[newbie] siblings and surnames

From: Fabio (dont_at_spam.please)
Date: 07/21/04 

Date: Wed, 21 Jul 2004 21:47:40 GMT

Hello,

I'm a prolog newbie, studying it for my IA course, and trying to
implement some things I need for an exam...

I've read trough the archives, and I saw that the commutativity/simetry
topic was already covered, and that's been of great help to me, however
i'm now unable to solve another problem.

My problem doesn't concern siblings and surnames, but to not introduce
even more confusion in the discussion I thought I'd simply reuse the
terms used in the past thread about simmetry rules.

Let's assume that we already have this simmetric predicate called
siblings/2, which basically states that

     siblings(X,Y) <==> siblings(Y,X)

Now, I want to code the rule which says that "All siblings share the
same surname". Question is: how the heck do I do that without incurring
in a loop?

What I've come up with is this:

     %all siblings share the same surname
     surname(X, Y) :- siblings(X, Z), surname(Z, Y).

     %john's surname is smith.
     surname(john, smith).

     %john and paul are sibligs
     siblings(john, paul).

But of course the first clause causes a loop, if I type

     surname(paul, X).

If I put the first clause as last, that is this way:

     %john's surname is smith.
     surname(john, smith).

     %john and paul are sibligs
     siblings(john, paul).

     %all siblings share the same surname
     surname(X, Y) :- siblings(X, Z), surname(Z, Y).

then, for how prolog does the search, it works, somwhat, because when it
encounters the surname predicate in the last clause's body it starts its
search from the first clause in the program and thus fids surname(john,
smith). However, if I press ';' when it gives me the answer then I can
see that it went into another loop. I can fix this if I modify the last
clause this way:

     %all siblings share the same surname
     surname(X, Y) :- siblings(X, Z), surname(Z, Y), !.

However, 1) I don't want to put such a clause after all the facts about
the siblings relationship, and 2) this still doesn't allow me to get all
siblings with a given surname.

Any suggestions?

Fabio

Relevant Pages

  • Re: [newbie] siblings and surnames
    ... > My problem doesn't concern siblings and surnames, ... > %john's surname is smith. ... > But of course the first clause causes a loop, ...
    (comp.lang.prolog)
  • Re: A double wedding
    ... the persons living in the same house with the same surname are ... children in the same house with the same surname are siblings. ... possibilities are that they are either siblings or cousins. ...
    (soc.genealogy.britain)
  • Re: A double wedding
    ... the persons living in the same house with the same surname are ... children in the same house with the same surname are siblings. ... The two weddings were not a coincidence, ...
    (soc.genealogy.britain)
  • Re: [newbie] siblings and surnames
    ... >> %john's surname is smith. ... > are siblings, as long as they are not the same person? ... Cesar Rabak ...
    (comp.lang.prolog)