Simple Arithmetic in prolog

Question

I am trying to write a program that gives a list and back a newer list that have even numbers in the first list, as below:

evennom([1,3,2,4,3,2,1,2],out)
out = [2,4,2,2]

My Code is as below:

evennom([],[]).  
evennom(H1|T1],[H2|T2):- H2 is H1 mod 2.

Answer 1

You are close - there needs to be something that removes the first element when the mod 2 operation fails. Also, your "copy" rule was not entirely correct.

Try this:

evennom([], []).
evennom([H|T1], [H|T2]) :- H rem 2 =:= 0, !, evennom(T1, T2).
evennom([_|T1], T2) :- evennom(T1, T2).

Answer 2

Prolog does the recursion for you..

evennom(ListIn, ListOut) :-
    findall(Even, (nth0(_, ListIn, Even), 0 is (Even mod 2)), ListOut).