Module Hashtbl
: sigend
Hash tables and hash functions.
Hash tables are hashed association tables, with in-place modification. Because most operations on a hash
table modify their input, they're more commonly used in imperative code. The lookup of the value
associated with a key (see Hashtbl.find , Hashtbl.find_opt ) is normally very fast, often faster than the
equivalent lookup in Map .
The functors Hashtbl.Make and Hashtbl.MakeSeeded can be used when performance or flexibility are key.
The user provides custom equality and hash functions for the key type, and obtains a custom hash table
type for this particular type of key.
Warning a hash table is only as good as the hash function. A bad hash function will turn the table into a
degenerate association list, with linear time lookup instead of constant time lookup.
The polymorphic Hashtbl.t hash table is useful in simpler cases or in interactive environments. It uses
the polymorphic Hashtbl.hash function defined in the OCaml runtime (at the time of writing, it's
SipHash), as well as the polymorphic equality (=) .
See Hashtbl.examples .
Alertunsynchronized_access. Unsynchronized accesses to hash tables are a programming error.
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 ).
Genericinterfacetype(!'a,!'b)t
The type of hash tables from type 'a to type 'b .
valcreate : ?random:bool->int->('a,'b)tHashtbl.createn creates a new, empty hash table, with initial size greater or equal to the suggested
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. If n is very small or negative
then it is disregarded and a small default size is used.
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 the ~random parameter was not present and all hash tables were created in non-randomized mode.
valclear : ('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.
valreset : ('a,'b)t->unit
Empty a hash table and shrink the size of the bucket table to its initial size.
Since 4.00
valcopy : ('a,'b)t->('a,'b)t
Return a copy of the given hashtable.
valadd : ('a,'b)t->'a->'b->unitHashtbl.addtblkeydata 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
Hashtbl.removetblkey , 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 Hashtbl.replace .
valfind : ('a,'b)t->'a->'bHashtbl.findtblx returns the current binding of x in tbl , or raises Not_found if no such binding
exists.
valfind_opt : ('a,'b)t->'a->'boptionHashtbl.find_opttblx returns the current binding of x in tbl , or None if no such binding exists.
Since 4.05
valfind_all : ('a,'b)t->'a->'blistHashtbl.find_alltblx 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.
valmem : ('a,'b)t->'a->boolHashtbl.memtblx checks if x is bound in tbl .
valremove : ('a,'b)t->'a->unitHashtbl.removetblx 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 .
valreplace : ('a,'b)t->'a->'b->unitHashtbl.replacetblkeydata 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.removetblkey followed by Hashtbl.addtblkeydata .
valiter : ('a->'b->unit)->('a,'b)t->unitHashtbl.iterftbl 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.
valfilter_map_inplace : ('a->'b->'boption)->('a,'b)t->unitHashtbl.filter_map_inplaceftbl 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 Somenew_val , the
binding is update to associate the key to new_val .
Other comments for Hashtbl.iter apply as well.
Since 4.03
valfold : ('a->'b->'acc->'acc)->('a,'b)t->'acc->'accHashtbl.foldftblinit computes (fkNdN...(fk1d1init)...) , 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.
vallength : ('a,'b)t->intHashtbl.lengthtbl 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.
valrandomize : 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
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 Hashtbl.create . This is intentional. Non-randomized hash tables can still be created using
Hashtbl.create~random:false .
Since 4.00
valis_randomized : unit->bool
Return true if the tables are currently created in randomized mode by default, false otherwise.
Since 4.03
valrebuild : ?random:bool->('a,'b)t->('a,'b)t
Return a copy of the given hashtable. Unlike Hashtbl.copy , Hashtbl.rebuildh 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
typestatistics = {
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 : intarray ; (* 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
valstats : ('a,'b)t->statisticsHashtbl.statstbl returns statistics about the table tbl : number of buckets, size of the biggest bucket,
distribution of buckets by size.
Since 4.00
HashtablesandSequencesvalto_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 specified if the hash table is modified during the iteration.
