Queue/Usage

Queue/Usage
You are encouraged to solve this task according to the task description, using any language you may know.

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

You may see other such structures in the Data Structures category.

Illustration of FIFO behavior

Create a queue data structure and demonstrate its operations. (For implementations of queues, see the FIFO task.)

Operations:

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

Ada

<lang ada>with FIFO; with Ada.Text_Io; use Ada.Text_Io;

procedure Queue_Test is

  package Int_FIFO is new FIFO (Integer);
  use Int_FIFO;
  Queue : FIFO_Type;
  Value : Integer;

begin

  Push (Queue, 1);
  Push (Queue, 2);   
  Push (Queue, 3);
  Pop (Queue, Value);
  Pop (Queue, Value);
  Push (Queue, 4);
  Pop (Queue, Value);
  Pop (Queue, Value);
  Push (Queue, 5);
  Pop (Queue, Value);
  Put_Line ("Is_Empty " & Boolean'Image (Is_Empty (Queue)));

end Queue_Test;</lang> Sample output:

Is_Empty TRUE

AutoHotkey

ahk forum: discussion <lang AutoHotkey>push("st",2),push("st",4)  ; TEST: push 2 and 4 onto stack named "st" While !empty("st")  ; Repeat until stack is not empty

  MsgBox % pop("st")       ; Print popped values (4, 2)

MsgBox % pop("st")  ; Empty MsgBox %ErrorLevel%  ; ErrorLevel = 1: popped too much

1. Include fifo.ahk</lang>

C

See FIFO for the needed code. <lang c>#include <stdio.h>

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

1. include <sys/queue.h>

/* #include "fifolist.h" */

int main() {

 int i;
 FIFOList head;

 TAILQ_INIT(&head);

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

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

 fprintf(stderr, "FIFO list %s\n",

( m_dequeue(&i, &head) ) ? "had still an element" : "is void!");

 exit(0);

}</lang>

C++

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

1. include <cassert> // for run time assertions

int main() {

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

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

 q.push(2);                  // add another element
 assert( !q.empty() );       // it's of course not empty again
 assert( q.front() == 1 );   // the first element didn't change

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

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

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

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

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

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

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

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

}</lang>

Note that the container used to store the queue elements can be specified explicitly; to use a linked linst instead of a deque (the latter is the default), just replace the definition of q to <lang cpp> std::queue<int, std::list<int> ></lang>

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

C#

In C# we can use the Queue<T> class in the .NET 2.0 framework. <lang csharp>using System; using System.Collections.Generic;

namespace RosettaCode {

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

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

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

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

}</lang>

Common Lisp

Using the implementation from FIFO.

<lang lisp>(let ((queue (make-queue)))

 (enqueue 38 queue)
 (assert (not (queue-empty-p queue)))
 (enqueue 23 queue)
 (assert (eql 38 (dequeue queue)))
 (assert (eql 23 (dequeue queue)))
 (assert (queue-empty-p queue)))</lang>

E

Using the implementation from FIFO.

<lang e>def [reader, writer] := makeQueue() require(escape empty { reader.dequeue(empty); false } catch _ { true }) writer.enqueue(1) writer.enqueue(2) require(reader.dequeue(throw) == 1) writer.enqueue(3) require(reader.dequeue(throw) == 2) require(reader.dequeue(throw) == 3) require(escape empty { reader.dequeue(empty); false } catch _ { true })</lang>

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

Erlang

All functions, from the shell: <lang Erlang>1> Q = fifo:new(). {fifo,[],[]} 2> fifo:empty(Q). true 3> Q2 = fifo:push(Q,1). {fifo,[1],[]} 4> Q3 = fifo:push(Q2,2). {fifo,[2,1],[]} 5> fifo:empty(Q3). false 6> fifo:pop(Q3). {1,{fifo,[],[2]}} 7> {Popped, Q} = fifo:pop(Q2). {1,{fifo,[],[]}} 8> fifo:pop(fifo:new()).

