Prolog: Unification or Backtracking errors in program

Question

I have a simple knowledge base that encodes a family tree. Some important rules in this representation are as follows:

% fathers
father(michael,cathy).
father(michael,sharon).
father(charles_gordon,michael).
father(charles_gordon,julie).
father(charles,charles_gordon).
father(jim,melody).
father(jim,crystal).
father(elmo,jim).
father(greg,stephanie).
father(greg,danielle).
% mothers
mother(melody,cathy).
mother(melody,sharon).
mother(hazel,michael).
mother(hazel,julie).
mother(eleanor,melody).
mother(eleanor,crystal).
mother(crystal,stephanie).
mother(crystal,danielle).
% parents
parent(X,Y) :- father(X,Y).
parent(X,Y) :- mother(X,Y).
% men
male(michael).
male(charles_gordon).
male(charles).
male(jim).
male(elmo).
male(greg).
% women
female(cathy).
female(sharon).
female(julie).
female(hazel).
female(eleanor).
female(melody).
female(crystal).
female(stephanie).
female(danielle). 

person(X) :- male(X) ; female(X).
parent(X,Y) :- father(X,Y) ; mother(X,Y).          % X is parent of Y
child(X,Y) :- parent(Y,X).
elder(X,Y) :-   parent(X,Y).                       % X is an elder of Y, meaning X is a parent or an ancestor of Y
elder(X,Y) :-   parent(X,Z), elder(Z,Y).
junior(X,Y) :-  child(X,Y).                        % X is a junior of Y, meaning X is a child or some descendant of Y
junior(X,Y) :-  child(X,Z), junior(Z,Y).

I am attempting to find the nearest elder between two individuals(predicate ne(X,Y,Z)). This individual Z is the elder of both X and Y, and no junior of Z is also an elder of BOTH X and Y.

My attempt looks like this:

ne(X,Y,Z) :-    person(X),
            person(Y),
            X \= Y,
            elder(Z,X),
            elder(Z,Y),
            junior(A,Z),
            not(elder(A,X)),
            not(elder(A,Y)).

but this is somehow incorrect, because whenever I run ?- ne(stephanie,cathy,Z). i get

Z = jim ;
Z = jim ;
Z = jim ;
Z = jim ;
Z = elmo ;
Z = elmo ;
Z = elmo ;
Z = elmo ;
Z = eleanor ;
Z = eleanor ;
Z = eleanor ;
Z = eleanor ;

but i'm only supposed to get one answer, and I can't figure out what's wrong. Thanks!

There is quite a bit of redundancy in your program. You have defined parent/2 twice. Also, a "person" is a "male" or a "female", but then why then do you have a father/2 and a mother/2 and not just parent/2? Or can the same "person" be a "male" and a "mother"? Either way, trace your program and you might "figure out" what's wrong. And don't ignore compile time warnings. — user1812457
– user1812457, Commented Sep 9, 2014 at 12:00
Thanks for the comment. I suppose I should look into the singleton variable warnings and such — DarkyTheOdd
– DarkyTheOdd, Commented Sep 9, 2014 at 14:02

CapelliC · Accepted Answer · 2014-09-10 08:47:23Z

2

from this graph

enter image description here seems this answer is correct

?- ne(stephanie,cathy,A).
A = eleanor ;
A = jim.

here is my attempt to ne/3

ne(X,Y,Z) :-
    setof(A, (
        elder(A, X),
        elder(A, Y),
        X \= Y,
        \+ (elder(A, T), elder(T, X) , elder(T, Y) ))
    , As), member(Z, As).

not sure it's the better way...

Setof/3 (joined with member/2) is used to eliminate duplicate answers, since we get

?- aggregate(count,A^ne(stephanie,cathy,A),N).
N = 32.

with this core logic

ne(X,Y,A) :-
        elder(A, X),
        elder(A, Y),
        X \= Y,
        \+ (elder(A, T), elder(T, X) , elder(T, Y)).

note variable A replaces locally the original Z

edit

I didn't took in account the savvy comment by @Boris, but after removing the duplicate parent/2 definition, the setof/3+member/2 trick become useless.

edited Sep 10, 2014 at 8:47

answered Sep 9, 2014 at 15:09

CapelliC

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1 Comment

DarkyTheOdd Over a year ago

Thanks for the answer. My attempt was incorrect because it prints eleanor, jim and elmo, which are all of the elders that they (stephanie and cathy) have in common (eleanor,jim,and elmo). What it should do is just print eleanor and jim. I tried your implementation, but it didn't seem to bind any of the variables? (it prints 'true' infinitely).

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