enqueuing into the resulting empty queue will correctly reset the tail pointer.

(defstruct (queue (:constructor %make-queue))
  (items '() :type list)
  (tail '() :type list))

(defun make-queue ()
  "Returns an empty queue."
  (%make-queue))

(defun queue-empty-p (queue)
  "Returns true if the queue is empty."
  (endp (queue-items queue)))

(defun enqueue (item queue)
  "Enqueue item in queue. Returns the queue."
  (prog1 queue
    (if (queue-empty-p queue)
      (setf (queue-items queue) (list item)
            (queue-tail queue) (queue-items queue))
      (setf (cdr (queue-tail queue)) (list item)
            (queue-tail queue) (cdr (queue-tail queue))))))

(defun dequeue (queue)
  "Dequeues an item from queue. Signals an error if queue is empty."
  (if (queue-empty-p queue)
    (error "Cannot dequeue from empty queue.")
    (pop (queue-items queue))))

Component Pascal[edit]

BlackBox Component Builder

MODULE Queue;
IMPORT 
	Boxes;
TYPE
	Instance* = POINTER TO LIMITED RECORD
		size: LONGINT;
		first,last: LONGINT;
		_queue: POINTER TO ARRAY OF Boxes.Box;
	END;
	
	PROCEDURE (self: Instance) Initialize(capacity: LONGINT),NEW;
	BEGIN
		self.size := 0;
		self.first := 0;
		self.last := 0;
		NEW(self._queue,capacity)
	END Initialize;
	
	PROCEDURE New*(capacity: LONGINT): Instance;
	VAR
		aQueue: Instance;
	BEGIN
		NEW(aQueue);aQueue.Initialize(capacity);RETURN aQueue
	END New;
	
	PROCEDURE (self: Instance) IsEmpty*(): BOOLEAN, NEW;
	BEGIN
		RETURN self.size = 0;
	END IsEmpty;
	
	PROCEDURE (self: Instance) Capacity*(): LONGINT, NEW;
	BEGIN
		RETURN LEN(self._queue)
	END Capacity;
	
	PROCEDURE (self: Instance) Size*(): LONGINT, NEW;
	BEGIN
		RETURN self.size
	END Size;
	
	PROCEDURE (self: Instance) IsFull*(): BOOLEAN, NEW;
	BEGIN
		RETURN self.size = self.Capacity()
	END IsFull;
	
	PROCEDURE (self: Instance) Push*(b: Boxes.Box), NEW;
	VAR
		i, j, newCapacity, oldSize: LONGINT;
		queue: POINTER TO ARRAY OF Boxes.Box;
	BEGIN
		INC(self.size);
		self._queue[self.last] := b;
		self.last := (self.last + 1) MOD self.Capacity();
		IF self.IsFull() THEN
			(* grow queue *)
			newCapacity := self.Capacity() + (self.Capacity() DIV 2);
			(* new queue *)
			NEW(queue,newCapacity);
			(* move data from old to new queue *)
			i := self.first; j := 0; oldSize := self.Capacity() - self.first + self.last;
			WHILE (j < oldSize) & (j < newCapacity - 1) DO
				queue[j] := self._queue[i];
				i := (i + 1) MOD newCapacity;INC(j)
			END;
			self._queue := queue;self.first := 0;self.last := j
		END
	END Push;
	
	PROCEDURE (self: Instance) Pop*(): Boxes.Box, NEW;
	VAR
		b: Boxes.Box;
	BEGIN
		ASSERT(~self.IsEmpty());
		DEC(self.size);
		b := self._queue[self.first];
		self._queue[self.first] := NIL;
		self.first := (self.first + 1) MOD self.Capacity();
		RETURN b
	END Pop;
	
END Queue.

Interface extracted from implementation

DEFINITION Queue;

	IMPORT Boxes;

	TYPE
		Instance = POINTER TO LIMITED RECORD 
			(self: Instance) Capacity (): LONGINT, NEW;
			(self: Instance) IsEmpty (): BOOLEAN, NEW;
			(self: Instance) IsFull (): BOOLEAN, NEW;
			(self: Instance) Pop (): Boxes.Box, NEW;
			(self: Instance) Push (b: Boxes.Box), NEW;
			(self: Instance) Size (): LONGINT, NEW
		END;

	PROCEDURE New (capacity: LONGINT): Instance;

END Queue.

Cowgol[edit]

This code should be put in a file called queue.coh, to be used with the Cowgol program at Queue/Usage. The queue is implemented by means of a linked list.

include "strings.coh";
include "malloc.coh";

# Define types. The calling code is expected to provide a QueueData type.
record QueueItem is
    data: QueueData;
    next: [QueueItem];
end record;

record QueueMeta is 
    head: [QueueItem];
    tail: [QueueItem];
end record;

typedef Queue is [QueueMeta]; 
const Q_NONE := 0 as [QueueItem];

# Allocate and free the queue datastructure.
sub MakeQueue(): (q: Queue) is
    q := Alloc(@bytesof QueueMeta) as Queue;
    q.head := Q_NONE;
    q.tail := Q_NONE;
end sub;

sub FreeQueue(q: Queue) is
    var cur := q.head;
    while cur != Q_NONE loop
        var next := cur.next;
        Free(cur as [uint8]);
        cur := next;
    end loop;
    Free(q as [uint8]);
end sub;

# Check if queue is empty.
sub QueueEmpty(q: Queue): (r: uint8) is 
    r := 0;
    if q.head == Q_NONE then
        r := 1;
    end if;
end sub;

# Enqueue and dequeue data. Cowgol has no exceptions, so the calling code
# should check QueueEmpty first.
sub Enqueue(q: Queue, d: QueueData) is
    var item := Alloc(@bytesof QueueItem) as [QueueItem];
    item.data := d;
    item.next := Q_NONE;
    if q.head == Q_NONE then
        q.head := item;
    else
        q.tail.next := item;
    end if;
    q.tail := item;
end sub;

sub Dequeue(q: Queue): (d: QueueData) is
    d := q.head.data;
    var cur := q.head;
    q.head := q.head.next;
    Free(cur as [uint8]);
    if q.head == Q_NONE then
        q.tail := Q_NONE;
    end if;
end sub;

D[edit]

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

Déjà Vu[edit]

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

queue:
	{ :start 0 :end 0 }

enqueue q item:
	set-to q q!end item
	set-to q :end ++ q!end

dequeue q:
	if empty q:
		Raise :value-error "popping from empty queue"
	q! q!start
	delete-from q q!start
	set-to q :start ++ q!start

empty q:
	= q!start q!end

Delphi[edit]

program QueueDefinition;

{$APPTYPE CONSOLE}

uses
  System.Generics.Collections;

type
  TQueue = System.Generics.Collections.TQueue<Integer>;

  TQueueHelper = class helper for TQueue
    function Empty: Boolean;
    function Pop: Integer;
    procedure Push(const NewItem: Integer);
  end;

{ TQueueHelper }

function TQueueHelper.Empty: Boolean;
begin
  Result := count = 0;
end;

function TQueueHelper.Pop: Integer;
begin
  Result := Dequeue;
end;

procedure TQueueHelper.Push(const NewItem: Integer);
begin
  Enqueue(NewItem);
end;

var
  Queue: TQueue;
  i: Integer;

begin
  Queue := TQueue.Create;

  for i := 1 to 1000 do
    Queue.push(i);

  while not Queue.Empty do
    Write(Queue.pop, ' ');
  Writeln;

  Queue.Free;
  Readln;
end.

E[edit]

This uses a linked list representation of queues, hanging onto both ends of the list, except that the next-link reference is an E promise rather than a mutable slot.

Also, according to E design principles, the read and write ends of the queue are separate objects. This has two advantages; first, it implements POLA by allowing only the needed end of the queue to be handed out to its users; second, if the reader end is garbage collected the contents of the queue automatically will be as well (rather than accumulating if the writer continues writing).

def makeQueue() {
  def [var head, var tail] := Ref.promise()

  def writer {
    to enqueue(value) {
      def [nh, nt] := Ref.promise()
      tail.resolve([value, nh])
      tail := nt
    }
  }

  def reader {
    to empty() { return !Ref.isResolved(head) }

    to dequeue(whenEmpty) {
      if (Ref.isResolved(head)) {
        def [value, next] := head
        head := next
        return value
      } else {
        throw.eject(whenEmpty, "pop() of empty queue")
      }
    }
  }
  
  return [reader, writer]
}

EchoLisp[edit]

There is no native queue type in EchoLisp. make-Q implements queues in message passing style, using vector operations. Conversions from-to lists are also provided.

;; put info string in permanent storage for later use
(info 'make-Q 
"usage: (define q (make-Q)) ;  (q '[top | empty? | pop | push value | to-list | from-list list])")

;; make-Q
(define (make-Q)
  (let ((q (make-vector 0)))
    (lambda (message . args)
      (case message
        ((empty?) (vector-empty? q))
        ((top) (if (vector-empty? q) (error  'Q:top:empty q) (vector-ref q 0)))
        ((push) (vector-push q (car args)))
        ((pop) (if (vector-empty? q) (error 'Q:pop:empty q) (vector-shift q)))
        ((to-list) (vector->list q))
        ((from-list) (set! q (list->vector (car args))) q )
        (else (info 'make-Q) (error  "Q:bad message:" message )))))) ; display info if unknown message
        
;; 
(define q (make-Q))
(q 'empty?)  #t
(q 'push 'first)  first
(q 'push 'second)  second
(q 'pop)  first
(q 'pop)  second
(q 'top)
"💬 error: Q:top:empty #()"
(q 'from-list '( 6 7 8))  #( 6 7 8)
(q 'top)  6
(q 'pop)  6
(q 'to-list) (7 8)
(q 'delete)
"💭 error: Q:bad message: delete"

;; save make-Q
(local-put 'make-Q)

Elena[edit]

ELENA 4.x :

import extensions;
 
template queue<T>
{
    T[]  theArray;
    int  theTop;
    int  theTale;
 
    constructor()
    {
        theArray := new T[](8);
        theTop := 0;
        theTale := 0;
    }
 
    bool empty()
        = theTop == theTale;
 
    push(T object)
    {
        if (theTale > theArray.Length)
        {
            theArray := theArray.reallocate(theTale)
        };
 
        theArray[theTale] := object;
 
        theTale += 1
    }
 
    T pop()
    {
        if (theTale == theTop)
            { InvalidOperationException.new:"Queue is empty".raise() };
 
        T item := theArray[theTop];
 
        theTop += 1;
 
        ^ item
    }
}
 
public program()
{
    queue<int> q := new queue<int>();
    q.push(1);
    q.push(2);
    q.push(3);
    console.printLine(q.pop());
    console.printLine(q.pop());
    console.printLine(q.pop());
    console.printLine("a queue is ", q.empty().iif("empty","not empty"));
    console.print("Trying to pop:");
    try
    {
        q.pop()
    }
    catch(Exception e)
    {
        console.printLine(e.Message)
    }
}
Output:
1
2
3
a queue is empty
Trying to pop:Queue is empty

Elisa[edit]

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

component GenericQueue ( Queue, Element );
 type Queue;
      Queue (MaxLength = integer) -> Queue;
      Length( Queue )           -> integer;
      Empty ( Queue )           -> boolean;
      Full ( Queue )            -> boolean;
      Push ( Queue, Element)    -> nothing;
      Pull ( Queue )            -> Element;
begin
      Queue (MaxLength) = Queue:[ MaxLength; length:=0; list=alist(Element) ];
      Length ( queue ) = queue.length;
      Empty ( queue ) = (queue.length <= 0);
      Full ( queue ) = (queue.length >= queue.MaxLength);

      Push ( queue, element ) = 
                   [ exception (Full(queue), "Queue Overflow");
                     queue.length:= queue.length + 1;
                     add (queue.list, element)];
      Pull ( queue ) = 
                   [ exception (Empty(queue), "Queue Underflow");
                     queue.length:= queue.length - 1;
                     remove(first(queue.list))]; 
end component GenericQueue;

