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Queue(3o) OCaml library Queue(3o)

NAME

Queue - First-in first-out queues.

Module

Module Queue

Documentation

Module Queue
: sig end

First-in first-out queues.

This module implements queues (FIFOs), with in-place modification. See Queue.examples below.

Alert unsynchronized_access. Unsynchronized accesses to queues are a programming error.

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 Queue.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 Queue.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
2024-08-29 OCamldoc