Queue/Definition: Difference between revisions

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(→‎{{header|Ruby}}: Add RDoc comments and a few more aliases. Usage remains as before.)
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=={{header|Ruby}}==
=={{header|Ruby}}==
The core class ''Array'' already implements all queue operations, so this class ''FIFO'' delegates everything to methods of ''Array''.
⚫
<lang ruby>class FIFO

⚫
def initialize
<lang ruby>require 'forwardable'
@fifo = []

# 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
end

⚫
# Creates an empty FIFO.
⚫
def push(*args)
⚫
def initialize; @ary = []; end
⚫
@fifo.push(*args)

# Appends _objects_ to the end of this FIFO. Returns self.
⚫
def push(*objects)
@ary.push(*objects)
self
self
end
end
Line 2,150: Line 2,164:
alias enqueue push
alias enqueue push


⚫
##
# popping an empty FIFO returns nil, or [] if a number is specified
def pop(*args)
# :method: pop
# :call-seq:
@fifo.shift(*args)
# pop -> obj or nil
end
alias dequeue pop
# pop(n) -> ary
#
# Removes an element from the front of this FIFO, and returns it.
⚫
def empty?
@fifo.empty?
# Returns nil if the FIFO is empty.
end
#
# 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 size
⚫
def to_s
@fifo.length
"FIFO#{@ary.inspect}"
end
end
alias inspect to_s
end
end</lang>


f = FIFO.new
<lang ruby>f = FIFO.new
f.empty? # => true
f.empty? # => true
f.pop # => nil
f.pop # => nil
f.pop(2) # => []
f.pop(2) # => []
f.push(14) # => #<FIFO:...>
f.push(14) # => FIFO[14]
f << "foo" << [1,2,3] # => #<FIFO:...>
f << "foo" << [1,2,3] # => FIFO[14, "foo", [1, 2, 3]]
f.enqueue("bar", Hash.new, "baz") # => #<FIFO:...>
f.enqueue("bar", Hash.new, "baz")
# => FIFO[14, "foo", [1, 2, 3], "bar", {}, "baz"]
f.size # => 6
f.size # => 6
f.pop(3) # => [14, "foo", [1, 2, 3]]
f.pop(3) # => [14, "foo", [1, 2, 3]]
f.dequeue # => "bar"
f.dequeue # => "bar"
f.empty? # => false</lang>
f.empty? # => false
g = FIFO[:a, :b, :c]
g.pop(2) # => [:a, :b]
g.pop(2) # => [:c]
g.pop(2) # => []</lang>


=={{header|Scala}}==
=={{header|Scala}}==

Revision as of 18:44, 15 September 2011

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

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 FIFO (usage) for the built-in FIFO or queue of your language or standard library.

Ada

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. <lang ada>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;</lang> The FIFO implementation is described in the package body: <lang ada>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;</lang> A "main" procedure for this program is: <lang ada>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;</lang> 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. <lang ada> 

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;

</lang> <lang ada> 

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;</lang>

<lang ada>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;</lang> 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. <lang ada>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;</lang> <lang ada>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;</lang> <lang ada>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;</lang>

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. <lang ada>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;</lang> 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. <lang ada>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;</lang> <lang ada>with Asynchronous_Fifo; with Ada.Text_Io; use Ada.Text_Io; 

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

AutoHotkey

<lang autohotkey>push("qu", 2), push("qu", 44), push("qu", "xyz") ; TEST

MsgBox % "Len = " len("qu") ; Number of entries While !empty("qu")  ; Repeat until queue is not empty 

   MsgBox % pop("qu")      ; Print popped values (2, 44, xyz)

MsgBox Error = %ErrorLevel% ; ErrorLevel = 0: OK MsgBox % pop("qu")  ; Empty MsgBox Error = %ErrorLevel% ; ErrorLevel = -1: popped too much MsgBox % "Len = " len("qu") ; Number of entries

push(queue,_) {  ; push _ onto queue named "queue" (!=_), _ string not containing | 

   Global
   %queue% .= %queue% = "" ? _ : "|" _

}

pop(queue) {  ; pop value from queue named "queue" (!=_,_1,_2) 

