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Vincen
#1 / 6
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 2 simple programs Hi everybody, I'm new user of prolog and I meet problems with doing 2 things :
1/ I want to modify all the occurences of the 'a' character by 'b' in a
list.
I do this but it returns "no" :
modify([], []).
modify([a|List], [b|Result]) :- modify(X, Result).
modify([Z|Y], [Z|Result]) :- modify(Y, Result).
What's wrong???
2/ I want to find the maximum of an integer list :
I try this one :
maxList([X], X).
maxList([X|List], Y) :- maxList(List, Z), max(X, Y, Z).
{max is a function wich put in Z the maximum between X and Y}.
In fact, I don't understand why this one work :
maxList([X], X).
maxList([X, Y|List], Result) :- maxList([Y|List], Maxi), max(X, Maxi,
Result).
I hope you can help me! (mainly on the 1/ )
Thanks.
--
Vincent
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| Sat, 22 May 2004 01:47:28 GMT |
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Martin Sondergaar
#2 / 6
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2 simple programs
Quote: > Hi everybody, > I'm new user of prolog and I meet problems with doing 2 things :
> 1/ I want to modify all the occurences of the 'a' character by 'b' in a
> list.
> I do this but it returns "no" :
> modify([], []).
> modify([a|List], [b|Result]) :- modify(X, Result).
> modify([Z|Y], [Z|Result]) :- modify(Y, Result).
> What's wrong???
The second clause is wrong. You have used two different variables, "List" and "X", which should be the same. It should be :
modify( [a|List], [b|Result] ) :- modify( List, Result ).
I would write it like this :
modify( [a|List], [b|List1] ) :-
modify( List, List1 ).
The other clauses look correct to me.
So your answer was very nearly right. :-)
You will find your programs easier to read
if you align them in the usual way,
instead of having a clause all on one line.
--
Martin Sondergaard,
London.
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| Sat, 22 May 2004 03:13:17 GMT |
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Vincent Peytavi
#3 / 6
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2 simple programs
Quote: > > 1/ I want to modify all the occurences of the 'a' character by 'b' in a > > list. > > I do this but it returns "no" : > > modify([], []). > > modify([a|List], [b|Result]) :- modify(X, Result). > > modify([Z|Y], [Z|Result]) :- modify(Y, Result). > > What's wrong???
> The second clause is wrong.
> You have used two different variables, "List" and "X",
> which should be the same.
Yes! This error occurs when I copied this clause with my keyboard. Quote: > --
> Martin Sondergaard,
> London.
-- Vincent, Grenoble - France FAQ de fr.comp.lang.java http://www.faqs.org/faqs/fr/comp/lang/faq-java/
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| Sat, 22 May 2004 05:38:55 GMT |
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Esp
#4 / 6
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2 simple programs Answer for 2/ Quote: > maxList([X|List], Y) :- maxList(List, Z), max(X, Y, Z).
means : X is the first, Z is the max of List, Z is the max between X and Y ??? maxList([X|List], Z) :- maxList(List, Y), max(X, Y, Z).
means : X is the first, Y is the max of List, Z is the max between X and Y -
right
and it should work (I can't test it)
hth
Esp
Quote: > Hi everybody, > I'm new user of prolog and I meet problems with doing 2 things :
> 1/ I want to modify all the occurences of the 'a' character by 'b' in a
> list.
> I do this but it returns "no" :
> modify([], []).
> modify([a|List], [b|Result]) :- modify(X, Result).
> modify([Z|Y], [Z|Result]) :- modify(Y, Result).
> What's wrong???
> 2/ I want to find the maximum of an integer list :
> I try this one :
> maxList([X], X).
> maxList([X|List], Y) :- maxList(List, Z), max(X, Y, Z).
> {max is a function wich put in Z the maximum between X and Y}.
> In fact, I don't understand why this one work :
> maxList([X], X).
> maxList([X, Y|List], Result) :- maxList([Y|List], Maxi), max(X, Maxi,
> Result).
> I hope you can help me! (mainly on the 1/ )
> Thanks.
> --
> Vincent
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| Sun, 23 May 2004 00:24:47 GMT |
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Mike Ringros
#5 / 6
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2 simple programs Your maxlist function looked correct; however, I didn't look at too closely. Here's one a wrote for a project a while back and it works. max(X,Y,X) :- X >= Y, !. max(_,Y,Y). maxlist([H],H).
maxlist([H1|T],M) :- maxlist(T,R),max(H1,R,M).
Quote: > Hi everybody, > I'm new user of prolog and I meet problems with doing 2 things :
> 1/ I want to modify all the occurences of the 'a' character by 'b' in a
> list.
> I do this but it returns "no" :
> modify([], []).
> modify([a|List], [b|Result]) :- modify(X, Result).
> modify([Z|Y], [Z|Result]) :- modify(Y, Result).
> What's wrong???
> 2/ I want to find the maximum of an integer list :
> I try this one :
> maxList([X], X).
> maxList([X|List], Y) :- maxList(List, Z), max(X, Y, Z).
> {max is a function wich put in Z the maximum between X and Y}.
> In fact, I don't understand why this one work :
> maxList([X], X).
> maxList([X, Y|List], Result) :- maxList([Y|List], Maxi), max(X, Maxi,
> Result).
> I hope you can help me! (mainly on the 1/ )
> Thanks.
> --
> Vincent
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| Sun, 23 May 2004 11:50:05 GMT |
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Graham Thwaite
#6 / 6
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2 simple programs ----------
Quote: >Your maxlist function looked correct; however, I didn't look at too closely.
>Here's one a wrote for a project a while back and it works.
>max(X,Y,X) :- X >= Y, !.
>max(_,Y,Y).
The problem with this definition is that it isn't 'steadfast'. For instance the query ?- max(10,0,0) will succeed because the match with the head of the first clause fails and so it never reaches the '>=' comparison. Consequently the query backtracks to the second clause and (erroneously) succeeds. The correct approach is to place output unifications after the cut e.g. max(X,Y,Z) :- X >= Y, !, Z=X.
max(_,Y,Y).
O'Keefe addresses this in chapter 3 of his book "The Craft of Prolog".
HTH
Graham Thwaites
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| Mon, 24 May 2004 23:35:02 GMT |
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