## table of contents

Hashtbl(3o) | OCaml library | Hashtbl(3o) |

# NAME¶

Hashtbl - Hash tables and hash functions.

# Module¶

Module Hashtbl

# Documentation¶

Module **Hashtbl**

: **sig end**

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* : **?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 ( **Hashtbl.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 **Hashtbl.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
**Hashtbl.fold** or **Hashtbl.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 **Hashtbl.randomize** or by setting the
**R** flag in the **OCAMLRUNPARAM** environment variable.

**Before4.00.0** the **~** **random** parameter was not
present and all hash tables were created in non-randomized mode.

*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.

**Since** 4.00.0

*val copy* : **('a, 'b) t -> ('a, 'b) t**

Return a copy of the given hashtable.

*val add* : **('a, 'b) t -> 'a -> 'b ->
unit**

**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 **Hashtbl.remove**
**tbl key** , the previous binding for **key** , 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_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.

**Since** 4.05

*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**

**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
**Hashtbl.remove** **tbl key** followed by **Hashtbl.add** **tbl
key data** .

*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 defined 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 **Hashtbl.iter** apply as well.

**Since** 4.03.0

*val fold* : **('a -> 'b -> 'c -> 'c) -> ('a,
'b) t -> 'c -> 'c**

**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 defined 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.

*val randomize* : **unit -> unit**

After a call to **Hashtbl.randomize()** , hash tables are
created in randomized mode by default: **Hashtbl.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
**Hashtbl.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
**Hashtbl.create** . This is intentional. Non-randomized hash tables can
still be created using **Hashtbl.create ~random:false** .

**Since** 4.00.0

*val is_randomized* : **unit -> bool**

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

**Since** 4.03.0

*val rebuild* : **?random:bool -> ('a, 'b) t -> ('a,
'b) t**

Return a copy of the given hashtable. Unlike **Hashtbl.copy** ,
**Hashtbl.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
**Hashtbl.create** for more information.

**Hashtbl.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 **Hashtbl.rebuild** to
produce a hash table for the current version of the **Hashtbl**
module.

**Since** 4.12.0

*type statistics* = {

num_bindings : **int** ; (* Number of bindings present in the table. Same
value as returned by **Hashtbl.length** .

*)

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** .

*)

}

**Since** 4.00.0

*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.

**Since** 4.00.0

## Hash tables and Sequences¶

*val to_seq* : **('a, 'b) t -> ('a * 'b) Seq.t**

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 defined if the hash table is modified during the iteration.

**Since** 4.07

*val to_seq_keys* : **('a, 'b) t -> 'a Seq.t**

Same as **Seq.map fst (to_seq m)**

**Since** 4.07

*val to_seq_values* : **('a, 'b) t -> 'b Seq.t**

Same as **Seq.map snd (to_seq m)**

**Since** 4.07

*val add_seq* : **('a, 'b) t -> ('a * 'b) Seq.t ->
unit**

Add the given bindings to the table, using **Hashtbl.add**

**Since** 4.07

*val replace_seq* : **('a, 'b) t -> ('a * 'b) Seq.t ->
unit**

Add the given bindings to the table, using
**Hashtbl.replace**

**Since** 4.07

*val of_seq* : **('a * 'b) Seq.t -> ('a, 'b) t**

Build a table from the given bindings. The bindings are added in
the same order they appear in the sequence, using **Hashtbl.replace_seq**
, which means that if two pairs have the same key, only the latest one will
appear in the table.

**Since** 4.07

## Functorial interface¶

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 =* **sig end**

The input signature of the functor **Hashtbl.Make** .

*module type S =* **sig end**

The output signature of the functor **Hashtbl.Make** .

*module Make :* **functor (H : HashedType) -> sig
end**

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 =* **sig end**

The input signature of the functor **Hashtbl.MakeSeeded** .

**Since** 4.00.0

*module type SeededS =* **sig end**

The output signature of the functor **Hashtbl.MakeSeeded**
.

**Since** 4.00.0

*module MakeSeeded :* **functor (H : SeededHashedType) ->
sig end**

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** ).

**Since** 4.00.0

## The polymorphic hash functions¶

*val hash* : **'a -> int**

**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.

*val seeded_hash* : **int -> 'a -> int**

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

**Since** 4.00.0

*val hash_param* : **int -> int -> 'a ->
int**

**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, **Hashtbl.hash**
and **Hashtbl.seeded_hash** take **meaningful = 10** and **total =
100** .

*val seeded_hash_param* : **int -> int -> int -> 'a
-> int**

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

**Since** 4.00.0

2022-01-19 | OCamldoc |