Generate List then Split into Two in Prolog

I am a complete amateur on Prolog so my question might be very basic. I want to automatically generate a list from 1 to N, then split it into even and odd, from just one integer input (so I don't input the list manually). Let's say I input 5, then the result should be like this: X = [1,3,5] Y = [2,4] Doesn't matter which one is X, which one is Y.

How should I tackle this problem?

I know the built-in function to generate list is numlist(1,5,L). I also found an answer on how to split the list here

I tried to combine those two like this separate_even_odd(N) :- numlist(1,N,L), separate_even_odd(L, X, Y). Then call the function separate_even_odd(5).

All i got is True.

Ultimately I want to append the odd list to the even list but let's put that on another story. For now, I just want it splitted.

Solution

SWI-Prolog has a library predicate partition/4 that seems it's done to fulfill your needs:

separate_even_odd(Integers, Even, Odd) :-
  partition(integer_is_even, Integers, Even, Odd).
integer_is_even(I) :- I mod 2 =:= 0.

Instead of providing the service predicate integer_is_even/1, we could as well use the lambda library(yall):

separate_even_odd(Integers, Even, Odd) :-
  partition([I] >> (I mod 2 =:= 0), Integers, Even, Odd).

and we get

?- numlist(1,5,L), separate_even_odd(L, Even, Odd).
L = [1, 2, 3, 4, 5],
Even = [2, 4],
Odd = [1, 3,

Just to illustrate some of the unusual constructs of Prolog (unification and if/then/else), take a look at a simple implementation, in procedural style, without library predicates:

list_with_separate_even_odd(IntegerLow, IntegerHigh, Even, Odd) :-
    (   IntegerLow > IntegerHigh
    ->  Even = [], Odd = []
    ;   (   IntegerLow mod 2 =:= 0
        ->  Even = [IntegerLow|RestEven], Odd = RestOdd
        ;   Even = RestEven, Odd = [IntegerLow|RestOdd]
        ),
        LowSucc is IntegerLow + 1,
        list_with_separate_even_odd(LowSucc, IntegerHigh, RestEven, RestOdd)
    ).

Note in particular how =/2 performs unification, not assigment.