Re: negotiation by failure- list operations

On Thu, 16 Jun 2005, ne0 wrote:

great that so many people are getting into my propblem.
i think i found a solution - maybe u guys can check it

delete1(E,L1,L2):-not(member(E,L1)),write(yes),L2=L1,!.
delete1(_, [], []).
delete1(Head, [Head | Tail], Result):-!, delete5(Head, Tail, Result).
delete1(Element, [Head | Tail], [Head | NewTail]):-!, delete5(Element,
Tail, NewTail).

what happens on swi ?
====
?- delete1(a,[b,d],X).
yes

X = [b, d] ;

No
===

i think thats what i searched for
or is there maybe a shorter way- especially for not getting to answer
the X question ? can i make a cut with a real stop! - and of course my
"yes" String ?


It would help if you specified better what exactly you want.

This is what you wrote before:

"all i need is a modified delete1 predicate that throws back a YES if x
 isnt in the List. - the point is the yes - not deleting all matching
 elements in the list.

 its like a if then construction

 delete1(x,[L1],Y).
 if x isnt in the list - then halt and give back a YES, if x is in the
 list- procede with delete"

Nowhere in this did you say you wanted the word "yes" to be printed
on the screen. A predicate call that causes the word "yes" to be
printed is not the same thing at all as a predicate call that
succeeds. I find this with novice programmers in other languages -
sometimes they can't see the difference between a method call which
returns a value and a method call which causes a value to be printed
on the output stream.

In your original wording, I think it would have been assumed that
when you wrote "give back a YES" and "throw back a YES" you meant
"succeed". Predicate calls in Prolog don't return values, they either
succeed or fail, but they may have the side-effect of binding an
unbound variable argument, which is how they return results. So it looked like you wanted a predicate whose third argument
is the result of deleting the first from the second assuming the
second is a list, and which always succeeded, with the third
argument equal to the second if the first does not occur in the second.

Matthew Huntbach
.

Relevant Pages