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

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.

The optional ~random parameter (a boolean) controls whether the internal organization of the hash table is randomized at each execution of Hashtbl.create or deterministic over all executions.

A hash table that is created with ~random set to false uses a fixed hash function (hash) to distribute keys among buckets. As a consequence, collisions between keys happen deterministically. In Web-facing applications or other security-sensitive applications, the deterministic collision patterns can be exploited by a malicious user to create a denial-of-service attack: the attacker sends input crafted to create many collisions in the table, slowing the application down.

A hash table that is created with ~random set to true uses the seeded hash function seeded_hash with a seed that is randomly chosen at hash table creation time. In effect, the hash function used is randomly selected among 2^{30} different hash functions. All these hash functions have different collision patterns, rendering ineffective the denial-of-service attack described above. However, because of randomization, enumerating all elements of the hash table using fold or iter is no longer deterministic: elements are enumerated in different orders at different runs of the program.

If no ~random parameter is given, hash tables are created in non-random mode by default. This default can be changed either programmatically by calling randomize or by setting the R flag in the OCAMLRUNPARAM environment variable.

• before 4.00.0

  the ~random parameter was not present and all hash tables were created in non-randomized mode.

Empty a hash table. Use reset instead of clear to shrink the size of the bucket table to its initial size.

Empty a hash table and shrink the size of the bucket table to its initial size.

• since 4.00.0

Return a copy of the given hashtable.

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

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

Hashtbl.find_opt tbl x returns the current binding of x in tbl, or None if no such binding exists.

• since 4.05

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.

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

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.

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

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.

If the hash table was created in non-randomized mode, the order in which the bindings are enumerated is reproducible between successive runs of the program, and even between minor versions of OCaml. For randomized hash tables, the order of enumeration is entirely random.

The behavior is not specified if the hash table is modified by f during the iteration.

Hashtbl.filter_map_inplace f tbl applies f to all bindings in table tbl and update each binding depending on the result of f. If f returns None, the binding is discarded. If it returns Some new_val, the binding is update to associate the key to new_val.

Other comments for iter apply as well.

• since 4.03.0

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.

If the hash table was created in non-randomized mode, the order in which the bindings are enumerated is reproducible between successive runs of the program, and even between minor versions of OCaml. For randomized hash tables, the order of enumeration is entirely random.

The behavior is not specified if the hash table is modified by f during the iteration.

Hashtbl.length tbl returns the number of bindings in tbl. It takes constant time. Multiple bindings are counted once each, so Hashtbl.length gives the number of times Hashtbl.iter calls its first argument.

After a call to Hashtbl.randomize(), hash tables are created in randomized mode by default: create returns randomized hash tables, unless the ~random:false optional parameter is given. The same effect can be achieved by setting the R parameter in the OCAMLRUNPARAM environment variable.

It is recommended that applications or Web frameworks that need to protect themselves against the denial-of-service attack described in create call Hashtbl.randomize() at initialization time.

Note that once Hashtbl.randomize() was called, there is no way to revert to the non-randomized default behavior of create. This is intentional. Non-randomized hash tables can still be created using Hashtbl.create ~random:false.

• since 4.00.0

Return true if the tables are currently created in randomized mode by default, false otherwise.

• since 4.03.0

Return a copy of the given hashtable. Unlike copy, rebuild h re-hashes all the (key, value) entries of the original table h. The returned hash table is randomized if h was randomized, or the optional random parameter is true, or if the default is to create randomized hash tables; see create for more information.

rebuild can safely be used to import a hash table built by an old version of the Hashtbl module, then marshaled to persistent storage. After unmarshaling, apply rebuild to produce a hash table for the current version of the Hashtbl module.

• since 4.12.0

• since 4.00.0

Hashtbl.stats tbl returns statistics about the table tbl: number of buckets, size of the biggest bucket, distribution of buckets by size.

• since 4.00.0

Iterate on the whole table. The order in which the bindings appear in the sequence is unspecified. However, if the table contains several bindings for the same key, they appear in reversed order of introduction, that is, the most recent binding appears first.

The behavior is not specified if the hash table is modified during the iteration.

• since 4.07

Same as Seq.map fst (to_seq m)

• since 4.07

Same as Seq.map snd (to_seq m)

• since 4.07

Add the given bindings to the table, using replace

• since 4.07

The input signature of the functor Make.

The output signature of the functor Make.

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. Since the hash function is not seeded, the create operation of the result structure always returns non-randomized hash tables.