   Global
   RegExMatch(%queue%, "([^\|]*)\|?(.*)", _)
   Return _1, ErrorLevel := -(%queue%=""), %queue% := _2

}

empty(queue) {  ; check if queue named "queue" is empty 

   Global
   Return %queue% = ""

}

len(queue) {  ; number of entries in "queue" 

   Global
   StringReplace %queue%, %queue%, |, |, UseErrorLevel
   Return %queue% = "" ? 0 : ErrorLevel+1

}</lang>

C

Dynamic array

Dynamic array working as a circular buffer. <lang c>#include <stdio.h>

  1. include <stdlib.h>
  2. 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; if (q->tail == q->head) { /* 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; }</lang>

Doubly linked list

<lang c>#include <stdio.h>

  1. include <stdlib.h>

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

  1. define HEAD(q) q->prev
  2. 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; } </lang>

Test code This main function works with both implementions above. <lang c>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; }</lang>

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

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)

<lang c>#include <stdio.h>

  1. include <stdlib.h>
  2. include <stdbool.h>
  1. 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;

}</lang>

C++

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. <lang cpp>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;
 }

}</lang>

C#

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. <lang csharp>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;
 }

}</lang>

Clojure

The "pop" function implies mutating the input, but since Clojure data structures are immutable we use a mutable reference to an immutable data structure; in this case an atom holding a vector:

<lang lisp>(defn make-queue [] 

 (atom []))

(defn enqueue [q x] 

 (swap! q conj x))

(defn dequeue [q] 

 (if (seq @q)
   (let [x (first @q)] 
     (swap! q subvec 1)
     x)
   (throw (IllegalStateException. "Can't pop an empty queue."))))

(defn queue-empty? [q] 

 (empty? @q))</lang>

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

Common Lisp

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.

<lang lisp>(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))))</lang>

D

Implemented a queue class, by reusing previous stack class definition. See Stack#D. <lang d>module stack ; class Stack(T){ ... 

 void push(T top) { ... }
 T pop() { ... }
 bool empty() { ... }

}</lang> <lang d>module fifo ; import stack ; class FIFO(T) : Stack!(T){ 

 override T pop() {
   if (empty)
     throw new Exception("FIFO Empty") ;
   T top = content[0] ;
   content = content[1..$] ;
   return top ;
 }
 alias push enqueue ;
 alias pop dequeue ;

}</lang> Statement content = content[1..$] is efficient enough, because no array content is moved/copyed, but pointer modified.

Using the Singly-Linked List (element): <lang d>module fifolink ; class FIFOLinked(T) { 

 alias Node!(T) Node;
 private Node head = null;
 private Node tail = null;
 void push(T last) {
   head = new Node(last, head);
   if (tail is null)
     tail = head;
 }
 T pop() {
   if(empty)
     throw new Exception("FIFO Empty") ;
   T first = head.data;
   if (head is tail) // is last one?
     tail = null;   // release tail reference so that GC can collect afterward
   head = head.next;
   return first;
 }
 bool empty() { return head is null; }
 alias push enqueue ;
 alias pop dequeue ;

}</lang>

E

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).

<lang e>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]

}</lang>

Erlang

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. <lang Erlang>-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.</lang>

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

Fantom

<lang fantom> 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
 }

} </lang>

Forth

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.

<lang forth>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 +! ;</lang>

Fortran

Works with: Fortran version 90 and later

See FIFO (usage) for an example of fifo_nodes

<lang fortran>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</lang>

Go

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

} </lang>

Haskell

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.

<lang haskell>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 </lang>

Icon and Unicon

Icon

The following works in both Icon and Unicon:

<lang icon>

  1. 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

  1. 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

  1. 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 </lang>

Output: 

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

Unicon

Unicon also provides classes:

<lang Unicon>

  1. 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 </lang>

Produces the same output as above.

J

Object oriented technique, using mutable state:

<lang J>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

)</lang>

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

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

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

notnull =: 0 ~: #</lang>

See also FIFO (usage)#J

Java

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: <lang java>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; } }</lang>

JavaScript

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

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

Custom constructor function

<lang javascript>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;

}</lang>

Lua

<lang lua>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</lang>

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. <lang MATLAB>%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</lang>

Sample usage: <lang MATLAB>>> 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</lang>

NetRexx

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. <lang NetRexx>/* 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

</lang> 

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

OCaml

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.

