lists:clumped/2

Availability::- use_module(library(lists)).(can be autoloaded)

Pairs is a list of Item-Count pairs that represents the run length encoding of Items. For example:

?- clumped([a,a,b,a,a,a,a,c,c,c], R).
R = [a-2, b-1, a-4, c-3].

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