How to make a condition to end the query in prolog?

Question

i'm new in prolog, so i have done a program which have a method to query something, but when i use it, it keeps yielding results even when it should end.

amigos(juanarjona,omarpacheco).
amigos(juanarjona,jasonancona).
amigos(juanarjona,jorgebolio).
amigos(juanarjona,gerardoadrian).

amigos(Amigo,Amigoslista):-!,amigos(Amigoslista,Amigo).

for example, when i execute

amigos(X,juanarjona).

it should give me just this answers:

X = omarpacheco ; X = jasonancona ; X = jorgebolio ; X = gerardoadrian ;

but what it really do is to repeat those answers without end like this

X = omarpacheco ; X = jasonancona ; X = jorgebolio ; X = gerardoadrian ; X = omarpacheco ; X = jasonancona ; X = jorgebolio ; X = gerardoadrian . . . . . . . . . . . . (repeating)...........

so, that's why i'm recurring to you guys, i need to know how to establish a condition which do not let repeating the answers. Sorry for my bad english by the way.

I Forgot to say that it must be with just one predicate in this case "amigos"

Please be more specific! What about other limitations of what you may or may not use? — repeat, Mar 02 '16 at 08:06

score 4 · Accepted Answer · edited May 23 '17 at 12:23

It's easy: Use two predicates instead of one!

The first predicate comprises facts only:

amigos(juanarjona, omarpacheco).
amigos(juanarjona, jasonancona).
amigos(juanarjona, jorgebolio).
amigos(juanarjona, gerardoadrian).

The second one—built on top of the first one—provides the symmetric closure¹:

sym_amigos(X, Y) :- amigos(X, Y).
sym_amigos(X, Y) :- amigos(Y, X).

Sample query using SICStus Prolog 4.3.2:

| ?- sym_amigos(P, Q).
P = juanarjona,    Q = omarpacheco   ? ;
P = juanarjona,    Q = jasonancona   ? ;
P = juanarjona,    Q = jorgebolio    ? ;
P = juanarjona,    Q = gerardoadrian ? ;
P = omarpacheco,   Q = juanarjona    ? ;
P = jasonancona,   Q = juanarjona    ? ;
P = jorgebolio,    Q = juanarjona    ? ;
P = gerardoadrian, Q = juanarjona    ? ;
no

As your fact base gets larger and larger, looking at all the concrete answers gets tiresome... So, to check if sym_amigos/2 terminates universally, we can run a query like this one:

| ?- sym_amigos(P, Q),      % looking for amigos ...
     false.                 % ... we discard every candidate solution.
no                          % "no" means "terminates universally"

Above query can never yield any solution—after all, it is a conjunction with the goal false/0:

It can, however, tell us if the above goal terminates universally.
If Prolog returns no—it does²—we know that sym_amigos/2 terminates universally.

^{Footnote 1:
meta-predicate symm/2 covers the common idiom "symmetric closure".

Footnote 2: SWI-Prolog does not reply no but false instead.}

How to make a condition to end the query in prolog?

1 Answers1