Question about the use of delete/3 for lists

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2 total messages Started by Lionel Ains Fri, 12 Apr 2002 13:52

Author: Lionel Ains
Date: Fri, 12 Apr 2002 13:52

59 lines
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Hello,

I've got a quick question about the use of delete/3
>From the SWI-Prolog manual, I though I was going to use it
to implement a cleanup predicate for a list:

The following works as I was expecting:
?- delete([a, b, c, d, a], a, Out).

Out = [b, c, d] ;

No

But, when I try to delete compounds from a list using the
following call:
?- delete([foo(a,b), foo(a,c), foo(c,a), foo(a,b),
bar(a,b)], foo(a, _), Out).

Out = [foo(a, c), foo(c, a), bar(a, b)] ;

No
I get [foo(a, c), foo(c, a), bar(a, b)], while I would have
expected: [foo(c,a), bar(a,b)].

It looks like delete internally binds my _ to c and only
deleted foo(a,c)

Can someone help on this?

Many thanks,

Lionel

______________________________________________________
Boîte aux lettres - Caramail - http://www.caramail.com


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Re: Question about the use of delete/3 for lists

Author: Jan Wielemaker
Date: Fri, 12 Apr 2002 13:27

52 lines
1469 bytes

On Friday 12 April 2002 14:52, Lionel Ains wrote:
> Hello,
>
> I've got a quick question about the use of delete/3
> From the SWI-Prolog manual, I though I was going to use it
> to implement a cleanup predicate for a list:
>
> The following works as I was expecting:
> ?- delete([a, b, c, d, a], a, Out).
>
> Out = [b, c, d] ;
>
> No
>
> But, when I try to delete compounds from a list using the
> following call:
> ?- delete([foo(a,b), foo(a,c), foo(c,a), foo(a,b),
> bar(a,b)], foo(a, _), Out).
>
> Out = [foo(a, c), foo(c, a), bar(a, b)] ;
>
> No
> I get [foo(a, c), foo(c, a), bar(a, b)], while I would have
> expected: [foo(c,a), bar(a,b)].
>
> It looks like delete internally binds my _ to c and only
> deleted foo(a,c)
>
> Can someone help on this?

After getting wiser, my advice is to stay away from delete/3.
Use select/3, which is nice and clean semantics or write
your own delete/3 that does what you want.  There has
been discussion about this in the past, leading to quite
a few definitions of delete/3 that were all sensible given
the right point of view.

	Cheers --- Jan


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