Question

Please help me to solve this problem:

subset(N, [1,2,3], L).

if N=2, I want the result is that:

[1,2];

[2,1];

[1,3];

[3,1];

[2,3];

[3,2];

(in any order)

Answer 1

Well, your base case is trivial:

subset(0,Lst,[]).

If N>0, you have 2 choices as to what to do with the first element of Lst:

1. You can ignore it, and look for your subset in what remains of Lst
2. You can include it in your subset, adding it to what you get for a 1-smaller subset of what remains of Lst.

You might think you have to worry about Lst being too short (or N being too big: same thing), but if you’ve coded the above properly, it should be taken care of for you.

Hoep that’s enough to get you started.