find all k-length subsets of list in prolog

Question

i need to find a k-length subset of list in prolog, i have this function:

    subset([], []).
    subset([E|Tail], [E|NTail]):-
    subset(Tail, NTail).
    subs         


        

Answer 1

Using your initial solution to subset/2, you might add another argument (Len) and:

• Base case holds when Len = 0
• Recursive step which adds element decrements Len and finishes recursion if new Len=0

That would look:

 subset(0, [], []).
 subset(Len, [E|Tail], [E|NTail]):-
   succ(PLen, Len),
   (PLen > 0 -> subset(PLen, Tail, NTail) ; NTail=[]).
 subset(Len, [_|Tail], NTail):-
   subset(Len, Tail, NTail).