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

use GenericQueue (QueueofPersons, Person);
type Person = text;
Q = QueueofPersons(25);

Push (Q, "Peter");
Push (Q, "Alice");
Push (Q, "Edward");
Q?
QueueofPersons:[MaxLength = 25;
                length = 3;
                list = { "Peter", 
                         "Alice", 
                         "Edward"}]
Pull (Q)?
"Peter"

Pull (Q)?
"Alice"

Pull (Q)?
"Edward"

Q?
QueueofPersons:[MaxLength = 25;
                length = 0;
                list = { }]

Pull (Q)?
***** Exception: Queue Underflow

Elixir[edit]

Translation of: Erlang
defmodule Queue do
  def new, do: {Queue, [], []}
  
  def push({Queue, input, output}, x), do: {Queue, [x|input], output}
  
  def pop({Queue, [], []}), do: (raise RuntimeError, message: "empty Queue")
  def pop({Queue, input, []}), do: pop({Queue, [], Enum.reverse(input)})
  def pop({Queue, input, [h|t]}), do: {h, {Queue, input, t}}
  
  def empty?({Queue, [], []}), do: true
  def empty?({Queue, _, _}), do: false
end

Example:

iex(1)> c("queue.ex")
[Queue]
iex(2)> q = Queue.new
{Queue, [], []}
iex(3)> Queue.empty?(q)
true
iex(4)> q2 = Queue.push(q,1)
{Queue, [1], []}
iex(5)> q3 = Queue.push(q2,2)
{Queue, [2, 1], []}
iex(6)> Queue.empty?(q3)
false
iex(7)> Queue.pop(q3)
{1, {Queue, [], [2]}}
iex(8)> {popped, ^q} = Queue.pop(q2)
{1, {Queue, [], []}}
iex(9)> Queue.pop(Queue.new)
** (RuntimeError) empty Queue
    queue.ex:6: Queue.pop/1

Erlang[edit]

The standard way to manage fifo in functional programming is to use a pair of list for the fifo queue, one is the input, the other is the output. When the output is empty just take the input list and reverse it.

-module(fifo).
-export([new/0, push/2, pop/1, empty/1]).

new() -> {fifo, [], []}.

push({fifo, In, Out}, X) -> {fifo, [X|In], Out}.

pop({fifo, [], []}) -> erlang:error('empty fifo');
pop({fifo, In, []}) -> pop({fifo, [], lists:reverse(In)});
pop({fifo, In, [H|T]}) -> {H, {fifo, In, T}}.

empty({fifo, [], []}) -> true;
empty({fifo, _, _}) -> false.

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

ERRE[edit]

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

PROGRAM CLASS_DEMO

CLASS QUEUE

   LOCAL SP
   LOCAL DIM STACK[100]

   FUNCTION ISEMPTY()
      ISEMPTY=(SP=0)
   END FUNCTION

   PROCEDURE INIT
      SP=0
   END PROCEDURE

   PROCEDURE POP(->XX)
      XX=STACK[SP]
      SP=SP-1
   END PROCEDURE

   PROCEDURE PUSH(XX)
      SP=SP+1
      STACK[SP]=XX
   END PROCEDURE

END CLASS

NEW PILA:QUEUE

BEGIN
    PILA_INIT  ! constructor
    FOR N=1 TO 4 DO  ! push 4 numbers
       PRINT("Push";N)
       PILA_PUSH(N)
    END FOR
    FOR I=1 TO 5 DO  ! pop 5 numbers
       IF NOT PILA_ISEMPTY() THEN
           PILA_POP(->N)
           PRINT("Pop";N)
         ELSE
           PRINT("Queue is empty!")
       END IF
    END FOR
    PRINT("* End *")
END PROGRAM
Output:
Push 1
Push 2
Push 3
Push 4
Pop 4
Pop 3
Pop 2
Pop 1
Queue is empty!
* End *

Factor[edit]

Translation of: Java
USING: accessors kernel ;
IN: rosetta-code.queue-definition

TUPLE: queue head tail ;
TUPLE: node value next ;

: <queue> ( -- queue ) queue new ;
: <node> ( obj -- node ) node new swap >>value ;

: empty? ( queue -- ? ) head>> >boolean not ;

: enqueue ( obj queue -- )
    [ <node> ] dip 2dup dup empty?
    [ head<< ] [ tail>> next<< ] if tail<< ;

: dequeue ( queue -- obj )
    dup empty? [ "Cannot dequeue empty queue." throw ] when
    [ head>> value>> ] [ head>> next>> ] [ head<< ] tri ;

Fantom[edit]

class Queue
{
  List queue := [,]

  public Void push (Obj obj)
  {
    queue.add (obj)  // add to right of list
  }

  public Obj pop ()
  {
    if (queue.isEmpty)
      throw (Err("queue is empty"))
    else
    {
      return queue.removeAt(0) // removes left-most item 
    }
  }

  public Bool isEmpty ()
  {
    queue.isEmpty
  }
}

Forth[edit]

This is a FIFO implemented as a circular buffer, as is often found between communicating processes such the interrupt and user parts of a device driver. In practice, the get/put actions would block instead of aborting if the queue is empty/full.

1024 constant size
create buffer size cells allot
here constant end
variable head  buffer head !
variable tail  buffer tail !
variable used       0 used !

: empty?  used @ 0= ;
: full?   used @ size = ;

: next ( ptr -- ptr )
  cell+  dup end = if drop buffer then ;

: put ( n -- )
  full? abort" buffer full"
  \ begin full? while pause repeat
  tail @ !  tail @ next tail !   1 used +! ;

: get ( -- n )
  empty? abort" buffer empty"
  \ begin empty? while pause repeat
  head @ @  head @ next head !  -1 used +! ;

Linked list version[edit]

Using Forth-2012 structure words and ALLOCATE/FREE. In spirit quite similar to the Java variant below, with one difference: Here we use addresses of fields (not possible in Java), which often makes things simpler than in Java (fewer special cases at boundaries), but in this case it does not. Where the Java version has a special case on enqueue, this version has a special case on dequeue:

0
  field: list-next
  field: list-val
constant list-struct

: insert ( x list-addr -- )
    list-struct allocate throw >r
    swap r@ list-val !
    dup @ r@ list-next !
    r> swap ! ;

: remove ( list-addr -- x )
    >r r@ @ ( list-node )
    r@ @ dup list-val @ ( list-node x )
    swap list-next @ r> !
    swap free throw ;

0
  field: queue-last \ points to the last entry (head of the list)
  field: queue-nextaddr \ points to the pointer to the next-inserted entry
constant queue-struct

: init-queue ( queue -- )
    >r 0 r@ queue-last !
    r@ queue-last r> queue-nextaddr ! ;

: make-queue ( -- queue )
    queue-struct allocate throw dup init-queue ;

: empty? ( queue -- f )
    queue-last @ 0= ;

: enqueue ( x queue -- )
    dup >r queue-nextaddr @ insert
    r@ queue-nextaddr @ @ list-next r> queue-nextaddr ! ;

: dequeue ( queue -- x )
    dup empty? abort" dequeue applied to an empty queue"
    dup queue-last remove ( queue x )
    over empty? if
        over init-queue then
    nip ;

Fortran[edit]

Works with: Fortran version 90 and later

See FIFO (usage) for an example of fifo_nodes

module FIFO
  use fifo_nodes
! fifo_nodes must define the type fifo_node, with the two field
! next and valid, for queue handling, while the field datum depends
! on the usage (see [[FIFO (usage)]] for an example)
!  type fifo_node
!     integer :: datum
!     ! the next part is not variable and must be present
!     type(fifo_node), pointer :: next
!     logical :: valid
!  end type fifo_node

  type fifo_head
     type(fifo_node), pointer :: head, tail
  end type fifo_head

contains

  subroutine new_fifo(h)
    type(fifo_head), intent(out) :: h
    nullify(h%head)
    nullify(h%tail)
  end subroutine new_fifo

  subroutine fifo_enqueue(h, n)
    type(fifo_head), intent(inout) :: h
    type(fifo_node), intent(inout), target :: n

    if ( associated(h%tail) ) then
       h%tail%next => n
       h%tail => n
    else
       h%tail => n
       h%head => n
    end if

    nullify(n%next)
  end subroutine fifo_enqueue

  subroutine fifo_dequeue(h, n)
    type(fifo_head), intent(inout) :: h
    type(fifo_node), intent(out), target :: n

    if ( associated(h%head) ) then
       n = h%head
       if ( associated(n%next) ) then
          h%head => n%next
       else
          nullify(h%head)
          nullify(h%tail)
       end if
       n%valid = .true.
    else
       n%valid = .false.
    end if
    nullify(n%next)
  end subroutine fifo_dequeue

  function fifo_isempty(h) result(r)
    logical :: r
    type(fifo_head), intent(in) :: h
    if ( associated(h%head) ) then
       r = .false.
    else
       r = .true.
    end if
  end function fifo_isempty

end module FIFO

Free Pascal[edit]

program queue;
 {$IFDEF FPC}{$MODE DELPHI}{$IFDEF WINDOWS}{$APPTYPE CONSOLE}{$ENDIF}{$ENDIF} 
 {$ASSERTIONS ON} 
uses Generics.Collections;
 
var
  lQueue: TQueue<Integer>;
begin
  lQueue := TQueue<Integer>.Create;
  try
    lQueue.EnQueue(1);
    lQueue.EnQueue(2);
    lQueue.EnQueue(3); 
    Write(lQueue.DeQueue:2);   // 1
    Write(lQueue.DeQueue:2);   // 2
    Writeln(lQueue.DeQueue:2); // 3
    Assert(lQueue.Count = 0, 'Queue is not empty'); // should be empty
  finally
    lQueue.Free;
  end;
end.
Output:
 1 2 3

FreeBASIC[edit]

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

' FB 1.05.0 Win64

' queue_rosetta.bi
' simple generic Queue type

#Define Queue(T) Queue_##T

#Macro Declare_Queue(T)
Type Queue(T)
 Public:
    Declare Constructor()
    Declare Destructor()
    Declare Property capacity As Integer
    Declare Property count As Integer 
    Declare Property empty As Boolean
    Declare Property front As T 
    Declare Function pop() As T   
    Declare Sub push(item As T)
  Private:
    a(any) As T 
    count_ As Integer = 0  
    Declare Function resize(size As Integer) As Integer    
End Type

Constructor Queue(T)()
  Redim a(0 To 0) '' create a default T instance for various purposes 
End Constructor

Destructor Queue(T)()
  Erase a
End Destructor

Property Queue(T).capacity As Integer
  Return UBound(a)
End Property
 
Property Queue(T).count As Integer
  Return count_
End Property

Property Queue(T).empty As Boolean
  Return count_ = 0
End Property

Property Queue(T).front As T
  If count_ > 0 Then
    Return a(1)
  End If
  Print "Error: Attempted to access 'front' element of an empty queue"
  Return a(0)  '' return default element 
End Property

Function Queue(T).pop() As T
  If count_ > 0 Then
    Dim value As T = a(1)
    If count_ > 1 Then  '' move remaining elements to fill space vacated 
      For i As Integer = 2 To count_
        a(i - 1) = a(i)
      Next 
    End If  
    a(count_) = a(0)  '' zero last element
    count_ -= 1
    Return value
  End If
  Print "Error: Attempted to remove 'front' element of an empty queue"
  Return a(0)  '' return default element
End Function

Sub Queue(T).push(item As T)
  Dim size As Integer = UBound(a)
  count_ += 1
  If count_ >  size Then
    size = resize(size)
    Redim Preserve a(0 to size)
  End If
  a(count_) = item   
End Sub

Function Queue(T).resize(size As Integer) As Integer
  If size = 0 Then
    size = 4
  ElseIf size <= 32 Then
    size  = 2 * size
  Else
    size += 32
  End If
  Return size
End Function
#EndMacro

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

' FB 1.05.0 Win64

#Include "queue_rosetta.bi"

