Cycles in the graph. You need to track what nodes you’re visiting, and check them. Try this, using a helper predicate with an accumulator to track the visited nodes: path(A,B) :- % two nodes are connected, if walk(A,B,[]) % – if we can walk from one to the other, . % first seeding the visited … Read more
It’s useful, but in my opinion not yet ideal because I cannot cut duplicate paths at the point of their creation. Consider, with the complete graph K_n: n_complete(N, Es) :- numlist(1, N, Ns), phrase(pairs(Ns), Es). adjacent(Edges, X, Y) :- member(edge(X, Y), Edges). pairs([]) –> []. pairs([N|Ns]) –> edges(Ns, N), pairs(Ns). edges([], _) –> []. edges([N|Ns], … Read more
How about defining path/4 like this? path(R_2, Xs, A,Z) :- % A path `Xs` from `A` to `Z` is … walk(R_2, Xs, A,Z), % … a walk `Xs` from `A` to `Z` … all_dif(Xs). % … with no duplicates in `Xs`. To aid universal termination, we swap the two goals in above conjunction … path(R_2, … Read more