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make-queue
enqueue!
dequeue!
queue->list
queue-length
queue-empty?
queue?
in-queue
queue/ c
nonempty-queue/ c
Version: 5.2

1 Imperative Queues

Carl Eastlund <cce@racket-lang.org>

 (require data/queue)

This module provides a simple mutable queue representation, first-in/first-out only. Operations on queues mutate it in a thread-unsafe way.

Produces an empty queue.

(enqueue! q v)  void?
  q : queue/c
  v : any/c
Adds an element to the back of a queue.

(dequeue! q)  any/c
  q : nonempty-queue/c
Removes an element from the front of a nonempty queue, and returns that element.

Examples:

(define q (make-queue))
> (enqueue! q 1)
> (dequeue! q)

1

> (enqueue! q 2)
> (enqueue! q 3)
> (dequeue! q)

2

> (dequeue! q)

3

(queue->list queue)  (listof any/c)
  queue : queue/c
Returns an immutable list containing the elements of the queue in the order the elements were added.

Examples:

(define queue (make-queue))
> (enqueue! queue 8)
> (enqueue! queue 9)
> (enqueue! queue 0)
> (queue->list queue)

'(8 9 0)

(queue-length queue)  integer?
  queue : queue/c
Returns the number of elements in the queue.

Examples:

(define queue (make-queue))
> (queue-length queue)

0

> (enqueue! queue 5)
> (enqueue! queue 12)
> (queue-length queue)

2

> (dequeue! queue)

5

> (queue-length queue)

1

(queue-empty? q)  boolean?
  q : queue/c
Recognizes whether a queue is empty or not.

Examples:

(define q (make-queue))
> (queue-empty? q)

#t

> (enqueue! q 1)
> (queue-empty? q)

#f

> (dequeue! q)

1

> (queue-empty? q)

#t

(queue? v)  boolean?
  v : any/c
This predicate recognizes queues.

Examples:

> (queue? (make-queue))

#t

> (queue? 'not-a-queue)

#f

(in-queue queue)  sequence?
  queue : queue?
Returns a sequence whose elements are the elements of queue.

These contracts recognize queues; the latter requires the queue to contain at least one value.