The input signature of the functor MakeSeeded.

The output signature of the functor MakeSeeded.

Functor building an implementation of the hashtable structure. The functor Hashtbl.MakeSeeded 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 seeded hashing and equality functions specified in the functor argument H instead of generic equality and hashing. The create operation of the result structure supports the ~random optional parameter and returns randomized hash tables if ~random:true is passed or if randomization is globally on (see Hashtbl.randomize).

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

A variant of hash that is further parameterized by an integer seed.

• since 4.00.0

Hashtbl.hash_param meaningful total x computes a hash value for x, with the same properties as for hash. The two extra integer parameters meaningful and total give more precise control over hashing. Hashing performs a breadth-first, left-to-right traversal of the structure x, stopping after meaningful meaningful nodes were encountered, or total nodes (meaningful or not) were encountered. If total as specified by the user exceeds a certain value, currently 256, then it is capped to that value. Meaningful nodes are: integers; floating-point numbers; strings; characters; booleans; and constant constructors. Larger values of meaningful and total 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 meaningful and total govern the tradeoff between accuracy and speed. As default choices, hash and seeded_hash take meaningful = 10 and total = 100.

A variant of hash_param that is further parameterized by an integer seed. Usage: Hashtbl.seeded_hash_param meaningful total seed x.

• since 4.00.0

get tbl k finds a binding for the key k if present, or returns None if no value is found. Safe version of Hashtbl.find.

get_or tbl k ~default returns the value associated to k if present, and returns default otherwise (if k doesn't belong in tbl).

• since 0.16

keys tbl f iterates on keys (similar order as Hashtbl.iter).

values tbl f iterates on values in the table tbl.

keys_list tbl is the list of keys in tbl. If the key is in the Hashtable multiple times, all occurrences will be returned.

• since 0.8

values_list tbl is the list of values in tbl.

• since 0.8

map_list f tbl maps on a tbl's items. Collect into a list.

incr ?by tbl x increments or initializes the counter associated with x. If get tbl x = None, then after update, get tbl x = Some 1; otherwise, if get tbl x = Some n, now get tbl x = Some (n+1).

• parameter by

  if specified, the int value is incremented by by rather than 1.

• since 0.16

decr ?by tbl x is like incr but subtract 1 (or the value of by). If the value reaches 0, the key is removed from the table. This does nothing if the key is not already present in the table.

• since 0.16

Iterate on bindings in the table.

• since 2.8

add_list tbl x y adds y to the list x is bound to. If x is not bound, it becomes bound to y.

• since 0.16

Add the corresponding pairs to the table, using Hashtbl.add.

• since 2.8

Add the corresponding pairs to the table, using Hashtbl.add. Renamed from add_std_seq since 3.0.

• since 3.0

From the given bindings, added in order.

• since 2.8

From the given bindings, added in order. Renamed from of_std_seq since 3.0.

• since 3.0

add_iter_count tbl i increments the count of each element of i by calling incr. This is useful for counting how many times each element of i occurs.

• since 2.8

add_seq_count tbl seq increments the count of each element of seq by calling incr. This is useful for counting how many times each element of seq occurs. Renamed from add_std_seq_count since 3.0.

• since 3.0

Like add_seq_count, but allocates a new table and returns it.

• since 2.8

Like add_seq_count, but allocates a new table and returns it. Renamed from of_std_seq_count since 3.0.

• since 3.0

to_list tbl returns the list of (key,value) bindings (order unspecified).

of_list l builds a table from the given list l of bindings k_i -> v_i, added in order using add. If a key occurs several times, it will be added several times, and the visible binding will be the last one.

update tbl ~f ~k updates key k by calling f k (Some v) if k was mapped to v, or f k None otherwise; if the call returns None then k is removed/stays removed, if the call returns Some v' then the binding k -> v' is inserted using Hashtbl.replace.

• since 0.14

get_or_add tbl ~k ~f finds and returns the binding of k in tbl, if it exists. If it does not exist, then f k is called to obtain a new binding v; k -> v is added to tbl and v is returned.

• since 1.0

pp ~pp_start ~pp_stop ~pp_sep ~pp arrow pp_k pp_v returns a table printer given a pp_k printer for individual key and a pp_v printer for individual value. pp_start and pp_stop control the opening and closing delimiters, by default print nothing. pp_sep control the separator between binding. pp_arrow control the arrow between the key and value. Renamed from print since 2.0.

• since 0.13