Type Cat
  name As String
  age As Integer
  Declare Constructor
  Declare Constructor(name_ As string, age_ As integer)
  Declare Operator Cast() As String
end type

Constructor Cat  '' default constructor
End Constructor

Constructor Cat(name_ As String, age_ As Integer)
  name = name_
  age = age_
End Constructor

Operator Cat.Cast() As String
  Return "[" + name + ", " + Str(age) + "]"
End Operator

Declare_Queue(Cat) '' expand Queue type for Cat instances

Dim CatQueue As Queue(Cat)

Var felix  = Cat("Felix", 8)
Var sheba  = Cat("Sheba", 4)
Var fluffy = Cat("Fluffy", 2)
With CatQueue  '' push these Cat instances into the Queue
  .push(felix)
  .push(sheba)
  .push(fluffy)
End With
Print "Number of Cats in the Queue :" ; CatQueue.count
Print "Capacity of Cat Queue       :" ; CatQueue.capacity
Print "Front Cat                   : "; CatQueue.front
CatQueue.pop()
Print "Front Cat now               : "; CatQueue.front
Print "Number of Cats in the Queue :" ; CatQueue.count
CatQueue.pop()
Print "Front Cat now               : "; CatQueue.front
Print "Number of Cats in the Queue :" ; CatQueue.count
Print "Is Queue empty now          : "; CatQueue.empty
catQueue.pop()
Print "Number of Cats in the Queue :" ; CatQueue.count
Print "Is Queue empty now          : "; CatQueue.empty
catQueue.pop()
Print
Print "Press any key to quit"
Sleep
Output:
Number of Cats in the Queue : 3
Capacity of Cat Queue       : 4
Front Cat                   : [Felix, 8]
Front Cat now               : [Sheba, 4]
Number of Cats in the Queue : 2
Front Cat now               : [Fluffy, 2]
Number of Cats in the Queue : 1
Is Queue empty now          : false
Number of Cats in the Queue : 0
Is Queue empty now          : true
Error: Attempted to remove 'front' element of an empty queue

GAP[edit]

Enqueue := function(v, x)
    Add(v[1], x);
end;
 
Dequeue := function(v)
    if IsEmpty(v[2]) then
        if IsEmpty(v[1]) then
            return fail;
        else
            v[2] := Reversed(v[1]);
            v[1] := [];
        fi;
    fi;
    return Remove(v[2]);
end;


# a new queue
v := [[], []];

Enqueue(v, 3);
Enqueue(v, 4);
Enqueue(v, 5);
Dequeue(v);
# 3
Enqueue(v, 6);
Dequeue(v);   
# 4
Dequeue(v);
# 5
Dequeue(v);
# 6
Dequeue(v);
# fail

Go[edit]

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

package queue

// int queue
// the zero object is a valid queue ready to be used.
// items are pushed at tail, popped at head.
// tail = -1 means queue is full
type Queue struct {
    b []string
    head, tail int
}

func (q *Queue) Push(x string) {
    switch {
    // buffer full. reallocate.
    case q.tail < 0:
        next := len(q.b)
        bigger := make([]string, 2*next)
        copy(bigger[copy(bigger, q.b[q.head:]):], q.b[:q.head])
        bigger[next] = x
        q.b, q.head, q.tail = bigger, 0, next+1
    // zero object. make initial allocation.
    case len(q.b) == 0:
        q.b, q.head, q.tail = make([]string, 4), 0 ,1
        q.b[0] = x
    // normal case
    default:
        q.b[q.tail] = x
        q.tail++
        if q.tail == len(q.b) {
            q.tail = 0
        }
        if q.tail == q.head {
            q.tail = -1
        }
    }
}

func (q *Queue) Pop() (string, bool) {
    if q.head == q.tail {
        return "", false
    }
    r := q.b[q.head]
    if q.tail == -1 {
        q.tail = q.head
    }
    q.head++
    if q.head == len(q.b) {
        q.head = 0
    }
    return r, true
}

func (q *Queue) Empty() bool {
    return q.head == q.tail
}

Groovy[edit]

Solution:

class Queue {
    private List buffer

    public Queue(List buffer =  new LinkedList()) {
        assert buffer != null
        assert buffer.empty
        this.buffer = buffer
    }

    def push (def item) { buffer << item }
    final enqueue = this.&push
    
    def pop() {
        if (this.empty) throw new NoSuchElementException('Empty Queue')
        buffer.remove(0)
    }
    final dequeue = this.&pop
    
    def getEmpty() { buffer.empty }
    
    String toString() { "Queue:${buffer}" }
}

Test:

def q = new Queue()
assert q.empty

['Crosby', 'Stills'].each { q.push(it) }
assert !q.empty
['Nash', 'Young'].each { q.enqueue(it) }
println q
assert !q.empty
assert q.pop() == 'Crosby'
println q
assert !q.empty
assert q.dequeue() == 'Stills'
println q
assert !q.empty
assert q.pop() == 'Nash'
println q
assert !q.empty
q.push('Crazy Horse')
println q
assert q.dequeue() == 'Young'
println q
assert !q.empty
assert q.pop() == 'Crazy Horse'
println q
assert q.empty
try { q.pop() } catch (NoSuchElementException e) { println e }
try { q.dequeue() } catch (NoSuchElementException e) { println e }
Output:
Queue:[Crosby, Stills, Nash, Young]
Queue:[Stills, Nash, Young]
Queue:[Nash, Young]
Queue:[Young]
Queue:[Young, Crazy Horse]
Queue:[Crazy Horse]
Queue:[]
java.util.NoSuchElementException: Empty Queue
java.util.NoSuchElementException: Empty Queue

Haskell[edit]

The standard way to manage fifo in functional programming is to use a pair of list for the fifo queue, one is the input, the other is the output. When the output is empty just take the input list and reverse it.

data Fifo a = F [a] [a]

emptyFifo :: Fifo a
emptyFifo = F [] []

push :: Fifo a -> a -> Fifo a
push (F input output) item = F (item:input) output

pop :: Fifo a -> (Maybe a, Fifo a)
pop (F input (item:output)) = (Just item, F input output)
pop (F []    []           ) = (Nothing, F [] [])
pop (F input []           ) = pop (F [] (reverse input))

isEmpty :: Fifo a -> Bool
isEmpty (F [] []) = True
isEmpty _         = False

Icon and Unicon[edit]

Icon[edit]

The following works in both Icon and Unicon:

# Use a record to hold a Queue, using a list as the concrete implementation
record Queue(items)

procedure make_queue ()
  return Queue ([])
end

procedure queue_push (queue, item)
  put (queue.items, item)
end

# if the queue is empty, this will 'fail' and return nothing
procedure queue_pop (queue)
  return pop (queue.items)
end

procedure queue_empty (queue)
  return *queue.items = 0
end

# procedure to test class
procedure main ()
  queue := make_queue()

  # add the numbers 1 to 5
  every (item := 1 to 5) do 
    queue_push (queue, item)
  
  # pop them in the added order, and show a message when queue is empty
  every (1 to 6) do {
    write ("Popped value: " || queue_pop (queue))
    if (queue_empty (queue)) then write ("empty queue")
  }
end
Output:
Popped value: 1
Popped value: 2
Popped value: 3
Popped value: 4
Popped value: 5
empty queue
empty queue

Unicon[edit]

Unicon also provides classes:

# Use a class to hold a Queue, with a list as the concrete implementation
class Queue (items)
  method push (item)
    put (items, item)
  end

  # if the queue is empty, this will 'fail' and return nothing
  method take ()
    return pop (items)
  end

  method is_empty ()
    return *items = 0
  end

  initially () # initialises the field on creating an instance
    items := []
end

procedure main ()
  queue := Queue ()

  every (item := 1 to 5) do 
    queue.push (item)
  
  every (1 to 6) do {
    write ("Popped value: " || queue.take ())
    if queue.is_empty () then write ("empty queue")
  }
end

Produces the same output as above.

J[edit]

Object oriented technique, using mutable state:

queue_fifo_=: ''

pop_fifo_=: verb define
  r=. {. ::] queue
  queue=: }.queue
  r
)

push_fifo_=: verb define
  queue=: queue,y
  y
)

isEmpty_fifo_=: verb define
  0=#queue
)

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

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

make_empty =: a: , a: [ ]
onto       =: [ ; }.@]

notnull    =: 0 ~: #

See also FIFO (usage)#J

Java[edit]

Works with: Java version 1.5+

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

public class Queue<E>{
    Node<E> head = null, tail = null;

    static class Node<E>{
        E value;
        Node<E> next;

        Node(E value, Node<E> next){
            this.value= value;
            this.next= next;
        }

    }

    public Queue(){
    }

    public void enqueue(E value){ //standard queue name for "push"
        Node<E> newNode= new Node<E>(value, null);
        if(empty()){
            head= newNode;
        }else{
            tail.next = newNode;
        }
        tail= newNode;
    }

    public E dequeue() throws java.util.NoSuchElementException{//standard queue name for "pop"
        if(empty()){
            throw new java.util.NoSuchElementException("No more elements.");
        }
        E retVal= head.value;
        head= head.next;
        return retVal;
    } 

    public boolean empty(){
        return head == null;
    }
}

JavaScript[edit]

Most of the time, the built-in Array suffices. However, if you explicitly want to limit the usage to FIFO operations, it's easy to implement such a constructor.

Using built-in Array[edit]

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

Custom constructor function[edit]

function FIFO() {
    this.data = new Array();

    this.push  = function(element) {this.data.push(element)}
    this.pop   = function() {return this.data.shift()}
    this.empty = function() {return this.data.length == 0}

    this.enqueue = this.push;
    this.dequeue = this.pop;
}

jq[edit]

Note that since jq is a purely functional language, the entity representing a queue must be presented as an input to any function that is to operate on it.

The definition of pop as given below is idiomatic in jq but implies that popping an empty queue yields [null, []] rather than an error. An alternative definition, pop_or_error, is also given to illustrate how an error condition can be generated.

# An empty queue:
def fifo: [];

def push(e): [e] + .;

def pop: [.[0], .[1:]];

def pop_or_error: if length == 0 then error("pop_or_error") else pop end;

def empty: length == 0;

Examples:

fifo | pop  # produces [null,[]]

fifo
 | push(42) # enqueue
 | push(43) # enqueue
 | pop      # dequeue
 | .[0]     # the value
# produces 43

fifo|push(1) as $q1
| fifo|push(2) as $q2
| [($q1|pop|.[0]), ($q2|pop|.[0])]
# produces: [1, 2]

Julia[edit]

Julia provides a variety of queue-like methods for vectors, making the solution to this task rather straightforward. Define a Queue in terms of a one dimensional array, and provide its methods using the appropriate vector operations. To adhere to Julia naming conventions, the queue operations are named "push!", "pop!" and "isempty" rather than "push", "pop" and "empty".

struct Queue{T}
    a::Array{T,1}
end

Queue() = Queue(Any[])
Queue(a::DataType) = Queue(a[])
Queue(a) = Queue(typeof(a)[])

Base.isempty(q::Queue) = isempty(q.a)

function Base.pop!(q::Queue{T}) where {T}
    !isempty(q) || error("queue must be non-empty")
    pop!(q.a)
end

function Base.push!(q::Queue{T}, x::T) where {T}
    pushfirst!(q.a, x)
    return q
end

function Base.push!(q::Queue{Any}, x::T) where {T}
    pushfirst!(q.a, x)
    return q
end
Output:

It is easiest to use the REPL to show a Queue in action.

julia> q = Queue()
Queue{Any}(Any[])

julia> isempty(q)
true

julia> push!(q, 1)
Queue{Any}(Any[1])

julia> isempty(q)
false

julia> push!(q, "two")
Queue{Any}(Any["two",1])

julia> push!(q, 3.0)
Queue{Any}(Any[3.0,"two",1])

julia> push!(q, false)
Queue{Any}(Any[false,3.0,"two",1])

julia> pop!(q)
1

julia> pop!(q)
"two"

julia> pop!(q)
3.0

julia> pop!(q)
false

julia> pop!(q)
ERROR: queue must be non-empty
Stacktrace:
 [1] error(s::String)
   @ Base ./error.jl:33
 [2] pop!(q::Queue{Any})
   @ Main /tmp/cmdline_1648668849_nacnudus/lines.jl:3
 [3] top-level scope
   @ REPL[11]:1

Klingphix[edit]

{ include ..\Utilitys.tlhy }
"..\Utilitys.tlhy" load

 
:push!   { l i -- l&i }
    0 put
;
 
:empty?  { l -- flag }
    len not   { len 0 equal }
;
 
:pop!    { l -- l-1 }
    empty? (
        ["Empty"]
        [pop swap]
    ) if
;
 
 
( ) { empty queue }
 
1 push! 2 push! 3 push!
pop! ? pop! ? pop! ? pop! ?