<lang ocaml>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</lang>

and a session in the top-level:

<lang ocaml># open FIFO;;

  1. let q = empty ;;

val q : '_a FIFO.fifo = <abstr>

  1. is_empty q ;;

- : bool = true

  1. let q = push q 1 ;;

val q : int FIFO.fifo = <abstr>

  1. is_empty q ;;

- : bool = false

  1. let q =
   List.fold_left push q [2;3;4] ;;

val q : int FIFO.fifo = <abstr>

  1. let v, q = pop q ;;

val v : int = 1 val q : int FIFO.fifo = <abstr>

  1. let v, q = pop q ;;

val v : int = 2 val q : int FIFO.fifo = <abstr>

  1. let v, q = pop q ;;

val v : int = 3 val q : int FIFO.fifo = <abstr>

  1. let v, q = pop q ;;

val v : int = 4 val q : int FIFO.fifo = <abstr>

  1. let v, q = pop q ;;

Exception: Failure "empty fifo".</lang>

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

Oz

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.

<lang oz>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}}</lang>

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

Pascal

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).

<lang 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.</lang>


Perl

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

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

Example:

<lang perl>my @fifo=qw(1 2 3 a b c);

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

Perl 6

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. <lang perl6>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;
   }

}</lang>

Example usage:

<lang perl6>my @queue does FIFO;

say @queue.is-empty; # -> Bool::True say @queue.enqueue: <A B C>; # -> 3 say @queue.enqueue: Any; # -> 1 say @queue.enqueue: 7, 8; # -> 2 say @queue.is-empty; # -> Bool::False say @queue.dequeue; # -> A say @queue.elems; # -> 5 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 \n 8 \n OHAI! say @queue.is-empty; # -> Bool::True say @queue.dequeue; # -></lang>

PHP

<lang PHP>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);
 }

}</lang>

Example:

<lang PHP>$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 </lang>

PicoLisp

The built-in function 'fifo' maintains a queue in a circular list, with direct access to the first and the last cell <lang PicoLisp>(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</lang> Output: 

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

PL/I

<lang PL/I> /* 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; </lang>

PostScript

Library: initlib

<lang postscript> % our queue is just [] and empty? is already defined. /push {exch tadd}. /pop {uncons exch}. </lang>

Prolog

Works with SWI-Prolog. One can push any data in queue. <lang Prolog>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). </lang>

PureBasic

For FIFO function PureBasic normally uses linked lists. Usage as described above could look like; <lang PureBasic>NewList MyStack()

Procedure Push(n) 

 Shared MyStack()
 LastElement(MyStack())
 AddElement(MyStack())
 MyStack()=n

EndProcedure

Procedure Pop() 

 Shared MyStack()
 Protected n
 If FirstElement(MyStack())  ; e.g. Stack not empty
   n=MyStack()
   DeleteElement(MyStack(),1)
 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()</lang>

Outputs 

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

Python

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:

<lang python> class FIFO(object): 

      def __init__(self, *args):
          self.contents = list()
          if len(args):
              self.contents.extend(*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.extend(*itemlist)
      def empty(self):
          if len(self.contents):
              return True
          else:
              return False
      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,</lang>

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:

<lang python>class FIFO: ## NOT a new-style class, must not derive from "object" 

  def __init__(self,*args):
      self.contents = list()
      if len(args):
          for i in args:
              self.contents.append(i)
  def __call__(self):
      return self.pop()
  def empty(self):
      if self.contents:
          return True
      else:
          return False
  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()</lang>

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.

<lang python>from collections import deque fifo = deque() fifo. appendleft(value) # push value = fifo.pop() not fifo # empty fifo.pop() # raises IndexError when empty</lang>

R

Simple functional implementation

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. <lang R>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")
  l1 <<- NULL
  print(l)
  invisible()

} l <- list() empty()

  1. [1] TRUE

push(3)

  1. 1
  2. [1] 3

push("abc")

  1. 1
  2. [1] 3
  3. 2
  4. [1] "abc"

push(matrix(1:6, nrow=2))

  1. 1
  2. [1] 3
  3. 2
  4. [1] "abc"
  5. 3
  6. [,1] [,2] [,3]
  7. [1,] 1 3 5
  8. [2,] 2 4 6

empty()

  1. [1] FALSE

pop()

  1. 1
  2. [1] 3
  3. 2
  4. [1] "abc"

pop()

  1. 1
  2. [1] 3

pop()

  1. list()

pop()

  1. Error in pop() : can't pop from an empty list</lang>

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.)

