Prolog length of a list

Question

How can I calculate the length of a list

?- size_sub([[b,a,g], [9,3,7,4], [6]],  X).
X = [3, 4, 1].

?- size_sub([[c,g,e,w], [7]],  X).
X = [4, 1].

?- size_sub(         


        

Answer 1

To map length/2 over a list of lists, we can use the meta-predicate maplist/3 like this:

size_sub(Xss,Ls):-
    maplist(length,Xss,Ls).