"End " input
Output:
1
2
3
Empty
End

Kotlin[edit]

// version 1.1.2

import java.util.LinkedList

class Queue<E> {
    private val data = LinkedList<E>()

    val size get() = data.size

    val empty get() = size == 0

    fun push(element: E) = data.add(element)

    fun pop(): E {
        if (empty) throw RuntimeException("Can't pop elements from an empty queue")
        return data.removeFirst()
    }

   val top: E
        get() {
            if (empty) throw RuntimeException("Empty queue can't have a top element")
            return data.first()
        }

    fun clear() = data.clear()

    override fun toString() = data.toString()
}

fun main(args: Array<String>) {
    val q = Queue<Int>()
    (1..5).forEach { q.push(it) }
    println(q)
    println("Size of queue = ${q.size}")
    print("Popping: ")
    (1..3).forEach { print("${q.pop()} ") }
    println("\nRemaining in queue: $q")
    println("Top element is now ${q.top}")
    q.clear()
    println("After clearing, queue is ${if(q.empty) "empty" else "not empty"}")
    try {
        q.pop()
    }
    catch (e: Exception) {
        println(e.message)
    }
}
Output:
[1, 2, 3, 4, 5]
Size of queue = 5
Popping: 1 2 3
Remaining in queue: [4, 5]
Top element is now 4
After clearing, queue is empty
Can't pop elements from an empty queue

LabVIEW[edit]

This image is a VI Snippet, an executable image of LabVIEW code. The LabVIEW version is shown on the top-right hand corner. You can download it, then drag-and-drop it onto the LabVIEW block diagram from a file browser, and it will appear as runnable, editable code.
LabVIEW Queue Definition.png

Lasso[edit]

Definition:

define myqueue => type {
    data store = list
    
    public onCreate(...) => {
        if(void != #rest) => {
            with item in #rest do .`store`->insert(#item)
        }
    }

    public push(value) => .`store`->insertLast(#value)

    public pop => {
        handle => {
            .`store`->removefirst
        }

        return .`store`->first
    }

    public isEmpty => (.`store`->size == 0)
}

Usage:

local(q) = myqueue('a')
#q->isEmpty
// => false

#q->push('b')
#q->pop
// => a
#q->pop
// => b

#q->isEmpty
// => true
#q->pop
// => void

Lua[edit]

Queue = {}

function Queue.new()
    return { first = 0, last = -1 }
end
    
function Queue.push( queue, value )
    queue.last = queue.last + 1
    queue[queue.last] = value
end

function Queue.pop( queue )
    if queue.first > queue.last then
        return nil
    end
    
    local val = queue[queue.first]
    queue[queue.first] = nil
    queue.first = queue.first + 1
    return val
end

function Queue.empty( queue )
    return queue.first > queue.last
end

M2000 Interpreter[edit]

A Stack object can be used as LIFO or FIFO. Data push to bottom of stack. Read pop a value to a variable from top of stack.

Module Checkit {
      a=Stack
      Stack a {
            Data 100,200, 300
      }
      Stack a {
            While not empty {
                  Read N
                  Print N
            }
      }
}
Checkit

Mathematica/Wolfram Language[edit]

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

MATLAB / Octave[edit]

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

myfifo = {};
   
% push 
myfifo{end+1} = x; 

% pop
x = myfifo{1};  myfifo{1} = [];

% empty 
isempty(myfifo)

Below is another solution, that encapsulates the fifo within the object-orientated "class" elements supported by Matlab. For this to work it must be saved in a file named "FIFOQueue.m" in a folder named "@FIFOQueue" in your current Matlab directory.

%This class impliments a standard FIFO queue.
classdef FIFOQueue
    
    properties  
        queue
    end
    
    methods
         
        %Class constructor
        function theQueue = FIFOQueue(varargin)
            
            if isempty(varargin) %No input arguments
                
                %Initialize the queue state as empty
                theQueue.queue = {};
            elseif (numel(varargin) > 1) %More than 1 input arg
                
                %Make the queue the list of input args
                theQueue.queue = varargin;
            elseif iscell(varargin{:}) %If the only input is a cell array
                
                %Make the contents of the cell array the elements in the queue 
                theQueue.queue = varargin{:};
            else %There is one input argument that is not a cell
                
                %Make that one arg the only element in the queue
                theQueue.queue = varargin;
            end
            
        end        
        
        %push() - pushes a new element to the end of the queue
        function push(theQueue,varargin)
            
            if isempty(varargin)
                theQueue.queue(end+1) = {[]};
            elseif (numel(varargin) > 1) %More than 1 input arg
                
                %Make the queue the list of input args
                theQueue.queue( end+1:end+numel(varargin) ) = varargin;
            elseif iscell(varargin{:}) %If the only input is a cell array
                
                %Make the contents of the cell array the elements in the queue 
                theQueue.queue( end+1:end+numel(varargin{:}) ) = varargin{:};
            else %There is one input argument that is not a cell
                
                %Make that one arg the only element in the queue
                theQueue.queue{end+1} = varargin{:};                
            end
            
            %Makes changes to the queue permanent
            assignin('caller',inputname(1),theQueue);  
            
        end
        
        %pop() - pops the first element off the queue
        function element = pop(theQueue)
           
            if empty(theQueue)
                error 'The queue is empty'
            else
                %Returns the first element in the queue
                element = theQueue.queue{1};
                
                %Removes the first element from the queue
                theQueue.queue(1) = [];
                
                %Makes changes to the queue permanent
                assignin('caller',inputname(1),theQueue);
            end
        end
        
        %empty() - Returns true if the queue is empty
        function trueFalse = empty(theQueue)
           
            trueFalse = isempty(theQueue.queue);
            
        end
        
    end %methods
end

Sample usage:

>> myQueue = FIFOQueue({'hello'})
 
myQueue =
 
    FIFOQueue

>> push(myQueue,'jello')
>> pop(myQueue)

ans =

hello

>> pop(myQueue)

ans =

jello

>> pop(myQueue)
??? Error using ==> FIFOQueue.FIFOQueue>FIFOQueue.pop at 61
The queue is empty

Maxima[edit]

defstruct(queue(in=[], out=[]))$

enqueue(x, q) := (q@in: cons(x, q@in), done)$

dequeue(q) := (if not emptyp(q@out) then first([first(q@out), q@out: rest(q@out)])
elseif emptyp(q@in) then 'fail
else (q@out: reverse(q@in), q@in: [], first([first(q@out), q@out: rest(q@out)])))$

q:new(queue);    /* queue([], []) */
enqueue(1, q)$
enqueue(2, q)$
enqueue(3, q)$
dequeue(q);      /* 1 */
enqueue(4, q)$
dequeue(q);      /* 2 */
dequeue(q);      /* 3 */
dequeue(q);      /* 4 */
dequeue(q);      /* fail */

Nanoquery[edit]

This is a fully-featured FIFO queue class definition. In addition to the functions required by the task, it also demonstrates redefining operators for Nanoquery classes by redefining +, *, and =.

class FIFO
	declare contents

	// define constructors for FIFO objects
	def FIFO()
		this.contents = {}
	end
	def FIFO(contents)
		this.contents = contents
	end

	// define methods for this class
	def push(value)
		contents.append(value)
	end
	def pop()
		if !this.empty()
			value = contents[len(contents) - 1]
			contents.remove(len(contents) - 1)
			return value
		else
			// we could throw our own exception here but
			// we'll return null instead
			return null
		end
	end
	def length()
		return len(contents)
	end
	def extend(itemlist)
		contents += itemlist
	end
	def empty()
		return len(contents) = 0
	end

	// define operators for this class
	def toString()
		return str(contents)
	end
	def operator+(other)
		return this.contents + other.contents
	end
	def operator*(n)
		return this.contents * n
	end
	def operator=(other)
		return this.contents = other.contents
	end
end

NetRexx[edit]

Unlike Rexx, NetRexx does not include built–in support for queues but the language's ability to access the Java SDK permits use of any number of Java's "Collection" classes. The following sample implements a stack via the ArrayDeque double–ended queue.

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

mqueue = ArrayDeque()

viewQueue(mqueue)

a = "Fred"
mqueue.push('')      /* Puts an empty line onto the queue */
mqueue.push(a 2)     /* Puts "Fred 2" onto the queue */
viewQueue(mqueue)

a = "Toft"
mqueue.add(a 2)      /* Enqueues "Toft 2" */
mqueue.add('')       /* Enqueues an empty line behind the last */
viewQueue(mqueue)

loop q_ = 1 while mqueue.size > 0
  parse mqueue.pop.toString line
  say q_.right(3)':' line
  end q_
viewQueue(mqueue)

return

method viewQueue(mqueue = Deque) private static

   If mqueue.size = 0 then do
    Say 'Queue is empty'
    end
  else do
    Say 'There are' mqueue.size 'elements in the queue'
    end

  return
Queue is empty
There are 2 elements in the queue
There are 4 elements in the queue
  1: Fred 2
  2: 
  3: Toft 2
  4: 
Queue is empty

Nim[edit]

type

  Node[T] = ref object
    value: T
    next: Node[T]

  Queue*[T] = object
    head, tail: Node[T]
    length: Natural

func initQueue*[T](): Queue[T] = Queue[T]()

func len*(queue: Queue): Natural =
  queue.length

func isEmpty*(queue: Queue): bool {.inline.} =
  queue.len == 0

func push*[T](queue: var Queue[T]; value: T) =
  let node = Node[T](value: value, next: nil)
  if queue.isEmpty: queue.head = node
  else: queue.tail.next = node
  queue.tail = node
  inc queue.length

func pop*[T](queue: var Queue[T]): T =
  if queue.isEmpty:
    raise newException(ValueError, "popping from empty queue.")
  result = queue.head.value
  queue.head = queue.head.next
  dec queue.length
  if queue.isEmpty: queue.tail = nil


when isMainModule:

  var fifo = initQueue[int]()

  fifo.push(26)
  fifo.push(99)
  fifo.push(2)
  echo "Fifo size: ", fifo.len()
  try:
    echo "Popping: ", fifo.pop()
    echo "Popping: ", fifo.pop()
    echo "Popping: ", fifo.pop()
    echo "Popping: ", fifo.pop()
  except ValueError:
    echo "Exception catched: ", getCurrentExceptionMsg()
Output:
Fifo size: 3
Popping: 26
Popping: 99
Popping: 2
Exception catched: popping from empty queue.

