Hashtbl(3)

NAME
Hashtbl - Hash tables and hash functions.

Module
Module Hashtbl

Documentation

Module Hashtbl
 : sig end

Hash tables and hash functions.

Hash tables are hashed association tables, with in-place modification.

=== Generic interface === type ('a, 'b) t

The type of hash tables from type 'a to type 'b

val create : int -> ('a, 'b) t

Hashtbl.create n creates a new, empty hash table, with initial size n n should be on the order of the expected number of elements that will be in the table. The table grows as needed, so n is just an initial guess.

val clear : ('a, 'b) t -> unit

Empty a hash table.

val add : ('a, 'b) t -> 'a -> 'b -> unit

Hashtbl.add tbl x y adds a binding of x to y in table tbl Previous bindings for x are not removed, but simply hidden. That is, after performing Hashtbl.remove tbl x , the previous binding for x , if any, is restored. (Same behavior as with association lists.)

val copy : ('a, 'b) t -> ('a, 'b) t

Return a copy of the given hashtable.

val find : ('a, 'b) t -> 'a -> 'b

Hashtbl.find tbl x returns the current binding of x in tbl , or raises Not_found if no such binding exists.

val find_all : ('a, 'b) t -> 'a -> 'b list

Hashtbl.find_all tbl x returns the list of all data associated with x in tbl The current binding is returned first, then the previous bindings, in reverse order of introduction in the table.

val mem : ('a, 'b) t -> 'a -> bool

Hashtbl.mem tbl x checks if x is bound in tbl

val remove : ('a, 'b) t -> 'a -> unit

Hashtbl.remove tbl x removes the current binding of x in tbl , restoring the previous binding if it exists. It does nothing if x is not bound in tbl

val replace : ('a, 'b) t -> 'a -> 'b -> unit

Hashtbl.replace tbl x y replaces the current binding of x in tbl by a binding of x to y x is unbound in tbl , a binding of x to y is added to tbl This is functionally equivalent to Hashtbl.remove tbl x followed by Hashtbl.add tbl x y

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

Hashtbl.iter f tbl applies f to all bindings in table tbl f receives the key as first argument, and the associated value as second argument. Each binding is presented exactly once to f The order in which the bindings are passed to f is unspecified. However, if the table contains several bindings for the same key, they are passed to f in reverse order of introduction, that is, the most recent binding is passed first.

val fold : ('a -> 'b -> 'c -> 'c) -> ('a, 'b) t -> 'c -> 'c

Hashtbl.fold f tbl init computes (f kN dN ... (f k1 d1 init)...) , where k1 ... kN are the keys of all bindings in tbl , and d1 ... dN are the associated values. Each binding is presented exactly once to f The order in which the bindings are passed to f is unspecified. However, if the table contains several bindings for the same key, they are passed to f in reverse order of introduction, that is, the most recent binding is passed first.

val length : ('a, 'b) t -> int

Hashtbl.length tbl returns the number of bindings in tbl Multiple bindings are counted multiply, so Hashtbl.length gives the number of times Hashtbl.iter calls it first argument.

=== Functorial interface === module type HashedType = sig end

The input signature of the functor Hashtbl.Make

module type S = sig end

The output signature of the functor Hashtbl.Make

module Make : functor (H : HashedType) -> sig end

Functor building an implementation of the hashtable structure. The functor Hashtbl.Make returns a structure containing a type key of keys and a type 'a t of hash tables associating data of type 'a to keys of type key The operations perform similarly to those of the generic interface, but use the hashing and equality functions specified in the functor argument H instead of generic equality and hashing.

=== The polymorphic hash primitive === val hash : 'a -> int

Hashtbl.hash x associates a positive integer to any value of any type. It is guaranteed that if x = y or Pervasives.compare x y = 0 , then hash x = hash y Moreover, hash always terminates, even on cyclic structures.

val hash_param : int -> int -> 'a -> int

Hashtbl.hash_param n m x computes a hash value for x , with the same properties as for hash n and m give more precise control over hashing. Hashing performs a depth-first, right-to-left traversal of the structure x , stopping after n meaningful nodes were encountered, or m nodes, meaningful or not, were encountered. Meaningful nodes are: integers; floating-point numbers; strings; characters; booleans; and constant constructors. Larger values of m and n means that more nodes are taken into account to compute the final hash value, and therefore collisions are less likely to happen. However, hashing takes longer. The parameters m and n govern the tradeoff between accuracy and speed.