Hashtbl.create n creates a new, empty hash table, with initial size n. For best results, 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.
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 its first argument.
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.)
Hashtbl.replace tbl x y replaces the current binding of x in tbl by a binding of x to y. If 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 modify_def : 'b->'a->('b->'b)->('a, 'b)t-> unit
Hashtbl.modify_def v k f tbl does the same as Hashtbl.modify k f tbl but f v is inserted in tbl if k was unbound.
since 2.1
val modify_opt : 'a->('b option->'b option)->('a, 'b)t-> unit
Hashtbl.modify_opt k f tbl allows to remove, modify or add a binding for k in tbl. f will be called with None if k was unbound. first previous binding of k in tbl will be deleted if f returns None. Otherwise, the previous binding is replaced by the value produced by f.
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.
A number of higher-order functions are provided to allow purely functional traversal or transformation of hashtables. These functions are similar to their counterparts in module BatEnum.
Whenever you wish to traverse or transfor a hashtable, you have the choice between using the more general functions of BatEnum, with keys, values, enum and of_enum, or the more optimized functions of this section.
If you are new to OCaml or unsure about data structure, using the functions of BatEnum is a safe bet. Should you wish to improve performance at the cost of generality, you will always be able to rewrite your code to make use of the functions of this section.
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.
Hashtbl.fold f tbl init computes (f kN dN ... (f k1 d1 (f k0 d0 init))...), where k0,k1..kN are the keys of all bindings in tbl, and d0,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.
map_inplace f x replace all values currently bound in x by f applied to each value.
since 2.1
val filter : ('a-> bool)->('key, 'a)t->('key, 'a)t
filter f m returns a new hashtable where only the values a of m such that f a = true remain.
val filter_inplace : ('a-> bool)->('key, 'a)t-> unit
filter_inplace f m removes from m all bindings that does not satisfy the predicate f.
since 2.1
val filteri : ('key->'a-> bool)->('key, 'a)t->('key, 'a)t
filteri f m returns a hashtbl where only the key, values pairs key, a of m such that f key a = true remain.
val filteri_inplace : ('key->'a-> bool)->('key, 'a)t-> unit
filteri_inplace f m performs as filter_inplace but f receive the value in additiuon to the key.
since 2.1
val filter_map : ('key->'a->'b option)->('key, 'a)t->('key, 'b)t
filter_map f m combines the features of filteri and map. It calls f key0 a0, f key1 a1, f keyn an where a0,a1..an are the elements of m and key0..keyn the corresponding keys. It returns a hashtbl with associations keyi,bi where f keyi ai =
Some bi. When f returns None, the corresponding element of m is discarded.
val filter_map_inplace : ('key->'a->'a option)->('key, 'a)t-> unit
filter_map_inplace f m performs like filter_map but modify m inplace instead of creating a new Hashtbl.
val merge :
('a->'b option->'c option->'d option)->('a, 'b)t->('a, 'c)t->('a, 'd)t
merge f a b returns a new Hashtbl which is build from the bindings of a and b according to the function f, that is given all defined keys one by one, along with the value from a (if defined) and the value from b (if defined), and has to return the (optional) resulting value.
It is assumed that each key is bound at most once in a and b. See merge_all for a more general alternative if this is not the case.
since 2.10.0
val merge_all :
('a->'b list->'c list->'d list)->('a, 'b)t->('a, 'c)t->('a, 'd)t
merge_all f a b is similar to merge, but passes to f all bindings for a key (most recent first, as returned by find_all). f must then return all the new bindings of the merged hashtable (or an empty list if that key should not be bound in the resulting hashtable). Those new bindings will be inserted in reverse, so that the head of the list will become the most recent binding in the merged hashtable.
since 2.10.0
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.
The following modules replace functions defined in Hashtbl with functions behaving slightly differently but having the same name. This is by design: the functions meant to override the corresponding functions of Hashtbl.
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.