Prolog Family Tree query issues

Question

% facts
mother(john, dana).
father(john, david).
mother(chelsea, dana).
father(chelsea, david).
mother(jared, dana).
father(jared, david).
% queries
father(X,Y) :- father(X,Y), write(Y). 
mother(X,Y) :- mother(X,Y), write(Y).
parent(X,Y) :- father(X,Y);mother(X,Y).
sibling(X,Y) :- parent(X,Z), parent(Y,Z), write(Y).

I am having trouble getting these queries to work. when I type in the father command, it will tell me yes or no correctly, but won't do the write command (same with mother). "parent" doesn't work at all for me (therefor sibling doesn't either). Also, if I type in sibling(X,Y). I need to get all siblings...for example, sibling(john, chelsea). I need to output all the possible siblings (jared as well). Let me know where I am going wrong, I really don't see an issue with my logic here. Thanks!

m09 m09 · Accepted Answer · 2011-12-06T01:16:50

Basically you can remove your mother and father predicates that are not facts. They are infinite loops. Since parent use them and sibling use parent, all your predicates are infinite loops.

To see what happens, you can do that :

?- trace, father(john, X).

and observe how prolog handles the query. You'll soon observe than to resolve father, he needs to solve father, and that to solve father, he needs to solve father, and that it never stops...

When the two problematic are removed, I obtain a correct behaviour :

?- father(john, X).
X = david.

?- parent(john, X).
X = david ;
X = dana.

?- sibling(john, X).
john
X = john ;
chelsea
X = chelsea ;
jared
X = jared ;
john
X = john ;
chelsea
X = chelsea ;
jared
X = jared.

Now, to make your sibling predicate better, you could say that someone is not its own sibling and that if you have one common parent it's enough (it will remove the duplicates) : sibling(X,Y) :- father(Y,Z), father(X, Z), X =\= Y.

Prolog Family Tree query issues

1 Answers