How to remove even numbers in List using Prolog

Question

I need to remove all even numbers in first list and save the rest to second list. My first non-working approach was:

remove_even([],[]).
remove_even([H1|T1],         


        

Answer 1

Let's look at one of your clauses:

remove_even([El|T],[T]) :- El mod 2 =:= 0.

First, in the notation [El|T] El is a single item, and T is a list. Then [T] will be list inside a list, which probably is not what you want. It should be just "remove_even([El|T],T)".

Next, your variant of the rule just copies T into the answer, not removing any even numbers from the tail. Only the first number (if it's even) will be removed. remove_even should be applied to the T also.

In the end we should have something like this:

remove_even([El|T], NewT) :- 
    El mod 2 =:= 0,
    remove_even(T, NewT).