Hash tables are one of the many key-value representations available to
This module implements a hash table as a mutable and backtrackable
data structure. The hash table is implemented as a closed hash table,
where the buckets array is implemented using an unbounded arity
compound term. Elements in this array are manipulated using setarg/3.
Hash tables allow for any Prolog data types as keys or values, except
that the key cannot be a variable. Applications that require a plain
variable as key can do so by wrapping all keys in a compound, e.g.,
- Create a new hash table.
- ht_is_hashtable(@HT) is semidet
- True when HT is a hash table.
- ht_size(+HT, -Count) is det
- True when Size is the number of key-value pairs in HT.
- ht_put(!HT, +Key, +Value) is det
- Add a Key-Value to HT. The binding is undone on backtracking.
- ht_put_new(!HT, +Key, +Value) is semidet
- As ht_put/3, but fails if Key is already in HT instead of updating
the associated value.
- ht_update(+HT, +Key, ?Old, +New) is semidet
- True when HT holds Key-Old before and Key-New after this call. Note
that it is possible to update to a variable and the instantiate
this. For example, a word-count update could be implemented as:
update_word_count(HT, Word) :-
( ht_update(HT, Word, Old, New)
-> New is Old+1
; ht_put(HT, Word, 1)
- ht_put(!HT, +Key, +Value, +IfNew, -Old) is det
- Add Key-Value to HT. Old is unified with the old value associated
with Key or, if Key is new, with IfNew. This can be used to
bootstrap managing a list of values, e.g.
ht_put_list(HT, Key, Value) :-
ht_put(HT, Key, [Value|Tail], , Tail).
- ht_del(!HT, +Key, -Value) is semidet
- Delete Key-Value from HT. Fails if Key does not appear in HT or
Value does not unify with the old associated value.
- ht_get(+HT, +Key, -Value) is semidet
- True when Key is in HT and associated with Value.
- ht_gen(+HT, ?Key, ?Value) is nondet
- True when Key-Value is in HT. Pairs are enumerated on backtracking
using the hash table order.
- ht_pairs(?HT, ?Pairs) is det
- True when Pairs and HT represent the same association. When used in
mode (+,-), Pairs is an ordered set.
- ht_keys(+HT, -Keys) is det
- True when Keys is an ordered set of all keys in HT.