%Source: https://sites.google.com/site/prologsite/prolog-problems %query:group3(g,f,f,f). % 1.27 (**) Group the elements of a set into disjoint subsets. % Problem a) % group3(G,G1,G2,G3) :- distribute the 9 elements of G into G1, G2, and G3, % such that G1, G2 and G3 contain 2,3 and 4 elements respectively group3(G,G1,G2,G3) :- selectN(2,G,G1), subtract(G,G1,R1), selectN(3,R1,G2), subtract(R1,G2,R2), selectN(4,R2,G3), subtract(R2,G3,[]). % selectN(N,L,S) :- select N elements of the list L and put them in % the set S. Via backtracking return all posssible selections, but % avoid permutations; i.e. after generating S = [a,b,c] do not return % S = [b,a,c], etc. selectN(0,_,[]) :- !. selectN(N,L,[X|S]) :- N > 0, el(X,L,R), N1 is N-1, selectN(N1,R,S). el(X,[X|L],L). el(X,[_|L],R) :- el(X,L,R).
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