Simple graph search in Prolog

Question

I\'m trying to code a simple graph search in SWI-Prolog. I came up with the following program:

adjacent(1,4). adjacent(4,2). adjacent(3,6).
adjacent(6,4). ad         


        

Answer 1

You are trying to use the returned path also as a check for avoiding loops. That won't work when searching for a path because the path is not yet instantiated at the negation.

The simplest solution is to add another input argument where you collect the nodes already visited and check these to avoid repetition.

Also, I think you should check A \== B instead of A \== C. You don't have loops on nodes, so the latter will never happen. The case A == B is handled in the first clause, so you don't want to do that again in the second clause.

You also got the arguments of member backwards and need to fix the list in the first clause, as Martin has written.

An advanced way to avoid infinite loops without an extra argument is to use freeze/2 on the negation.

Also take a look at how debugging works in Prolog, that might help you understand things better.