▼ |  | Data: Data Structures

1 | Imperative Queues
2 | Growable Vectors
3 | Orders and Ordered Dictionaries
4 | Splay Trees
5 | Skip Lists
6 | Interval Maps
7 | Binary Heaps

On this page:

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

Version: 5.3
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### 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.

> procedure (make-queue) → queue/c

Produces an empty queue.

> procedure (enqueue! q v) → void?
> q : queue/c
> v : any/c

Adds an element to the back of a queue.

> procedure (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

> procedure (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)

> procedure (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

> procedure (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

> procedure (queue? v) → boolean?
> v : any/c

This predicate recognizes queues.

Examples:
> > (queue? (make-queue))
> #t
> > (queue? 'not-a-queue)
> #f

> procedure (in-queue queue) → sequence?
> queue : queue?

Returns a sequence whose elements are the elements of queue.

> value queue/c : flat-contract?
> value nonempty-queue/c : flat-contract?

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