Search code examples
prologcombinations

How to generate all permutations from positional value-lists in Prolog?

I have a list of lists and I'd like to generate the possible permutations, the first element coming from the first sub-list, the second from the second sub-list and so on.

?- A = [[1], [a, b], [2]], magic_function(A, Ans).
   Ans = [1, a, 2]
;  Ans = [1, b, 2]
;  false.

?- B = [[1], [a, b], [4, 5]], magic_function(B, Ans).
   Ans = [1, a, 4]
;  Ans = [1, a, 5]
;  Ans = [1, b, 4]
;  Ans = [1, b, 5]
;  false

I suppose it's a pretty standard idea, but for some reason I just can't seem to come up with anything useful.

Solution

  • It can be done using maplist/3 and member/2:

    ?- maplist(member, Es, [[1],[a,b],[2]]).
   Es = [1,a,2]
;  Es = [1,b,2].