Objective Caml module ``Hashtbl''
Module Hashtbl: hash tables and hash functions
Hash tables are hashed association tables, with in-place modification.
type ('a, 'b) t
The type of hash tables from type
'a to type
val create : int -> ('a,'b) t
Hashtbl.create n creates a new, empty hash table, with
n. The table grows as needed, so
just an initial guess. Better results are said to be
n is a prime number.
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
y in table
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
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
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
val remove : ('a, 'b) t -> 'a -> unit
Hashtbl.remove tbl x removes the current binding of
restoring the previous binding if it exists.
It does nothing if
x is not bound in
val iter : ('a -> 'b -> unit) -> ('a, 'b) t -> unit
Hashtbl.iter f tbl applies
f to all bindings in table
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
module type HashedType =
val equal: t -> t -> bool
val hash: t -> int
The input signature of the functor
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
equal, then they must have identical hash values as computed
Examples: suitable (
hash) pairs for arbitrary key
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 =
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
module Make(H: HashedType): (S with type key = H.t)
Hashtbl.Make returns a structure containing
key of keys and a type
'a t of hash tables
associating data of type
'a to keys of type
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
x = y, then
hash x = hash y.
hash always terminates, even on cyclic
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
m give more precise control over hashing. Hashing performs a
depth-first, right-to-left traversal of the structure
n meaningful nodes were encountered, or
meaningful or not, were encountered. Meaningful nodes are: integers;
floating-point numbers; strings; characters; booleans; and constant
constructors. Larger values of
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
govern the tradeoff between accuracy and speed.
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