Queue (ocaml.Stdlib.Queue)

Unsynchronized accesses

Unsynchronized accesses to a queue may lead to an invalid queue state. Thus, concurrent accesses to queues must be synchronized (for instance with a Mutex.t).

type !'a t

The type of queues containing elements of type 'a.

exception Empty

Raised when Queue.take or Queue.peek is applied to an empty queue.

val create : unit -> 'a t

Return a new queue, initially empty.

val add : 'a -> 'a t -> unit

add x q adds the element x at the end of the queue q.

val push : 'a -> 'a t -> unit

push is a synonym for add.

val take : 'a t -> 'a

take q removes and returns the first element in queue q, or raises Empty if the queue is empty.

val take_opt : 'a t -> 'a option

take_opt q removes and returns the first element in queue q, or returns None if the queue is empty.

since 4.08

val pop : 'a t -> 'a

pop is a synonym for take.

val peek : 'a t -> 'a

peek q returns the first element in queue q, without removing it from the queue, or raises Empty if the queue is empty.

val peek_opt : 'a t -> 'a option

peek_opt q returns the first element in queue q, without removing it from the queue, or returns None if the queue is empty.

since 4.08

val top : 'a t -> 'a

top is a synonym for peek.

val clear : 'a t -> unit

Discard all elements from a queue.

val copy : 'a t -> 'a t

Return a copy of the given queue.

val is_empty : 'a t -> bool

Return true if the given queue is empty, false otherwise.

val length : 'a t -> int

Return the number of elements in a queue.

val iter : ('a -> unit) -> 'a t -> unit

iter f q applies f in turn to all elements of q, from the least recently entered to the most recently entered. The queue itself is unchanged.

val fold : ('acc -> 'a -> 'acc) -> 'acc -> 'a t -> 'acc

fold f accu q is equivalent to List.fold_left f accu l, where l is the list of q's elements. The queue remains unchanged.

val transfer : 'a t -> 'a t -> unit

transfer q1 q2 adds all of q1's elements at the end of the queue q2, then clears q1. It is equivalent to the sequence iter (fun x -> add x q2) q1; clear q1, but runs in constant time.

Iterators

val to_seq : 'a t -> 'a Seq.t

Iterate on the queue, in front-to-back order. The behavior is not specified if the queue is modified during the iteration.

since 4.07

val add_seq : 'a t -> 'a Seq.t -> unit

Add the elements from a sequence to the end of the queue.

since 4.07

val of_seq : 'a Seq.t -> 'a t

Create a queue from a sequence.

since 4.07

Examples

Basic Example

A basic example:

# let q = Queue.create ()
val q : '_weak1 Queue.t = <abstr>


# Queue.push 1 q; Queue.push 2 q; Queue.push 3 q
- : unit = ()

# Queue.length q
- : int = 3

# Queue.pop q
- : int = 1

# Queue.pop q
- : int = 2

# Queue.pop q
- : int = 3

# Queue.pop q
Exception: Stdlib.Queue.Empty.

Search Through a Graph

For a more elaborate example, a classic algorithmic use of queues is to implement a BFS (breadth-first search) through a graph.

 type graph = {
   edges: (int, int list) Hashtbl.t
 }

(* Search in graph [g] using BFS, starting from node [start].
   It returns the first node that satisfies [p], or [None] if
   no node reachable from [start] satisfies [p].
*)
let search_for ~(g:graph) ~(start:int) (p:int -> bool) : int option =
  let to_explore = Queue.create() in
  let explored = Hashtbl.create 16 in

  Queue.push start to_explore;
  let rec loop () =
    if Queue.is_empty to_explore then None
    else
      (* node to explore *)
      let node = Queue.pop to_explore in
      explore_node node

  and explore_node node =
    if not (Hashtbl.mem explored node) then (
      if p node then Some node (* found *)
      else (
        Hashtbl.add explored node ();
        let children =
          Hashtbl.find_opt g.edges node
          |> Option.value ~default:[]
        in
        List.iter (fun child -> Queue.push child to_explore) children;
        loop()
      )
    ) else loop()
  in
  loop()

(* a sample graph *)
let my_graph: graph =
  let edges =
    List.to_seq [
      1, [2;3];
      2, [10; 11];
      3, [4;5];
      5, [100];
      11, [0; 20];
    ]
    |> Hashtbl.of_seq
  in {edges}

# search_for ~g:my_graph ~start:1 (fun x -> x = 30)
- : int option = None

# search_for ~g:my_graph ~start:1 (fun x -> x >= 15)
- : int option = Some 20

# search_for ~g:my_graph ~start:1 (fun x -> x >= 50)
- : int option = Some 100