OCaml[edit]

The standard way to manage fifo in functional programming is to use a pair of list for the fifo queue, one is the input, the other is the output. When the output is empty just take the input list and reverse it.

module FIFO : sig
  type 'a fifo
  val empty: 'a fifo
  val push: fifo:'a fifo -> item:'a -> 'a fifo
  val pop: fifo:'a fifo -> 'a * 'a fifo
  val is_empty: fifo:'a fifo -> bool
end = struct
  type 'a fifo = 'a list * 'a list
  let empty = [], []
  let push ~fifo:(input,output) ~item = (item::input,output)
  let is_empty ~fifo =
    match fifo with
    | [], [] -> true
    | _ -> false
  let rec pop ~fifo =
    match fifo with
    | input, item :: output -> item, (input,output)
    | [], [] -> failwith "empty fifo"
    | input, [] -> pop ([], List.rev input)
end

and a session in the top-level:

# open FIFO;;
# let q = empty ;;
val q : '_a FIFO.fifo = <abstr>
# is_empty q ;;
- : bool = true
# let q = push q 1 ;;
val q : int FIFO.fifo = <abstr>
# is_empty q ;;
- : bool = false

# let q =
    List.fold_left push q [2;3;4] ;;
val q : int FIFO.fifo = <abstr>

# let v, q = pop q ;;
val v : int = 1
val q : int FIFO.fifo = <abstr>
# let v, q = pop q ;;
val v : int = 2
val q : int FIFO.fifo = <abstr>
# let v, q = pop q ;;
val v : int = 3
val q : int FIFO.fifo = <abstr>
# let v, q = pop q ;;
val v : int = 4
val q : int FIFO.fifo = <abstr>
# let v, q = pop q ;;
Exception: Failure "empty fifo".

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

Oforth[edit]

If queue is empty, null is returned.

Object Class new: Queue(mutable l)

Queue method: initialize  ListBuffer new := l ;
Queue method: empty       @l isEmpty ;
Queue method: push        @l add ;
Queue method: pop         @l removeFirst ;

OxygenBasic[edit]

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

 
'==========
Class Queue
'==========

 'FIRST IN FIRST OUT

bstring buf 'buffer to hold queue content
int    bg   'buffer base offset
int i       'indexer
int le      'length of buffer

method constructor()
====================
buf=""
le=0
bg=0
i=0
end method

method destructor()
===================
del buf
le=0
bg=0
i=0
end method
 
method Encodelength(int ls)
===========================
int p at (i+strptr buf)
p=ls
i+=sizeof int
end method
 
method push(string s)
=====================
int ls=len s
if i+ls+8>le then
  buf+=nuls 8000+ls*2 'extend buf
  le=len buf
end if
EncodeLength ls 'length of input s
mid buf,i+1,s   'append input s
i+=ls
end method
 
method popLength() as int
=========================
if bg>=i then return -1 'buffer empty
int p at (bg+strptr buf)
bg+=sizeof int
return p
end method
 
method pop(string *s) as int
============================
int ls=popLength
if ls<0 then s="" : return ls 'empty buffer
s=mid buf,bg+1,ls
bg+=ls
'cleanup buffer
if bg>1e6 then
  buf=mid buf,bg+1
  le=len buf 
  i-=bg 'shrink buf
  bg=0
end if
end method
 
method clear()
==============
buf=""
le=0
bg=0
i=0
end method
 
end class 'Queue


'====
'DEMO
'====
 
new Queue  fifo
string     s
'
fifo.push "HumptyDumpty"
fifo.push "Sat on a wall"
'
int er
do
  er=fifo.pop s
  if er then print "(buffer empty)" : exit do
  print s
loop

del fifo

Oz[edit]

The semantics of the built-in "Port" datatype is essentially that of a thread-safe queue. We can implement the specified queue type as operations on a pair of a port and a mutable reference to the current read position of the associated stream.

It seems natural to make "Pop" a blocking operation (i.e. it waits for a new value if the queue is currently empty).

The implementation is thread-safe if there is only one reader thread. When multiple reader threads exist, it is possible that a value is popped more than once.

declare
  fun {NewQueue}
     Stream
     WritePort = {Port.new Stream}
     ReadPos = {NewCell Stream}
  in
     WritePort#ReadPos
  end

  proc {Push WritePort#_ Value}
     {Port.send WritePort Value}
  end

  fun {Empty _#ReadPos}
     %% the queue is empty if the value at the current
     %% read position is not determined
     {Not {IsDet @ReadPos}}
  end

  fun {Pop _#ReadPos}
     %% blocks if empty
     case @ReadPos of X|Xr then
        ReadPos := Xr
        X
     end
  end

  Q = {NewQueue}
in
  {Show {Empty Q}}
  {Push Q 42}
  {Show {Empty Q}}
  {Show {Pop Q}}
  {Show {Empty Q}}

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

Pascal[edit]

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

This program should be Standard Pascal compliant (i.e. it doesn't make use of the advanced/non-standard features of FreePascal or GNU Pascal).

program fifo(input, output);

type
 pNode = ^tNode;
 tNode = record
          value: integer;
          next:  pNode;
         end;

 tFifo = record
          first, last: pNode;
         end;           

procedure initFifo(var fifo: tFifo);
 begin
  fifo.first := nil;
  fifo.last := nil
 end;

procedure pushFifo(var fifo: tFifo; value: integer);
 var
  node: pNode;
 begin
  new(node);
  node^.value := value;
  node^.next := nil;
  if fifo.first = nil
   then
    fifo.first := node
   else
    fifo.last^.next := node;
  fifo.last := node
 end;

function popFifo(var fifo: tFifo; var value: integer): boolean;
 var
  node: pNode;
 begin
  if fifo.first = nil
   then
    popFifo := false
   else
    begin
     node := fifo.first;
     fifo.first := fifo.first^.next;
     value := node^.value;
     dispose(node);
     popFifo := true
    end
 end;

procedure testFifo;
 var
  fifo: tFifo;
 procedure testpop(expectEmpty: boolean; expectedValue: integer);
  var
   i: integer;
  begin
   if popFifo(fifo, i)
    then
     if expectEmpty
      then
       writeln('Error! Expected empty, got ', i, '.')
      else
       if i = expectedValue
        then
         writeln('Ok, got ', i, '.')
        else
         writeln('Error! Expected ', expectedValue, ', got ', i, '.')
    else
     if expectEmpty
       then
        writeln('Ok, fifo is empty.')
       else
        writeln('Error! Expected ', expectedValue, ', found fifo empty.')
  end;
 begin
  initFifo(fifo);
  pushFifo(fifo, 2);
  pushFifo(fifo, 3);
  pushFifo(fifo, 5);
  testpop(false, 2);
  pushFifo(fifo, 7);
  testpop(false, 3);
  testpop(false, 5);
  pushFifo(fifo, 11);
  testpop(false, 7);
  testpop(false, 11);
  pushFifo(fifo, 13);
  testpop(false, 13);
  testpop(true, 0);
  pushFifo(fifo, 17);
  testpop(false, 17);
  testpop(true, 0)
 end;

begin
 writeln('Testing fifo implementation ...');
 testFifo;
 writeln('Testing finished.')
end.

Perl[edit]

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

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

Example:

my @fifo=qw(1 2 3 a b c);

mypush @fifo, 44, 55, 66;
mypop @fifo for 1 .. 6+3;
mypop @fifo; #empty now

Phix[edit]

with javascript_semantics
sequence queue = {}
 
procedure push_item(object what)
    queue = append(queue,what)
end procedure
 
function pop_item()
    object what = queue[1]
    queue = queue[2..$]
    return what
end function
 
function empty()
    return length(queue)=0
end function

As of 1.0.2 there are standard builtins for the above, named new_queue(), push(), and queue_empty() respectively, see docs.

Phixmonti[edit]

include ..\Utilitys.pmt

def push    /# l i -- l&i #/
    0 put
enddef

def empty?  /# l -- flag #/
    len 0 ==
enddef

def pop     /# l -- l-1 #/
    empty? if
        "Empty"
    else
        head swap tail nip swap
    endif
enddef


( ) /# empty queue #/

1 push 2 push 3 push
pop ? pop ? pop ? pop ?

PHP[edit]

class Fifo {
  private $data = array();
  public function push($element){
    array_push($this->data, $element);
  }
  public function pop(){
    if ($this->isEmpty()){
      throw new Exception('Attempt to pop from an empty queue');
    }
    return array_shift($this->data);
  }

  //Alias functions
  public function enqueue($element) { $this->push($element); }
  public function dequeue() { return $this->pop(); }

  //Note: PHP prevents a method name of 'empty'
  public function isEmpty(){
    return empty($this->data);
  }
}

Example:

$foo = new Fifo();
$foo->push('One');
$foo->enqueue('Two');
$foo->push('Three');

echo $foo->pop();     //Prints 'One'
echo $foo->dequeue(); //Prints 'Two'
echo $foo->pop();     //Prints 'Three'
echo $foo->pop();     //Throws an exception

Picat[edit]

First variant[edit]

go =>
  println("Test 1"),
  queue_test1,
  nl.

empty(Q) => Q = [].
push(Queue, Value) = Q2 =>
  Q2 = [Value] ++ Queue.
pop(Q,_) = _, Q==[] ; var(Q) => 
   throw $error(empty_queue,pop,'Q'=Q).
pop(Queue,Q2) = Queue.last() =>
   Q2 = [Queue[I] : I in 1..Queue.len-1].

queue_test1 =>
  
    % create an empty queue
    println("Start test 2"),
    empty(Q0),
    printf("Create queue %w%n%n", Q0),
 
    % add numbers 1 and 2
    println("Add numbers 1 and 2 : "),
    Q1 = Q0.push(1),
    Q2 = Q1.push(2),
 
    % display queue
    printf("Q2: %w\n\n", Q2),
 
    % pop element
    V = Q2.pop(Q3),
 
    % display results
    printf("Pop : Value: %w Queue: %w\n\n", V, Q3),
 
    % test the queue
    print("Test of the queue: "),
    ( Q3.empty() -> println("Queue empty"); println("Queue not empty") ), 
    nl,
 
    % pop the elements
    print("Pop the queue : "),
    V1 = Q3.pop(Q4),
    printf("Value %w Queue : %w%n%n", V1, Q4),
 
    println("Pop empty queue:"),
    catch(_V = Q4.pop(_Q5),Exception,println(Exception)),
    nl,

    println("\nEnd of tests.").
Output:
Test 1

Create queue []

Add numbers 1 and 2 : 
Q2: [2,1]

Pop : Value: 1 Queue: [2]

Test of the queue: Queue not empty

Pop the queue : Value 2 Queue : []

Pop empty queue:
error(empty_queue,pop,Q = [])

End of tests.

Always returning the queue[edit]

Another approach is to always returns the queue which makes command chaining possible, e,g,

   Q := Q.push2(1).push2(2),
   Q := Q.pop2(V1).pop2(V2)
go2 => 
  println("Test 2"),
  queue_test2,
  nl.

empty2() = [].
push2(Queue, Value) = Q2 =>
  Q2 = [Value] ++ Queue.
pop2(Q,_) = _, Q==[] ; var(Q) => 
   throw $error(empty_queue,pop,'Q'=Q).
pop2(Queue,V) = [Queue[I] : I in 1..Queue.len-1] =>
   V = Queue.last().

queue_test2 =>
    % create an empty queue
    Q = empty2(),
    printf("Create queue %w%n%n", Q),
 
    % add numbers 1 and 2
    println("Add numbers 1 and 2 : "),
    Q := Q.push2(1).push2(2),
 
    % display queue
    printf("Q: %w\n\n", Q),
 
    % pop element
    Q := Q.pop2(V),
 
    % display results
    printf("Pop : Value: %w Queue: %w\n\n", V, Q),
 
    % test the queue
    print("Test of the queue: "),
    ( Q.empty() -> println("Queue empty"); println("Queue not empty") ), 
    nl,
 
    % pop the elements
    print("Pop the queue : "),
    Q := Q.pop2(V2),
    printf("Value %w Queue : %w%n%n", V2, Q),
 
    println("Pop empty queue:"),
    catch(_ = Q.pop2(_V),Exception,println(Exception)),

    % command chaining
    println("\nCommand chaining: "),
    Q := Q.push2(3).push2(4),
    Q := Q.pop2(V3).pop2(V4),
    printf("V3: %w V4: %w\n", V3, V4),
    nl,
    println(q=Q).
Output:
Test 2

Create queue []

Add numbers 1 and 2 : 
Q: [2,1]

Pop : Value: 1 Queue: [2]

Test of the queue: Queue not empty

Pop the queue : Value 2 Queue : []

Pop empty queue:
error(empty_queue,pop,Q = [])

Command chaining: 
V3: 3 V4: 4

q = []

PicoLisp[edit]

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

(off Queue)                # Clear Queue
(fifo 'Queue 1)            # Store number '1'
(fifo 'Queue 'abc)         # an internal symbol 'abc'
(fifo 'Queue "abc")        # a transient symbol "abc"
(fifo 'Queue '(a b c))     # and a list (a b c)
Queue                      # Show the queue
Output:
->((a b c) 1 abc "abc" .)

