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hash_update(+Table, ++Key, -OldValue, +NewValue, +InitValue)
Lookup and replace the value stored under Key, or initialise it
- Table
- A hash table
- Key
- a ground term
- OldValue
- Any term
- NewValue
- Any term
- InitValue
- Any term
Description
Lookup the old value stored under Key (defaulting to InitValue if none),
and store NewValue in its place. Behaves like
hash_update(Table, Key, Old, New, Init) :-
( hash_get(Table, Key, Old) ->
hash_set(Table, Key, New)
;
Old = Init,
hash_set(Table, Key, New)
).
Note that OldValue can be used to construct NewValue, see examples.
Modes and Determinism
- hash_update(+, ++, -, +, +) is det
Examples
?- hash_create(H),
hash_update(H, k, A, hello, first),
hash_update(H, k, B, world, first),
hash_get(H, k, C).
A = first
B = hello
C = world
Yes (0.00s cpu)
% Example: increment a numerical table entry, initialising with 0
hash_inc(H, Key) :-
hash_update(H, Key, N0, N1, 0),
N1 is N0+1.
% Example: add element to a list entry, starting with []
hash_prepend(H, Key, Value) :-
hash_update(H, Key, Values, [Value|Values], []).
See Also
hash_get / 3, hash_set / 3, hash_update / 4, hash_map / 3