Std.Hashtbl
include module type of struct include Hashtbl end
Unsynchronized accesses
Unsynchronized accesses to a hash table may lead to an invalid hash table state. Thus, concurrent accesses to a hash tables must be synchronized (for instance with a Mutex.t
).
type (!'a, !'b) t = ('a, 'b) Hashtbl.t
The type of hash tables from type 'a
to type 'b
.
val create : ?random:bool -> int -> ('a, 'b) t
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.
val clear : ('a, 'b) t -> unit
Empty a hash table. Use reset
instead of clear
to shrink the size of the bucket table to its initial size.
val reset : ('a, 'b) t -> unit
Empty a hash table and shrink the size of the bucket table to its initial size.
val add : ('a, 'b) t -> 'a -> 'b -> unit
Hashtbl.add tbl key data
adds a binding of key
to data
in table tbl
.
Warning: 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.)
If you desire the classic behavior of replacing elements, see replace
.
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_opt : ('a, 'b) t -> 'a -> 'b option
Hashtbl.find_opt tbl x
returns the current binding of x
in tbl
, or None
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
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.
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.
val filter_map_inplace : ('a -> 'b -> 'b option) -> ('a, 'b) t -> unit
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.
val fold : ('a -> 'b -> 'acc -> 'acc) -> ('a, 'b) t -> 'acc -> 'acc
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.
val length : ('a, 'b) t -> int
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 before any domains are created.
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
.
Return true
if the tables are currently created in randomized mode by default, false
otherwise.
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.
type statistics = Hashtbl.statistics = {
num_bindings : int;
num_buckets : int;
Number of buckets in the table.
*)max_bucket_length : int;
Maximal number of bindings per bucket.
*)bucket_histogram : int array;
Histogram of bucket sizes. This array histo
has length max_bucket_length + 1
. The value of histo.(i)
is the number of buckets whose size is i
.
}
val stats : ('a, 'b) t -> statistics
Hashtbl.stats tbl
returns statistics about the table tbl
: number of buckets, size of the biggest bucket, distribution of buckets by size.
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.
Add the given bindings to the table, using replace
Build a table from the given bindings. The bindings are added in the same order they appear in the sequence, using replace_seq
, which means that if two pairs have the same key, only the latest one will appear in the table.
The functorial interface allows the use of specific comparison and hash functions, either for performance/security concerns, or because keys are not hashable/comparable with the polymorphic builtins.
For instance, one might want to specialize a table for integer keys:
module IntHash =
struct
type t = int
let equal i j = i=j
let hash i = i land max_int
end
module IntHashtbl = Hashtbl.Make(IntHash)
let h = IntHashtbl.create 17 in
IntHashtbl.add h 12 "hello"
This creates a new module IntHashtbl
, with a new type 'a
IntHashtbl.t
of tables from int
to 'a
. In this example, h
contains string
values so its type is string IntHashtbl.t
.
Note that the new type 'a IntHashtbl.t
is not compatible with the type ('a,'b) Hashtbl.t
of the generic interface. For example, Hashtbl.length h
would not type-check, you must use IntHashtbl.length
.
module type HashedType = Hashtbl.HashedType
The input signature of the functor Make
.
module Make = Hashtbl.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.
module type SeededHashedType = Hashtbl.SeededHashedType
The input signature of the functor MakeSeeded
.
module type SeededS = Hashtbl.SeededS
The output signature of the functor MakeSeeded
.
module MakeSeeded = Hashtbl.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.
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
.
(* 0...99 *)
let seq = Seq.ints 0 |> Seq.take 100
(* build from Seq.t *)
# let tbl =
seq
|> Seq.map (fun x -> x, string_of_int x)
|> Hashtbl.of_seq
val tbl : (int, string) Hashtbl.t = <abstr>
# Hashtbl.length tbl
- : int = 100
# Hashtbl.find_opt tbl 32
- : string option = Some "32"
# Hashtbl.find_opt tbl 166
- : string option = None
# Hashtbl.replace tbl 166 "one six six"
- : unit = ()
# Hashtbl.find_opt tbl 166
- : string option = Some "one six six"
# Hashtbl.length tbl
- : int = 101
Given a sequence of elements (here, a Seq.t
), we want to count how many times each distinct element occurs in the sequence. A simple way to do this, assuming the elements are comparable and hashable, is to use a hash table that maps elements to their number of occurrences.
Here we illustrate that principle using a sequence of (ascii) characters (type char
). We use a custom Char_tbl
specialized for char
.
# module Char_tbl = Hashtbl.Make(struct
type t = char
let equal = Char.equal
let hash = Hashtbl.hash
end)
(* count distinct occurrences of chars in [seq] *)
# let count_chars (seq : char Seq.t) : _ list =
let counts = Char_tbl.create 16 in
Seq.iter
(fun c ->
let count_c =
Char_tbl.find_opt counts c
|> Option.value ~default:0
in
Char_tbl.replace counts c (count_c + 1))
seq;
(* turn into a list *)
Char_tbl.fold (fun c n l -> (c,n) :: l) counts []
|> List.sort (fun (c1,_)(c2,_) -> Char.compare c1 c2)
val count_chars : Char_tbl.key Seq.t -> (Char.t * int) list = <fun>
(* basic seq from a string *)
# let seq = String.to_seq "hello world, and all the camels in it!"
val seq : char Seq.t = <fun>
# count_chars seq
- : (Char.t * int) list =
[(' ', 7); ('!', 1); (',', 1); ('a', 3); ('c', 1); ('d', 2); ('e', 3);
('h', 2); ('i', 2); ('l', 6); ('m', 1); ('n', 2); ('o', 2); ('r', 1);
('s', 1); ('t', 2); ('w', 1)]
(* "abcabcabc..." *)
# let seq2 =
Seq.cycle (String.to_seq "abc") |> Seq.take 31
val seq2 : char Seq.t = <fun>
# String.of_seq seq2
- : String.t = "abcabcabcabcabcabcabcabcabcabca"
# count_chars seq2
- : (Char.t * int) list = [('a', 11); ('b', 10); ('c', 10)]
val sexp_of_t :
('a -> Sexplib0.Sexp.t) ->
('b -> Sexplib0.Sexp.t) ->
('a, 'b) Hashtbl.t ->
Sexplib0.Sexp.t
val t_of_sexp :
(Sexplib0.Sexp.t -> 'a) ->
(Sexplib0.Sexp.t -> 'b) ->
Sexplib0.Sexp.t ->
('a, 'b) Hashtbl.t