prolog - 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

Here is a general predicate that scans a list to find a single member defined by a given goal.

select_element(Goal, [Head | Tail], Selected) :-
    select_element(Goal, Tail, Head, Selected).


select_element(_Goal, [], Selected, Selected).

select_element(Goal, [Head | Tail], Current, FinalSelected) :-
    call(Goal, Head, Current, Selected),
    select_element(Goal, Tail, Selected, FinalSelected).

Lets say you define a predicate

get_bigger_number(N1, N2, N) :-
    N is max(N1, N2).

Now you can execute:

?- select_element(get_bigger_number, [5, 1, -2, 10, 3.2, 0], Selected).

Selected = 10

So all you need to do now is define a predicate get_longer_list(L1, L2, L), and use it instead of get_bigger_number/3.

Of course, using a general predicate like select_element/3 might not be very efficient. For example, you should try to avoid calculating the length of the same list several times, because this calculation is slow in Prolog (at least if implemented in Prolog in the standard way).