• • exception error: 'empty fifo'

    in function  fifo:pop/1</lang>

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

Fortran

Works with: Fortran version 90 and later

<lang fortran>module fifo_nodes

 type fifo_node
    integer :: datum
    ! the next part is not variable and must be present
    type(fifo_node), pointer :: next
    logical :: valid
 end type fifo_node

end module fifo_nodes

program FIFOTest

 use fifo
 implicit none

 type(fifo_head) :: thehead
 type(fifo_node), dimension(5) :: ex, xe
 integer :: i
 
 call new_fifo(thehead)

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

 i = 1
 do
    call fifo_dequeue(thehead, xe(i))
    print *, xe(i)%datum
    i = i + 1
    if ( fifo_isempty(thehead) ) exit
 end do

end program FIFOTest</lang>

J

This is an interactive J session:

<lang J> queue=: conew 'fifo'

  isEmpty__queue 

1

  push__queue 9

9

  push__queue 8

8

  push__queue 7

7

  isEmpty__queue 

0

  pop__queue 

9

  pop__queue 

8

  pop__queue 

7

  isEmpty__queue 

1</lang>

Java

Works with: Java version 1.5+

LinkedList can always be used as a queue or stack, but not in conjunction with the Stack object provided by Java. To use a LinkedList as a stack, use the push and pop methods. A LinkedList can also be used as a double-ended queue (deque); LinkedList has implemented the Deque interface since Java 1.6+. <lang java>import java.util.LinkedList; import java.util.Queue; ... Queue<Integer> queue = new LinkedList<Integer>(); System.out.println(queue.isEmpty()); // empty test - true // queue.remove(); // would throw NoSuchElementException queue.add(1); queue.add(2); queue.add(3); System.out.println(queue); // [1, 2, 3] System.out.println(queue.remove()); // 1 System.out.println(queue); // [2, 3] System.out.println(queue.isEmpty()); // false</lang>

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

Works with: Java version 1.4

<lang java>import java.util.LinkedList; ... LinkedList queue = new LinkedList(); System.out.println(queue.isEmpty()); // empty test - true queue.add(new Integer(1)); queue.add(new Integer(2)); queue.add(new Integer(3)); System.out.println(queue); // [1, 2, 3] System.out.println(queue.removeFirst()); // 1 System.out.println(queue); // [2, 3] System.out.println(queue.isEmpty()); // false</lang>

JavaScript

<lang javascript>var f = new FIFO(); print(f.empty()); f.push(1); f.push(2); f.pop(); print(f.empty()); print(f.dequeue()) print(f.empty()); print(f.dequeue());</lang>

outputs:

true
false
2
true
undefined

Logo

Works with: UCB Logo

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

<lang logo> make "fifo []

print empty? :fifo    ; true
queue "fifo 1
queue "fifo 2
queue "fifo 3
show :fifo            ; [1 2 3]
print dequeue "fifo   ; 1
show :fifo            ; [2 3]
print empty? :fifo    ; false</lang>

OCaml

<lang ocaml># let q = Queue.create ();; val q : '_a Queue.t = <abstr>

1. Queue.is_empty q;;

- : bool = true

1. Queue.add 1 q;;

- : unit = ()

1. Queue.is_empty q;;

- : bool = false

1. Queue.add 2 q;;

- : unit = ()

1. Queue.add 3 q;;

- : unit = ()

1. Queue.peek q;;

- : int = 1

1. Queue.length q;;

- : int = 3

1. Queue.iter (Printf.printf "%d, ") q; print_newline ();;

1, 2, 3, - : unit = ()

1. Queue.take q;;

- : int = 1

1. Queue.take q;;

- : int = 2

1. Queue.peek q;;

- : int = 3

1. Queue.length q;;

- : int = 1

1. Queue.add 4 q;;

- : unit = ()

1. Queue.take q;;

- : int = 3

1. Queue.peek q;;

- : int = 4

1. Queue.take q;;

- : int = 4

1. Queue.is_empty q;;

- : bool = true</lang>

Python

<lang python>import Queue my_queue = Queue.Queue() my_queue.put("foo") my_queue.put("bar") my_queue.put("baz") print my_queue.get() # foo print my_queue.get() # bar print my_queue.get() # baz</lang>

Ruby

Sample usage at FIFO#Ruby

Tcl

See FIFO for operation implementations: <lang tcl>set Q [list] empty Q  ;# ==> 1 (true) push Q foo empty Q  ;# ==> 0 (false) push Q bar peek Q  ;# ==> foo pop Q  ;# ==> foo peek Q  ;# ==> bar</lang>