PL/I[edit]

/* To push a node onto the end of the queue. */
push: procedure (tail);
   declare tail handle (node), t handle (node);
   t = new(:node:);
   get (t => value);
   if tail ^= bind(:null, node:) then
      tail => link = t;
      /* If the queue was non-empty, points the tail of the queue */
      /* to the new node. */
   tail = t; /* Point "tail" at the end of the queue. */
   tail => link = bind(:node, null:);
end push;

/* To pop a node from the head of the queue. */
pop: procedure (head, val);
   declare head handle (node), val fixed binary;
   if head = bind(:node, null:) then signal error;
   val = head => value;
   head = head => pointer; /* pops the top node. */
   if head = bind(:node, null:) then tail = head;
      /* (If the queue is now empty, make tail null also.) */
end pop;

/* Queue status: the EMPTY function, returns true for empty queue. */
empty: procedure (h) returns (bit(1));
   declare h handle (Node);
   return (h = bind(:Node, null:) );
end empty;

PostScript[edit]

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

PowerShell[edit]

Works with: PowerShell version 2

PowerShell can natively use the .Net Queue class.

 
$Q = New-Object System.Collections.Queue
 
$Q.Enqueue( 1 )
$Q.Enqueue( 2 )
$Q.Enqueue( 3 )
 
$Q.Dequeue()
$Q.Dequeue()
 
$Q.Count -eq 0
$Q.Dequeue()
$Q.Count -eq 0

try
{ $Q.Dequeue() }
catch [System.InvalidOperationException]
{ If ( $_.Exception.Message -eq 'Queue empty.' ) { 'Caught error' } }
Output:
1
2
False
3
True
Caught error

Prolog[edit]

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

empty(U-V) :-
    unify_with_occurs_check(U, V).

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

% when queue is empty pop fails.
pop([X|V]-U, X, V-U) :-
    \+empty([X|V]-U).

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

PureBasic[edit]

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

NewList MyStack()

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

Procedure Pop()
  Shared MyStack()
  Protected n
  If FirstElement(MyStack())  ; e.g. Stack not empty
    n=MyStack()
    DeleteElement(MyStack(),1)
  Else
    Debug "Pop(), out of range. Error at line "+str(#PB_Compiler_Line)
  EndIf
  ProcedureReturn n
EndProcedure

Procedure Empty()
  Shared MyStack()
  If  ListSize(MyStack())=0
    ProcedureReturn #True
  EndIf
  ProcedureReturn #False
EndProcedure

;----   Example of implementation ----
Push(3)
Push(1)
Push(4)
While Not Empty()
  Debug Pop()
Wend
;----   Now an extra Pop(), e.g. one to many ----
Debug Pop()
Output:
 3
 1
 4
 Pop(), out of range. Error at line 17
 0

Python[edit]

A python list can be used as a simple FIFO by simply using only it's .append() and .pop() methods and only using .pop(0) to consistently pull the head off the list. (The default .pop() pulls off the tail, and using that would treat the list as a stack.

To encapsulate this behavior into a class and provide the task's specific API we can simply use:

   class FIFO(object):
       def __init__(self, *args):
           self.contents = list(args)
       def __call__(self):
           return self.pop()
       def __len__(self):
           return len(self.contents)
       def pop(self):
           return self.contents.pop(0)
       def push(self, item):
           self.contents.append(item)
       def extend(self,*itemlist):
           self.contents += itemlist
       def empty(self):
           return bool(self.contents)
       def __iter__(self):
           return self
       def next(self):
           if self.empty():
               raise StopIteration
           return self.pop()

if __name__ == "__main__":
    # Sample usage:
    f = FIFO()
    f.push(3)
    f.push(2)
    f.push(1)
    while not f.empty():
        print f.pop(),
    # >>> 3 2 1
    # Another simple example gives the same results:
    f = FIFO(3,2,1)
    while not f.empty():
        print f(),
    # Another using the default "truth" value of the object
    # (implicitly calls on the length() of the object after
    # checking for a __nonzero__ method
    f = FIFO(3,2,1)
    while f:
        print f(),
    # Yet another, using more Pythonic iteration:
    f = FIFO(3,2,1)
    for i in f:
        print i,

This example does add to a couple of features which are easy in Python and allow this FIFO class to be used in ways that Python programmers might find more natural. Our __init__ accepts and optional list of initial values, we add __len__ and extend methods which simply wrap the corresponding list methods; we define a __call__ method to show how one can make objects "callable" as functions, and we define __iter__ and next() methods to facilitate using these FIFO objects with Python's prevalent iteration syntax (the for loop). The empty method could be implemented as simply an alias for __len__ --- but we've chosen to have it more strictly conform to the task specification. Implementing the __len__ method allows code using this object to test of emptiness using normal Python idioms for "truth" (any non-empty container is considered to be "true" and any empty container evaluates as "false").

These additional methods could be omitted and some could have been dispatched to the "contents" object by defining a __getattr__ method. (All methods that are note defined could be relayed to the contained list). This would allow us to skip our definitions of extend, __iter__, and __len__, and would allow contents of these objects to be access by indexes and slices as well as supporting all other list methods.

That sort of wrapper looks like:

class FIFO:  ## NOT a new-style class, must not derive from "object"
   def __init__(self,*args):
       self.contents = list(args)
   def __call__(self):
       return self.pop()
   def empty(self):
       return bool(self.contents)
   def pop(self):
       return self.contents.pop(0)
   def __getattr__(self, attr):
       return getattr(self.contents,attr)
   def next(self):
       if not self:
           raise StopIteration
       return self.pop()

As noted in the contents this must NOT be a new-style class, it must NOT but sub-classed from object nor any of its descendents. (A new-style implementation using __getattribute__ would be possible)

Works with: Python version 2.4+

Python 2.4 and later includes a deque class, supporting thread-safe, memory efficient appends and pops from either side of the deque with approximately the same O(1) performance in either direction. For other options see Python Cookbook.

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

Quackery[edit]

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

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

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

  [ dup empty? if
      [ $ "Queue unexpectedly empty." 
        fail ]
    behead ]                          is pop    (   [ --> [ x )
Output:

Testing in the Quackery shell.

/O> queue
... 1111 push
... 2222 push
... 3333 push
... pop echo cr
... pop echo cr
... pop echo cr
... dup empty? if [ say "queue is enpty" cr ]
... pop echo cr
... 
1111
2222
3333
queue is enpty

       Problem: Queue unexpectedly empty.
Quackery Stack: [ ]
  Return stack: {[...] 0} {quackery 1} {[...] 11} {shell 5} {quackery 1} {[...] 20} {pop 3}

R[edit]

Simple functional implementation[edit]

This simple implementation provides three functions that act on a variable in the global environment (user workspace) named l. the push and pop functions display the new status of l, but return NULL silently.

empty <- function() length(l) == 0
push <- function(x) 
{
   l <<- c(l, list(x))
   print(l)
   invisible()
}
pop <- function() 
{
   if(empty()) stop("can't pop from an empty list")
   l[[1]] <<- NULL
   print(l)
   invisible()
}
l <- list() 
empty()
# [1] TRUE
push(3)
# [[1]]
# [1] 3
push("abc")
# [[1]]
# [1] 3
# [[2]]
# [1] "abc"
push(matrix(1:6, nrow=2))
# [[1]]
# [1] 3
# [[2]]
# [1] "abc"
# [[3]]
#      [,1] [,2] [,3]
# [1,]    1    3    5
# [2,]    2    4    6
empty()
# [1] FALSE
pop()
# [[1]]
# [1] 3
# [[2]]
# [1] "abc"
pop()
# [[1]]
# [1] 3
pop()
# list()
pop()
# Error in pop() : can't pop from an empty list

The problem with this is that the functions aren't related to the FIFO object (the list l), and they require the list to exist in the global environment. (This second problem is possible to get round by passing l into the function and then returning it, but that is extra work.)

Message passing[edit]

# The usual Scheme way : build a function that takes commands as parameters (it's like message passing oriented programming)
queue <- function() {
    v <- list()
    f <- function(cmd, val=NULL) {
        if(cmd == "push") {
            v <<- c(v, val)
            invisible()
        } else if(cmd == "pop") {
            if(length(v) == 0) {
                stop("empty queue")
            } else {
                x <- v[[1]]
                v[[1]] <<- NULL
                x
            }
        } else if(cmd == "length") {
            length(v)
        } else if(cmd == "empty") {
            length(v) == 0
        } else {
            stop("unknown command")
        }
    }
    f
}

# Create two queues
a <- queue()
b <- queue()
a("push", 1)
a("push", 2)
b("push", 3)
a("push", 4)
b("push", 5)

a("pop")
# [1] 1
b("pop")
# [1] 3

Object oriented implementation[edit]

Library: proto

A better solution is to use the object oriented facility in the proto package. (R does have it's own native object oriented code, though the proto package is often nicer to use.)

library(proto)

fifo <- proto(expr = {
   l <- list()
   empty <- function(.) length(.$l) == 0
   push <- function(., x) 
   {
      .$l <- c(.$l, list(x))
      print(.$l)
      invisible()
   }
   pop <- function(.) 
   {
      if(.$empty()) stop("can't pop from an empty list")
      .$l[[1]] <- NULL
      print(.$l)
      invisible()
   }
})

#The following code provides output that is the same as the previous example.
fifo$empty()
fifo$push(3)
fifo$push("abc")
fifo$push(matrix(1:6, nrow=2))
fifo$empty()
fifo$pop()
fifo$pop()
fifo$pop()
fifo$pop()

Racket[edit]

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

#lang racket

(define (make-queue) (mcons #f #f))
(define (push! q x)
  (define new (mcons x #f))
  (if (mcar q) (set-mcdr! (mcdr q) new) (set-mcar! q new))
  (set-mcdr! q new))
(define (pop! q)
  (define old (mcar q))
  (cond [(eq? old (mcdr q)) (set-mcar! q #f) (set-mcdr! q #f)]
        [else (set-mcar! q (mcdr old))])
  (mcar old))
(define (empty? q)
  (not (mcar q)))

(define Q (make-queue))
(empty? Q) ; -> #t
(push! Q 'x)
(empty? Q) ; -> #f
(for ([x 3]) (push! Q x))
(pop! Q)   ; -> 'x
(list (pop! Q) (pop! Q) (pop! Q)) ; -> '(0 1 2)

And this is an implementation of a functional queue.

