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Colu
#1 / 7
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 LARGEST IN A LIST How do you get the largest integer in a list , the code beolow doesnt seem to work Any help would be much appreciated larger(A,B):-
B < A.
largest([],X).
largest([Head|Tail],X):-
Head < X,
X is Head,
largest(Tail).
largest([Head|Tail],X):-
largest(Tail).
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| Sat, 29 May 2004 05:22:41 GMT |
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Mike Ringros
#2 / 7
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LARGEST IN A LIST You can't call largest(tail) because there is no predicate called largest that takes 1 argument. What you need to do is the following: larger(X,Y,X) :- X >= Y, !.
larger(X,Y,Y).
largest([X],X).
largest([H|T],X) :- largest(T,X2), larger(H, X2, X).
Quote: > How do you get the largest integer in a list , the code beolow doesnt seem > to work > Any help would be much appreciated > larger(A,B):-
> B < A.
> largest([],X).
> largest([Head|Tail],X):-
> Head < X,
> X is Head,
> largest(Tail).
> largest([Head|Tail],X):-
> largest(Tail).
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| Sat, 29 May 2004 23:26:52 GMT |
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vanno..
#3 / 7
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LARGEST IN A LIST Quote:
> You can't call largest(tail) because there is no predicate called largest
> that takes 1 argument. What you need to do is the following:
> larger(X,Y,X) :- X >= Y, !.
> larger(X,Y,Y).
huh? ?- larger(4,3,3).
yes
--
Gertjan van Noord Alfa-informatica, RUG, Postbus 716, 9700 AS Groningen
vannoord at let dot rug dot nl http://www.let.rug.nl/~vannoord
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| Sun, 30 May 2004 23:05:48 GMT |
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Andrew Eremi
#4 / 7
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LARGEST IN A LIST Quote:
> > You can't call largest(tail) because there is no predicate called largest
> > that takes 1 argument. What you need to do is the following:
> > larger(X,Y,X) :- X >= Y, !.
> > larger(X,Y,Y).
> huh?
> ?- larger(4,3,3).
> yes
That should be larger(X,Y,Z) :- X >= Y, !, Z = X.
larger(X,Y,Y).
--
Andrew Eremin
IC Parc, William Penney Lab., Tel: +44 (0)20 7594 8299
Imperial College Fax: +44 (0)20 7594 8432
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| Sun, 30 May 2004 23:32:00 GMT |
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Mike Ringros
#5 / 7
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LARGEST IN A LIST The Z = X is not needed. The way I had it was correct, Prolog unifies the instantiated X if X >= Y. This is correct!
larger(X,Y,X) :- X >= Y, !.
larger(X,Y,Y) :- X =< Y.
Quote:
> > > You can't call largest(tail) because there is no predicate called
largest
> > > that takes 1 argument. What you need to do is the following:
> > > larger(X,Y,X) :- X >= Y, !.
> > > larger(X,Y,Y).
> > huh?
> > ?- larger(4,3,3).
> > yes
> That should be
> larger(X,Y,Z) :- X >= Y, !, Z = X.
> larger(X,Y,Y).
> --
> Andrew Eremin
> IC Parc, William Penney Lab., Tel: +44 (0)20 7594 8299
> Imperial College Fax: +44 (0)20 7594 8432
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| Mon, 31 May 2004 02:36:23 GMT |
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Andrew Eremi
#6 / 7
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LARGEST IN A LIST Quote:
> The Z = X is not needed. The way I had it was correct, Prolog unifies the
> instantiated X if X >= Y.
> This is correct!
> larger(X,Y,X) :- X >= Y, !.
> larger(X,Y,Y) :- X =< Y.
Now that you've added the test to the second clause it's correct, but for what you originally wrote the unification with the head of the first would fail, the cut would not be reached and unification with the second clause would succeed. Quote:
> > > > You can't call largest(tail) because there is no predicate called
> largest
> > > > that takes 1 argument. What you need to do is the following:
> > > > larger(X,Y,X) :- X >= Y, !.
> > > > larger(X,Y,Y).
> > > huh?
> > > ?- larger(4,3,3).
> > > yes
> > That should be
> > larger(X,Y,Z) :- X >= Y, !, Z = X.
> > larger(X,Y,Y).
-- Andrew Eremin IC Parc, William Penney Lab., Tel: +44 (0)20 7594 8299 Imperial College Fax: +44 (0)20 7594 8432
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| Mon, 31 May 2004 04:59:35 GMT |
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vanno..
#7 / 7
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LARGEST IN A LIST Quote:
> The Z = X is not needed. The way I had it was correct, Prolog unifies the
> instantiated X if X >= Y.
the way yioyou had it was incorrect. Andrew's variant is correct. Quote: > This is correct!
> larger(X,Y,X) :- X >= Y, !.
> larger(X,Y,Y) :- X =< Y.
and this one is correct too, but is less efficient (not that you'd ever notice) in that in some cases you check whether X is smaller than Y if you already established that X is not larger or equal to Y. Seems silly to me. of course my prefered solution would use if-then-else construct - but
we've been through that here on this list too often already.
Gj
Quote:
>> > > You can't call largest(tail) because there is no predicate called
> largest
>> > > that takes 1 argument. What you need to do is the following:
>> > > larger(X,Y,X) :- X >= Y, !.
>> > > larger(X,Y,Y).
>> > huh?
>> > ?- larger(4,3,3).
>> > yes
>> That should be
>> larger(X,Y,Z) :- X >= Y, !, Z = X.
>> larger(X,Y,Y).
-- Gertjan van Noord Alfa-informatica, RUG, Postbus 716, 9700 AS Groningen vannoord at let dot rug dot nl http://www.let.rug.nl/~vannoord
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| Mon, 31 May 2004 15:25:23 GMT |
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