Object oriented implementation

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.)

<lang R>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(.$empty()) stop("can't pop from an empty list")
     .$l1 <- NULL
     print(.$l)
     invisible()
  }

})

  1. 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()</lang>

REBOL

<lang REBOL>rebol [ 

   Title: "FIFO"
   Author: oofoe
   Date: 2009-12-11
   URL: http://rosettacode.org/wiki/FIFO

]

Define fifo class

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

Create and populate a FIFO

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

Dump it out, with narrative

print rejoin ["Queue is " either q/empty [""]["not "] "empty."] while [not q/empty][print [" " q/pop]] print rejoin ["Queue is " either q/empty [""]["not "] "empty."] print ["Trying to pop an empty queue yields:" q/pop]</lang>

Output: 

Queue is not empty.
   a
   2
   USD$12.34
   Athos
   Porthos
   Aramis
   Huey Dewey Lewey
Queue is empty.
Trying to pop an empty queue yields: none

REXX

Support for LIFO & FIFO queues is built into the Rexx language. The "PUSH" (LIFO), "QUEUE" (FIFO), "PULL" (and/or "PARSE PULL ..."; "PULL" is a synonym for "PARSE UPPER PULL ...") (dequeue) keywords in conjunction with the QUEUED() built–in function deliver this capability. <lang Rexx>/* Rexx */

Do 

 Call viewQueue

 a = "Fred"
 Push      /* Puts a null line onto the queue */
 Push a 2  /* Puts "Fred 2" onto the queue */
 Call viewQueue

 a = "Toft"
 Queue a 2 /* Enqueues "Toft 2" */
 Queue     /* Enqueues a null line behind the last */
 Call viewQueue

 Do q_ = 1 while queued() > 0
   Parse pull line
   Say right(q_, 3)':' line
   End q_
 Call viewQueue

 Return

End Exit

viewQueue: Procedure Do 

 If queued() = 0 then Do
   Say 'Queue is empty'
   End
 else Do
   Say 'There are' queued() 'elements in the queue'
   End

 Return

End Exit </lang>

Output: 

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

Ruby

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

<lang ruby>require 'forwardable'

  1. A FIFO queue contains elements in first-in, first-out order.
  2. FIFO#push adds new elements to the end of the queue;
  3. 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</lang>

<lang ruby>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")

  1. => 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) # => []</lang>

Scala

<lang scala>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(", ", ", ")")

}</lang> Usage: <lang scala>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)</lang> Output: 

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

Scheme

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.

<lang scheme>(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)))

</lang>


Slate

Toy code based on Slate's Queue standard library (which is optimized for FIFO access): <lang slate>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].</lang>

Smalltalk

Works with: GNU Smalltalk

An OrderedCollection can be easily used as a FIFO queue.

<lang smalltalk>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"</lang>

Standard ML

Here is the signature for a basic queue: <lang Standard ML> 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; </lang> A very basic implementation of this signature backed by a list is as follows: <lang Standard ML> 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; </lang>


Tcl

Here's a simple implementation using a list: <lang tcl>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</lang>

Library: Tcllib (Package: struct::queue)

<lang tcl>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</lang>

UnixPipes

Uses moreutils <lang bash>init() {echo > fifo} push() {echo $1 >> fifo } pop() {head -1 fifo ; (cat fifo | tail -n +2)|sponge fifo} empty() {cat fifo | wc -l}</lang> Usage: <lang bash>push me; push you; push us; push them |pop;pop;pop;pop me you us them</lang>

V

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

<lang v>[fifo_create []]. [fifo_push swap cons]. [fifo_pop [[*rest a] : [*rest] a] view]. [fifo_empty? dup empty?].</lang>

Using it <lang v>|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</lang> 

=true
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