#lang racket
;; Invariants:
;; The elements in the queue are (append front (reverse back)).
;; Front is always non-empty (except for the empty queue).
(struct queue (front back))

(define empty (queue '() '()))

(define (push x q)
  (if (null? (queue-front q))
      (queue (reverse (cons x (queue-back q))) '())
      (queue (queue-front q) (cons x (queue-back q)))))

(define (empty? q)
  (null? (queue-front q)))

(define (pop q)
  (cond [(empty? q) (error 'pop "the queue is empty")]
        [(not (null? (queue-front q)))
         (if (null? (rest (queue-front q)))
             (queue (reverse (queue-back q)) '())
             (queue (rest (queue-front q)) (queue-back q)))]
        [else (queue (reverse (queue-back q)) '())]))

(define (first q)
  (cond [(empty? q) (error 'first "the queue is empty")]
        [(car (queue-front q))]))

;; Example:
(first (pop (pop (for/fold ([q empty]) ([x '(1 2 3 4)])
                   (push x q)))))
;; => 3

Raku[edit]

(formerly Perl 6)

Works with: rakudo version 2018.03

We could build a new container class to do FIFO pretty easily, but Arrays already do everything needed by a FIFO queue, so it is easier to just compose a Role on the existing Array class.

role FIFO {
    method enqueue ( *@values ) { # Add values to queue, returns the number of values added.
        self.push: @values;
        return @values.elems;
    }
    method dequeue ( ) {         # Remove and return the first value from the queue.
                                 # Return Nil if queue is empty.
        return self.elems ?? self.shift !! Nil;
    }
    method is-empty ( ) {        # Check to see if queue is empty. Returns Boolean value.
        return self.elems == 0;
    }
}

# Example usage:

my @queue does FIFO;

say @queue.is-empty;                         # -> Bool::True
for <A B C> -> $i { say @queue.enqueue: $i } # 1 \n 1 \n 1
say @queue.enqueue: Any;                     # -> 1
say @queue.enqueue: 7, 8;                    # -> 2
say @queue.is-empty;                         # -> Bool::False
say @queue.dequeue;                          # -> A
say @queue.elems;                            # -> 4
say @queue.dequeue;                          # -> B
say @queue.is-empty;                         # -> Bool::False
say @queue.enqueue('OHAI!');                 # -> 1
say @queue.dequeue until @queue.is-empty;    # -> C \n Any() \n [7 8] \n OHAI!
say @queue.is-empty;                         # -> Bool::True
say @queue.dequeue;                          # ->

REBOL[edit]

rebol [
    Title: "FIFO"
    URL: http://rosettacode.org/wiki/FIFO
]

; Define fifo class:

fifo: make object! [
    queue: copy []
    push:  func [x][append queue x]
    pop:   func [/local x][   ; Make 'x' local so it won't pollute global namespace.
        if empty [return none]
        x: first queue  remove queue  x]
    empty: does [empty? queue]
]

; Create and populate a FIFO:

q: make fifo []
q/push 'a
q/push 2
q/push USD$12.34              ; Did I mention that REBOL has 'money!' datatype?
q/push [Athos Porthos Aramis] ; List elements pushed on one by one.
q/push [[Huey Dewey Lewey]]   ; This list is preserved as a list.

; Dump it out, with narrative:

print rejoin ["Queue is "  either q/empty [""]["not "]  "empty."]
while [not q/empty][print ["  " q/pop]]
print rejoin ["Queue is "  either q/empty [""]["not "]  "empty."]
print ["Trying to pop an empty queue yields:" q/pop]
Output:
Queue is not empty.
   a
   2
   USD$12.34
   Athos
   Porthos
   Aramis
   Huey Dewey Lewey
Queue is empty.
Trying to pop an empty queue yields: none

REXX[edit]

Support for   LIFO   &   FIFO   queues is built into the Rexx language.

The following are supported in REXX:

  •   PUSH     (lifo)
  •   QUEUE   (fifo)
  •   PULL   --- which is a short version of:
  •   PARSE UPPER PULL
  •   PARSE LOWER PULL   --- supported by some newer REXXes
  •   PARSE PULL
  •   QUEUED()   [a BIF which returns the number of queued entries.]


/*REXX program to demonstrate FIFO queue usage by some simple operations*/
call viewQueue
a="Fred"
push                                   /*puts a  "null" on top of queue.*/
push a 2                               /*puts  "Fred 2" on top of queue.*/
call viewQueue

queue "Toft 2"                         /*put  "Toft 2"  on queue bottom.*/
queue                                  /*put a "null"   on queue bottom.*/
call viewQueue
                  do n=1  while queued()\==0
                  parse pull xxx
                  say "queue entry" n': ' xxx
                  end   /*n*/
call viewQueue
exit                                   /*stick a fork in it, we're done.*/
/*──────────────────────────────────viewQueue subroutine────────────────*/
viewQueue:  if queued()==0 then say 'Queue is empty'
                           else say 'There are' queued() 'elements in the queue'
return

output

Queue is empty
There are 2 elements in the queue
There are 4 elements in the queue
  queue entry 1:  Fred 2
  queue entry 2:
  queue entry 3:  Toft 2
  queue entry 4:
Queue is empty

Ring[edit]

# Project : Queue/Definition

load "stdlib.ring"
oQueue = new Queue 
for n = 5 to 7
     see "Push: " + n + nl
     oQueue.add(n) 
next
see "Pop: " + oQueue.remove() + nl
see "Push: 8" + nl
oQueue.add(8)
see "Pop: " + oQueue.remove() + nl
see "Pop: " + oQueue.remove() + nl
see "Pop: " + oQueue.remove() + nl 
if len(oQueue) != 0
   oQueue.print()
else
   see "Error: queue is empty" + nl
ok

Output:

Push: 5
Push: 6
Push: 7
Pop: 5
Push: 8
Pop: 6
Pop: 7
Pop: 8
Error: queue is empty

Ruby[edit]

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

require 'forwardable'

# A FIFO queue contains elements in first-in, first-out order.
# FIFO#push adds new elements to the end of the queue;
# FIFO#pop or FIFO#shift removes elements from the front.
class FIFO
  extend Forwardable

  # Creates a FIFO containing _objects_.
  def self.[](*objects)
    new.push(*objects)
  end

  # Creates an empty FIFO.
  def initialize; @ary = []; end

  # Appends _objects_ to the end of this FIFO. Returns self.
  def push(*objects)
    @ary.push(*objects)
    self
  end
  alias << push
  alias enqueue push

  ##
  # :method: pop
  # :call-seq:
  #   pop -> obj or nil
  #   pop(n) -> ary
  #
  # Removes an element from the front of this FIFO, and returns it.
  # Returns nil if the FIFO is empty.
  #
  # If passing a number _n_, removes the first _n_ elements, and returns
  # an Array of them. If this FIFO contains fewer than _n_ elements,
  # returns them all. If this FIFO is empty, returns an empty Array.
  def_delegator :@ary, :shift, :pop
  alias shift pop
  alias dequeue shift

  ##
  # :method: empty?
  # Returns true if this FIFO contains no elements.
  def_delegator :@ary, :empty?

  ##
  # :method: size
  # Returns the number of elements in this FIFO.
  def_delegator :@ary, :size
  alias length size

  # Converts this FIFO to a String.
  def to_s
    "FIFO#{@ary.inspect}"
  end
  alias inspect to_s
end
f = FIFO.new
f.empty?                           # => true
f.pop                              # => nil
f.pop(2)                           # => []
f.push(14)                         # => FIFO[14]
f << "foo" << [1,2,3]              # => FIFO[14, "foo", [1, 2, 3]]
f.enqueue("bar", Hash.new, "baz")
# => FIFO[14, "foo", [1, 2, 3], "bar", {}, "baz"]
f.size                             # => 6
f.pop(3)                           # => [14, "foo", [1, 2, 3]]
f.dequeue                          # => "bar"
f.empty?                           # => false
g = FIFO[:a, :b, :c]
g.pop(2)                           # => [:a, :b]
g.pop(2)                           # => [:c]
g.pop(2)                           # => []

Rust[edit]

Using the standard library[edit]

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

use std::collections::VecDeque;
fn main() {
    let mut stack = VecDeque::new();
    stack.push_back("Element1");
    stack.push_back("Element2");
    stack.push_back("Element3");

    assert_eq!(Some(&"Element1"), stack.front());
    assert_eq!(Some("Element1"), stack.pop_front());
    assert_eq!(Some("Element2"), stack.pop_front());
    assert_eq!(Some("Element3"), stack.pop_front());
    assert_eq!(None, stack.pop_front());
}

A simple implementation[edit]

This shows the implementation of a singly-linked queue with dequeue and enqueue. There are two peek implementations, one returns an immutable reference, the other returns a mutable one. This implementation also shows iteration over the Queue by value (consumes queue), immutable reference, and mutable reference.

use std::ptr;

pub struct Queue<T> {
    head: Link<T>,
    tail: *mut Item<T>, // Raw, C-like pointer. Cannot be guaranteed safe
}

type Link<T> = Option<Box<Item<T>>>;

struct Item<T> {
    elem: T,
    next: Link<T>,
}

pub struct IntoIter<T>(Queue<T>);

pub struct Iter<'a, T:'a> {
    next: Option<&'a Item<T>>,
}

pub struct IterMut<'a, T: 'a> {
    next: Option<&'a mut Item<T>>,
}


impl<T> Queue<T> {
    pub fn new() -> Self {
        Queue { head: None, tail: ptr::null_mut() }
    }

    pub fn enqueue(&mut self, elem: T) {
        let mut new_tail = Box::new(Item {
            elem: elem,
            next: None,
        });

        let raw_tail: *mut _ = &mut *new_tail;

        if !self.tail.is_null() {
            unsafe {
                (*self.tail).next = Some(new_tail);
            }
        } else {
            self.head = Some(new_tail);
        }

        self.tail = raw_tail;
    }

    pub fn dequeue(&mut self) -> Option<T> {
        self.head.take().map(|head| {
            let head = *head;
            self.head = head.next;

            if self.head.is_none() {
                self.tail = ptr::null_mut();
            }

            head.elem
        })
    }

    pub fn peek(&self) -> Option<&T> {
        self.head.as_ref().map(|item| {
            &item.elem
        })
    }

    pub fn peek_mut(&mut self) -> Option<&mut T> {
        self.head.as_mut().map(|item| {
            &mut item.elem
        })
    }

    pub fn into_iter(self) -> IntoIter<T> {
        IntoIter(self)
    }

    pub fn iter(&self) -> Iter<T> {
        Iter { next: self.head.as_ref().map(|item| &**item) }
    }

    pub fn iter_mut(&mut self) -> IterMut<T> {
        IterMut { next: self.head.as_mut().map(|item| &mut **item) }
    }
}

impl<T> Drop for Queue<T> {
    fn drop(&mut self) {
        let mut cur_link = self.head.take();
        while let Some(mut boxed_item) = cur_link {
            cur_link = boxed_item.next.take();
        }
    }
}

impl<T> Iterator for IntoIter<T> {
    type Item = T;
    fn next(&mut self) -> Option<Self::Item> {
        self.0.dequeue()
    }
}

impl<'a, T> Iterator for Iter<'a, T> {
    type Item = &'a T;

    fn next(&mut self) -> Option<Self::Item> {
        self.next.map(|item| {
            self.next = item.next.as_ref().map(|item| &**item);
            &item.elem
        })
    }
}

impl<'a, T> Iterator for IterMut<'a, T> {
    type Item = &'a mut T;

    fn next(&mut self) -> Option<Self::Item> {
        self.next.take().map(|item| {
            self.next = item.next.as_mut().map(|item| &mut **item);
            &mut item.elem
        })
    }
}

Scala[edit]

class Queue[T] {
  private[this] class Node[T](val value:T) {
    var next:Option[Node[T]]=None
    def append(n:Node[T])=next=Some(n)
  }
  private[this] var head:Option[Node[T]]=None
  private[this] var tail:Option[Node[T]]=None
  
  def isEmpty=head.isEmpty
  
  def enqueue(item:T)={
    val n=new Node(item)
    if(isEmpty) head=Some(n) else tail.get.append(n)
    tail=Some(n)
  }
     
  def dequeue:T=head match {
    case Some(item) => head=item.next; item.value
    case None => throw new java.util.NoSuchElementException()
  }

  def front:T=head match {
    case Some(item) => item.value
    case None => throw new java.util.NoSuchElementException()
  }
  
  def iterator: Iterator[T]=new Iterator[T]{
    private[this] var it=head;
    override def hasNext=it.isDefined
    override def next:T={val n=it.get; it=n.next; n.value}
  }
  
  override def toString()=iterator.mkString("Queue(", ", ", ")")
}

Usage:

val q=new Queue[Int]()
println("isEmpty = " + q.isEmpty)
try{q dequeue} catch{case _:java.util.NoSuchElementException => println("dequeue(empty) failed.")}
q enqueue 1
q enqueue 2
q enqueue 3
println("queue   = " + q)
println("front   = " + q.front)
println("dequeue = " + q.dequeue)
println("dequeue = " + q.dequeue)
println("isEmpty = " + q.isEmpty)
Output:
isEmpty = true
dequeue(empty) failed.
queue   = Queue(1, 2, 3)
front   = 1
dequeue = 1
dequeue = 2
isEmpty = false

Scheme[edit]

Using a vector for mutable data. Can be optimized by using an extra slot in the vector to hold tail pointer to avoid the append call.

