Split a list in half

Question

I need to define divide so that List [1,2,3,4,5] divides into:

a = [1,2,3}

b = [4,5]

I\'m getting an error that says \"

Answer 1

Let's give the predicate a more relational name: list_half_half/3

list_half_half(Xs, Ys, Zs) :-
   length(Xs, N),
   H is N - N // 2,
   length(Ys, H),
   append(Ys, Zs, Xs).

length/2 and append/3 are predefined in practically all recent Prologs.

This is GNU Prolog:

| ?- append(L,_,[a,b,c,d]), list_half_half(L,H1,H2).

H1 = []
H2 = []
L = [] ? ;

H1 = [a]
H2 = []
L = [a] ? ;

H1 = [a]
H2 = [b]
L = [a,b] ? ;

H1 = [a,b]
H2 = [c]
L = [a,b,c] ? ;

H1 = [a,b]
H2 = [c,d]
L = [a,b,c,d]