Since 4.07
valto_seq_keys : ('a,'b)t->'aSeq.t
Same as Seq.mapfst(to_seqm)Since 4.07
valto_seq_values : ('a,'b)t->'bSeq.t
Same as Seq.mapsnd(to_seqm)Since 4.07
valadd_seq : ('a,'b)t->('a*'b)Seq.t->unit
Add the given bindings to the table, using Hashtbl.addSince 4.07
valreplace_seq : ('a,'b)t->('a*'b)Seq.t->unit
Add the given bindings to the table, using Hashtbl.replaceSince 4.07
valof_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
Functorialinterface
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:
moduleIntHash=structtypet=intletequalij=i=jlethashi=ilandmax_intendmoduleIntHashtbl=Hashtbl.Make(IntHash)leth=IntHashtbl.create17inIntHashtbl.addh12"hello"
This creates a new module IntHashtbl , with a new type 'aIntHashtbl.t of tables from int to 'a . In this example, h contains string values so its type is
stringIntHashtbl.t .
Note that the new type 'aIntHashtbl.t is not compatible with the type ('a,'b)Hashtbl.t of the generic
interface. For example, Hashtbl.lengthh would not type-check, you must use IntHashtbl.length .
moduletypeHashedType=sigend
The input signature of the functor Hashtbl.Make .
moduletypeS=sigend
The output signature of the functor Hashtbl.Make .
moduleMake:(H:HashedType)->sigend
Functor building an implementation of the hashtable structure. The functor Hashtbl.Make returns a
structure containing a type key of keys and a type 'at 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.
moduletypeSeededHashedType=sigend
The input signature of the functor Hashtbl.MakeSeeded .
Since 4.00
moduletypeSeededS=sigend
The output signature of the functor Hashtbl.MakeSeeded .
Since 4.00
moduleMakeSeeded:(H:SeededHashedType)->sigend
Functor building an implementation of the hashtable structure. The functor Hashtbl.MakeSeeded returns a
structure containing a type key of keys and a type 'at 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
Thepolymorphichashfunctionsvalhash : 'a->intHashtbl.hashx associates a nonnegative integer to any value of any type. It is guaranteed that if x=y
or Stdlib.comparexy=0 , then hashx=hashy . Moreover, hash always terminates, even on cyclic
structures.
valseeded_hash : int->'a->int
A variant of Hashtbl.hash that is further parameterized by an integer seed.
Since 4.00
valhash_param : int->int->'a->intHashtbl.hash_parammeaningfultotalx 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 .
valseeded_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_parammeaningfultotalseedx .
Since 4.00
ExamplesBasicExample(*0...99*)letseq=Seq.ints0|>Seq.take100(*buildfromSeq.t*)#lettbl=seq|>Seq.map(funx->x,string_of_intx)|>Hashtbl.of_seqvaltbl:(int,string)Hashtbl.t=<abstr>#Hashtbl.lengthtbl-:int=100#Hashtbl.find_opttbl32-:stringoption=Some"32"#Hashtbl.find_opttbl166-:stringoption=None#Hashtbl.replacetbl166"onesixsix"-:unit=()#Hashtbl.find_opttbl166-:stringoption=Some"onesixsix"#Hashtbl.lengthtbl-:int=101CountingElements
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 .
#moduleChar_tbl=Hashtbl.Make(structtypet=charletequal=Char.equallethash=Hashtbl.hashend)(*countdistinctoccurrencesofcharsin[seq]*)#letcount_chars(seq:charSeq.t):_list=letcounts=Char_tbl.create16inSeq.iter(func->letcount_c=Char_tbl.find_optcountsc|>Option.value~default:0inChar_tbl.replacecountsc(count_c+1))seq;(*turnintoalist*)Char_tbl.fold(funcnl->(c,n)::l)counts[]|>List.sort(fun(c1,_)(c2,_)->Char.comparec1c2)valcount_chars:Char_tbl.keySeq.t->(Char.t*int)list=<fun>(*basicseqfromastring*)#letseq=String.to_seq"helloworld,andallthecamelsinit!"valseq:charSeq.t=<fun>#count_charsseq-:(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..."*)#letseq2=Seq.cycle(String.to_seq"abc")|>Seq.take31valseq2:charSeq.t=<fun>#String.of_seqseq2-:String.t="abcabcabcabcabcabcabcabcabcabca"#count_charsseq2-:(Char.t*int)list=[('a',11);('b',10);('c',10)]
OCamldoc 2025-06-12 Hashtbl(3o)
Install Now
PNG Icons
