|
»
RE: Who likes whom?
|
View:
New views
4 Messages
—
Rating Filter:
Alert me
|
 |
|
|
|
Re: RE: Who likes whom?
by Bart Demoen-2
::
Rate this Message:
> >likes(X,Z) :- likes(X,Y), likes (Y,Z). ^ there is a space just before the ( shouldn't be there please, do not ignore the SWI syntax error messages ! Cheers Bart Demoen Disclaimer: http://www.kuleuven.be/cwis/email_disclaimer.htm _______________________________________________ SWI-Prolog mailing list SWI-Prolog@... https://mailbox.iai.uni-bonn.de/mailman/listinfo.cgi/swi-prolog |
|
|
Re: RE: Who likes whom?
by Andrew Koster
::
Rate this Message:
Your predicate won't work. Even if you fix the syntax errors it will result in
?- likes(a,d). yes This is quite easy to see. To solve likes(a,d), it will use the clause likes(X,Z) :- likes(X,Y), likes(Y,Z) in the following manner: likes(a,d) :- likes(a, Y), likes(Y,d). It will then try to bind Y to anything for which likes(a,Y) holds. It will find likes(a,b) and try to verify likes(b,d). It can do this by using that same predicate again (recursion). Recursion is a very powerful tool in Prolog, but it isn't always the right tool to use. In this case, rewrite it as follows: likes2(X,Y) :- likes(X,Y). likes2(X,Y) :- likes2(X, Z, Y). likes2(X, Z, Y) :- likes(X,Z), likes(Z,Y). This way, you can query: likes2(a,c) and it will answer yes. The first clause for likes2 will fail (there is no direct likes predicate). However, the second clause calls likes2/3. This succeeds for obvious reasons. Similarly, likes2(a,d) will fail, because there is no recursion. Cheers, Andrew Quoting Steve Kirkby <winkley192@...>: > Anyone? I would really like to know. > > Steve Kirkby > >> The following does not work - SWI-Prolog just replies "no": > >> likes(a,b). >> likes(b,c). >> likes(c,d). > >> likes(X,Z) :- likes(X,Y), likes (Y,Z). > >> ?- likes(a.c). >> no > >> 1: Why not please? > >> 2: And how to achieve the results: > >> ?- likes(a,c). >> yes > >> ?- likes(a,d). >> no > >> A person likes a third person if he likes someone who likes that >> third person, > but the person should not like a fourth person if he likes a someone > who likes a > third person who likes that fourth person - a reasonable model of human > relations. > >> When I used micro-prolog in the early 1990s the If-Then worked. > >> Thanks, Steve Kirkby. > > _______________________________________________ > SWI-Prolog mailing list > SWI-Prolog@... > https://mailbox.iai.uni-bonn.de/mailman/listinfo.cgi/swi-prolog > ---------------------------------------------------------------- This message was sent using IMP, the Internet Messaging Program. _______________________________________________ SWI-Prolog mailing list SWI-Prolog@... https://mailbox.iai.uni-bonn.de/mailman/listinfo.cgi/swi-prolog |
|
|
Re: RE: Who likes whom?
by Steve Kirkby
::
Rate this Message:
Thanks. That is really helpful. I would never have thought of including a
second relationship. Steve Kirkby On 3/11/09 00:55, "Andrew Koster" <andrew@...> wrote: > Your predicate won't work. Even if you fix the syntax errors it will result in > > ?- likes(a,d). > yes > > This is quite easy to see. To solve likes(a,d), it will use the clause > likes(X,Z) :- likes(X,Y), likes(Y,Z) in the following manner: > > likes(a,d) :- likes(a, Y), likes(Y,d). It will then try to bind Y to > anything for which likes(a,Y) holds. It will find likes(a,b) and try > to verify likes(b,d). It can do this by using that same predicate > again (recursion). > > Recursion is a very powerful tool in Prolog, but it isn't always the > right tool to use. In this case, rewrite it as follows: > > likes2(X,Y) :- likes(X,Y). > likes2(X,Y) :- likes2(X, Z, Y). > > likes2(X, Z, Y) :- likes(X,Z), likes(Z,Y). > > This way, you can query: likes2(a,c) and it will answer yes. The first > clause for likes2 will fail (there is no direct likes predicate). > However, the second clause calls likes2/3. This succeeds for obvious > reasons. > > Similarly, likes2(a,d) will fail, because there is no recursion. > > Cheers, > Andrew > > > Quoting Steve Kirkby <winkley192@...>: > >> Anyone? I would really like to know. >> >> Steve Kirkby >> >>> The following does not work - SWI-Prolog just replies "no": >> >>> likes(a,b). >>> likes(b,c). >>> likes(c,d). >> >>> likes(X,Z) :- likes(X,Y), likes (Y,Z). >> >>> ?- likes(a.c). >>> no >> >>> 1: Why not please? >> >>> 2: And how to achieve the results: >> >>> ?- likes(a,c). >>> yes >> >>> ?- likes(a,d). >>> no >> >>> A person likes a third person if he likes someone who likes that >>> third person, >> but the person should not like a fourth person if he likes a someone >> who likes a >> third person who likes that fourth person - a reasonable model of human >> relations. >> >>> When I used micro-prolog in the early 1990s the If-Then worked. >> >>> Thanks, Steve Kirkby. >> >> _______________________________________________ >> SWI-Prolog mailing list >> SWI-Prolog@... >> https://mailbox.iai.uni-bonn.de/mailman/listinfo.cgi/swi-prolog >> > > > > ---------------------------------------------------------------- > This message was sent using IMP, the Internet Messaging Program. > > _______________________________________________ > SWI-Prolog mailing list > SWI-Prolog@... > https://mailbox.iai.uni-bonn.de/mailman/listinfo.cgi/swi-prolog _______________________________________________ SWI-Prolog mailing list SWI-Prolog@... https://mailbox.iai.uni-bonn.de/mailman/listinfo.cgi/swi-prolog |
| Free embeddable forum powered by Nabble | Forum Help |
