Objective Caml module ``Hashtbl''

Module Hashtbl: 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. The table grows as needed, so n is just an initial guess. Better results are said to be achieved when n is a prime number. Raise Invalid_argument if n is less than 1.

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 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 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. The order in which the bindings are passed to f is unspecified. Each binding is presented exactly once to f.

Functorial interface

module type HashedType =
  sig
    type t
    val equal: t -> t -> bool
    val hash: t -> int
  end

The input signature of the functor Hashtbl.Make. t is the type of keys. equal is the equality predicate used to compare keys. hash is a hashing function on keys, returning a non-negative integer. It must be such that if two keys are equal according to equal, then they must have identical hash values as computed by hash. Examples: suitable (equal, hash) pairs for arbitrary key types include ((=), Hashtbl.hash) for comparing objects by structure, and ((==), Hashtbl.hash) for comparing objects by addresses (e.g. for mutable or cyclic keys).

module type S =
  sig
    type key
    type 'a t
    val create: int -> 'a t
    val clear: 'a t -> unit
    val add: 'a t -> key -> 'a -> unit
    val remove: 'a t -> key -> unit
    val find: 'a t -> key -> 'a
    val find_all: 'a t -> key -> 'a list
    val mem: 'a t -> key -> bool
    val iter: (key -> 'a -> unit) -> 'a t -> unit
  end
module Make(H: HashedType): (S with type key = H.t)

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, 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. The two extra parameters 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.

This document was translated from L^AT_EX by H^EV^EA.