Predicate to unzip a list

Question

List1=[(x,1),(y,1),(z,1)]

I'm attempting to split this list:

into two lists:

List3=[x,y,z]
List4=[1,1,1]

So I have written this predicate to try to do it:

splt([], [], []).
splt([X|Xs], [Y|Ys], [X,Y|Zs]) :-
    splt(Xs,Ys,Zs).

However instead of the desired result, the predicate returns:

1 ?- splt([(x,1),(y,2),(z,3)],L3,L4).
L3 = [_G1760, _G1769, _G1778],
L4 = [ (z, 1), _G1760, (y, 2), _G1769, (z, 3), _G1778].

Answer 1

First, the term you have chosen. This: (a, b), is most definitely not how you would usually represent a "tuple" in Prolog. You almost always use a-b for a "pair", and pairs are used throughout the standard libraries.

So your initial list would look like this: [x-1, y-1, z-1].

This should also explain why you are having your problem. You write (a, b), but your predicate says a, b, and you consume two elements when you expect to get one ,(a,b) term. So, to fix your current predicate you would write:

split([], [], []).
split([X|Xs], [Y|Ys], [(X,Y)|XYs]) :-
    split(Xs, Ys, XYs).

?- split(Xs, Ys, [(x,1), (y,1), (z,1)]).
Xs = [x, y, z],
Ys = [1, 1, 1].

But instead, using a more conventional name, term order, and Prolog pairs:

zip([], [], []).
zip([X-Y|XYs], [X|Xs], [Y|Ys]) :-
    zip(XYs, Xs, Ys).

?- zip([x-1, y-1, z-1], Xs, Ys).
Xs = [x, y, z],
Ys = [1, 1, 1].

And of course, SWI-Prolog at least has a library(pairs), and it comes with a pairs_keys_values/3:

?- pairs_keys_values([x-1, y-1, z-1], Xs, Ys).
Xs = [x, y, z],
Ys = [1, 1, 1].

Answer 2

I find comfortable using library(yall):

?- maplist([(X,Y),X,Y]>>true, [(x,1),(y,2),(z,3)],L3,L4).
L3 = [x, y, z],
L4 = [1, 2, 3].

or, maybe clearer

?- maplist([A,B,C]>>(A=(B,C)), [(x,1),(y,2),(z,3)],L3,L4).
L3 = [x, y, z],
L4 = [1, 2, 3].

Answer 3

You're matching the tuple as a whole, rather than it's component parts.

You should match on [(X1,Y1)|XS], instead of [X|XS] and [Y|Ys].

splt([],[],[]).
splt([(X1,Y1)|Xs],[X1|T1],[Y1|T2]):-
  splt(Xs,T1,T2).

Here the first term is used as input, the second and third as output.

Ideone example, using SWI-Prolog, here.