How do I find the longest list in a list of lists?

Question

I have a list of lists, and I need to find the longest one of them. If there are more than one with the same length it\'s the same which it returns. Thanks.

Answer 1

Please consider my aproach.

longest([L], L) :-
   !.
longest([H|T], H) :- 
   length(H, N),
   longest(T, X),
   length(X, M),
   N > M,
   !.
longest([H|T], X) :-
   longest(T, X),
   !.

Then you can consult it:

?- longest([[1]], N).
N = [1] ;

?- longest([[1],[2]], N).
N = [2] .

?- longest([[1],[2], [3,3,3], [2]], N).
N = [3, 3, 3] ;

?- longest([[1],[2], [3,3,3], [2]], N).
N = [3, 3, 3].

?- longest([[1],[2], [3,3,3], [2], [4,4,4,4]], N).
N = [4, 4, 4, 4] .

?- longest([[1],[2], [3,3,3], [2], [4,4,4,4]], N).
N = [4, 4, 4, 4] ;

Greets!