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Predicate downcase_atom/2
Availability:built-in
downcase_atom(+AnyCase, -LowerCase)
Converts the characters of AnyCase into lowercase as char_type/2 does (i.e. based on the defined locale if Prolog provides locale support on the hosting platform) and unifies the lowercase atom with LowerCase.

Examples

Counting word frequency

Counting word frequency

word_frequency_count(Words, Counts) :-
    maplist(downcase_atom, Words, LwrWords),
    msort(LwrWords, Sorted),
    clumped(Sorted, Counts).
?- word_frequency_count([a,b,'A',c,d,'B',b,e], Counts).
Counts = [a-2, b-3, c-1, d-1, e-1].
See also
- tokenize_atom/2 or split_string/4 may be used to split a text into tokens
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LogicalCaptain said (2021-05-22T22:03:02):0 upvotes 0 0 downvotes
Picture of user LogicalCaptain. 
?- downcase_atom('aAbB12',X).
X = aabb12.
?- downcase_atom("aAbB12",X).
X = aabb12.

For strings, there is the differently named string_lower/2

?- string_lower('aAbB12',X).
X = "aabb12".

?- string_lower("aAbB12",X).
X = "aabb12".