How to count only the number elements in a list (PROLOG) [duplicate]

Question

count([],0).
count([_|Tail], N) :- count(Tail, N1), N is N1 + 1.

This count all the elements, but I need to count only the numbers.

Answer 1

Prolog has an ISO builtin predicate number/1 that checks whether the given parameter is a number.

We can simply use an if-then-else statement that either increments N is N1+1, or sets N = N1, like:

count([],0).
count([H|Tail], N) :-
    count(Tail, N1),
    (  number(H)
    -> N is N1 + 1
    ;  N = N1
    ).