(define (make-queue)
  (make-vector 1 '()))

(define (push a queue)
  (vector-set! queue 0 (append (vector-ref queue 0) (list a))))

(define (empty? queue)
  (null? (vector-ref queue 0)))

(define (pop queue)
  (if (empty? queue)
      (error "can not pop an empty queue")
      (let ((ret (car (vector-ref queue 0))))
        (vector-set! queue 0 (cdr (vector-ref queue 0)))
        ret)))

Message passing[edit]

(define (make-queue)
(let ((q (cons '() '())))
(lambda (cmd . arg)
(case cmd
    ((empty?) (null? (car q)))
    ((put) (let ((a (cons (car arg) '())))
        (if (null? (car q))
            (begin (set-car! q a) (set-cdr! q a))
            (begin (set-cdr! (cdr q) a) (set-cdr! q a)))))
    ((get) (if (null? (car q)) 'empty
        (let ((x (caar q)))
            (set-car! q (cdar q))
            (if (null? (car q)) (set-cdr! q '()))
            x)))
))))

(define q (make-queue))
(q 'put 1)
(q 'put 6)
(q 'get)
; 1
(q 'get)
; 6
(q 'get)
; empty

SenseTalk[edit]

A queue in SenseTalk is implemented using push and pull operations on a list.

set myFoods to be an empty list

push "grapes" into myFoods
push "orange" into myFoods
push "apricot" into myFoods

put "The foods in my queue are: " & myFoods

pull from myFoods into firstThingToEat

put "The first thing to eat is: " & firstThingToEat

if myFoods is empty then
    put "The foods list is empty!"
else
    put "The remaining foods are: " & myFoods
end if

Output:

The foods in my queue are: (grapes,orange,apricot)
The first thing to eat is: grapes
The remaining foods are: (orange,apricot)

Sidef[edit]

Implemented as a class:

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

Slate[edit]

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

collections define: #Queue &parents: {ExtensibleArray}.

q@(Queue traits) isEmpty [resend].
q@(Queue traits) push: obj [q addLast: obj].
q@(Queue traits) pop [q removeFirst].
q@(Queue traits) pushAll: c [q addAllLast: c].
q@(Queue traits) pop: n [q removeFirst: n].

Smalltalk[edit]

Works with: GNU Smalltalk

An OrderedCollection can be easily used as a FIFO queue.

OrderedCollection extend [
   push: obj [ ^(self add: obj) ]
   pop [
       (self isEmpty) ifTrue: [
          SystemExceptions.NotFound signalOn: self
                reason: 'queue empty'
       ] ifFalse: [
          ^(self removeFirst)
       ]
   ]
]

|f|
f := OrderedCollection new.
f push: 'example'; push: 'another'; push: 'last'.
f pop printNl.
f pop printNl.
f pop printNl.
f isEmpty printNl.
f pop. "queue empty error"

Standard ML[edit]

Here is the signature for a basic queue:

signature QUEUE =
sig
  type 'a queue
  
  val empty_queue: 'a queue
  
  exception Empty
  
  val enq: 'a queue -> 'a -> 'a queue
  val deq: 'a queue -> ('a * 'a queue)
  val empty: 'a queue -> bool
end;

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

structure Queue:> QUEUE =
struct
  type 'a queue = 'a list
  
  val empty_queue = nil
  
  exception Empty
  
  fun enq q x = q @ [x]
  
  fun deq nil = raise Empty
  |   deq (x::q) = (x, q)
  
  fun empty nil = true
  |   empty _ = false
end;

Stata[edit]

See Singly-linked list/Element definition#Stata.

Tcl[edit]

Here's a simple implementation using a list:

proc push {stackvar value} {
    upvar 1 $stackvar stack
    lappend stack $value
}
proc pop {stackvar} {
    upvar 1 $stackvar stack
    set value [lindex $stack 0]
    set stack [lrange $stack 1 end]
    return $value
}
proc size {stackvar} {
    upvar 1 $stackvar stack
    llength $stack
}
proc empty {stackvar} {
    upvar 1 $stackvar stack
    expr {[size stack] == 0}
}
proc peek {stackvar} {
    upvar 1 $stackvar stack
    lindex $stack 0
}

set Q [list]
empty Q ;# ==> 1 (true)
push Q foo
empty Q ;# ==> 0 (false)
push Q bar
peek Q ;# ==> foo
pop Q ;# ==> foo
peek Q ;# ==> bar
Library: Tcllib (Package: struct::queue)
package require struct::queue
struct::queue Q
Q size ;# ==> 0
Q put a b c d e
Q size ;# ==> 5
Q peek ;# ==> a
Q get ;# ==> a
Q peek ;# ==> b
Q pop 4 ;# ==> b c d e
Q size ;# ==> 0

UNIX Shell[edit]

Works with: ksh93
queue_push() { 
    typeset -n q=$1
    shift
    q+=("$@")
}

queue_pop() { 
    if queue_empty $1; then 
        print -u2 "queue $1 is empty"
        return 1
    fi
    typeset -n q=$1
    print "${q[0]}"     # emit the value of the popped element
    q=( "${q[@]:1}" )   # and remove the first element from the queue
}

queue_empty() { 
    typeset -n q=$1
    (( ${#q[@]} == 0 ))
}

queue_peek() { 
    typeset -n q=$1
    print "${q[0]}"
}

Usage:

# any valid variable name can be used as a queue without initialization

queue_empty foo && echo foo is empty || echo foo is not empty

queue_push foo bar
queue_push foo baz
queue_push foo "element with spaces"

queue_empty foo && echo foo is empty || echo foo is not empty

print "peek: $(queue_peek foo)"; queue_pop foo
print "peek: $(queue_peek foo)"; queue_pop foo
print "peek: $(queue_peek foo)"; queue_pop foo
print "peek: $(queue_peek foo)"; queue_pop foo
Output:
foo is empty
foo is not empty
peek: bar
peek: baz
peek: element with spaces
peek: 
queue foo is empty

UnixPipes[edit]

Uses moreutils

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

Usage:

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

V[edit]

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

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

Using it

|fifo_create 3 fifo_push 4 fifo_push 5 fifo_push ??
=[5 4 3]
|fifo_empty? puts
=false
|fifo_pop put fifo_pop put fifo_pop put
=3 4 5
|fifo_empty? puts
 =true

VBA[edit]

Public queue As New Collection

Private Sub push(what As Variant)
    queue.Add what
End Sub

Private Function pop() As Variant
    If queue.Count > 0 Then
        what = queue(1)
        queue.Remove 1
    Else
        what = CVErr(461)
    End If
    pop = what
End Function

Private Function empty_()
    empty_ = queue.Count = 0
End Function

VBScript[edit]

Using an ArrayList.

' Queue Definition - VBScript
Option Explicit
Dim queue, i, x
Set queue = CreateObject("System.Collections.ArrayList")
If Not empty_(queue) Then Wscript.Echo queue.Count
push queue, "Banana"
push queue, "Apple"
push queue, "Pear"
push queue, "Strawberry"
Wscript.Echo "Count=" & queue.Count
Wscript.Echo pull(queue) & " - Count=" & queue.Count '
Wscript.Echo "Head=" & queue.Item(0)
Wscript.Echo "Tail=" & queue.Item(queue.Count-1)
Wscript.Echo queue.IndexOf("Pear", 0)
For i=1 To queue.Count
	Wscript.Echo join(queue.ToArray(), ", ")
	x = pull(queue)
Next 'i

Sub push(q, what)
    q.Add what
End Sub 'push
 
Function pull(q)
	Dim what
    If q.Count > 0 Then
        what = q(0)
        q.RemoveAt 0
    Else
        what = ""
    End If
    pull = what
End Function 'pull
 
Function empty_(q)
    empty_ = q.Count = 0
End Function 'empty_
Output:
Count=4
Banana - Count=3
Head=Apple
Tail=Strawberry
1
Apple, Pear, Strawberry
Pear, Strawberry
Strawberry

Vlang[edit]

Updated to Vlang version 0.2.2

const max_tail = 256
 
struct Queue<T> {
mut:
	data  []T
	tail int
	head  int
}
 
fn (mut queue Queue<T>) push(value T) {
	if queue.tail >= max_tail || queue.tail < queue.head {
		return
	}
	println('push: $value')
	queue.data << value
	queue.tail++
}
 
fn (mut queue Queue<T>) pop() !T {
	if queue.tail > 0 && queue.head < queue.tail {
		result := queue.data[queue.head]
		queue.head++
		println('Dequeue: top of Queue was $result')
		return result
	}
	return error('Queue Underflow!!')
}
 
fn (queue Queue<T>) peek() !T {
	if queue.tail > 0 && queue.head < queue.tail {
		result := queue.data[queue.head]
		println('Peek: top of Queue is $result')
		return result
	}
	return error('Out of Bounds...')
}
 
fn (queue Queue<T>) empty() bool {
	return queue.tail == 0
}
 
fn main() {
	mut queue := Queue<f64>{}
	println('Queue is empty? ' + if queue.empty() { 'Yes' } else { 'No' })
	queue.push(5.0)
	queue.push(4.2)
	println('Queue is empty? ' + if queue.empty() { 'Yes' } else { 'No' })
	queue.peek() or { return }
	queue.pop() or { return }
	queue.pop() or { return }
	queue.push(1.2)
}
Output:
Queue is empty? Yes
Enqueue: 5.00
Enqueue: 4.20
Queue is empty? No
Peek: top of Queue is 5.00
Dequeue: top of Queue was 5.00
Dequeue: top of Queue was 4.20
Enqueue: 1.20

Wart[edit]

Wart defines queues as lists with a pointer to the last element saved for constant-time enqueuing:

def (queue seq)
  (tag queue (list seq lastcons.seq len.seq))

def (enq x q)
  do1 x
    let (l last len) rep.q
      rep.q.2 <- (len + 1)
      if no.l
        rep.q.1 <- (rep.q.0 <- list.x)
        rep.q.1 <- (cdr.last <- list.x)

def (deq q)
  let (l last len) rep.q
    ret ans car.l
      unless zero?.len
        rep.q.2 <- (len - 1)
      rep.q.0 <- cdr.l

def (len q) :case (isa queue q)
  rep.q.2

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

Wren[edit]

Library: Wren-queue

The above module contains a suitable Queue class.

import "/queue" for Queue

var q = Queue.new()
var item = q.pop()
if (item == null) {
    System.print("ERROR: attempted to pop from an empty queue")
} else {
    System.print("'%(item)' was popped")
}
Output:
ERROR: attempted to pop from an empty queue

XLISP[edit]

A queue is similar to a stack, except that values are pushed onto and popped from different "ends" of it (whereas in a stack it is the same end for both operations). This implementation is based on the XLISP implementation of a stack, therefore, but with a push method that appends a new value to the end rather than sticking it onto the front. Attempting to pop from an empty queue will return the empty list, equivalent to Boolean "false".

(define-class queue
    (instance-variables vals))
 
(define-method (queue 'initialize)
    (setq vals '())
    self)
 
(define-method (queue 'push x)
    (setq vals (nconc vals (cons x nil))))
 
(define-method (queue 'pop)
    (define val (car vals))
    (setq vals (cdr vals))
    val)
 
(define-method (queue 'emptyp)
    (null vals))

A sample REPL session:

[1] (define my-queue (queue 'new))

MY-QUEUE
[2] (my-queue 'push 1)

(1)
[3] (my-queue 'push 2)

(1 2)
[4] (my-queue 'emptyp)

()
[5] (my-queue 'pop)

1
[6] (my-queue 'pop)

2
[7] (my-queue 'emptyp)

#T
[8] (my-queue 'pop)

()

XPL0[edit]

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

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

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

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

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

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

Output:

false
0
1
2
3
true
0
1
2
3

zkl[edit]

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