oracular (3) Stdlib.Queue.3o.gz
NAME
Stdlib.Queue - no description
Module
Module Stdlib.Queue
Documentation
Module Queue
: (module 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 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