There must be an easier way!

[Ed Keith-2]

I am using prolog again after a twenty year hiatus and I feel like I am working way too hard.

My problem seems simple. In it's most general form I want to test the truth of falsehood  the following:

For all X in S foo(X).

What I am doing is using findall to get a list of all X for which foo(X) is true. Then I get the size of the set S and I assert that the sizes are the same:

    forallXinS() :- findall(X,foo(X),B),
                    length(B,N),
            s(P),
            length(P,N).

This works, but it seems that there must be a better way.

Similarly to test :

For no X in S foo(X).

I use findall and check if it returns an empty set.

     fornoX() :- findall(X,foo(X),[]).

Again this seems like a hack.

What is the best way to make these tests?

     -EdK

Ed Keith
e_d_k@...

Blog: edkeith.blogspot.com


     
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[Paulo Moura]

On 2009/11/01, at 14:56, Ed Keith wrote:

> I am using prolog again after a twenty year hiatus and I feel like I  
> am working way too hard.
>
> My problem seems simple. In it's most general form I want to test  
> the truth of falsehood  the following:
>
> For all X in S foo(X).
>
> What I am doing is using findall to get a list of all X for which  
> foo(X) is true. Then I get the size of the set S and I assert that  
> the sizes are the same:
>
>    forallXinS() :- findall(X,foo(X),B),
>                    length(B,N),
>            s(P),
>            length(P,N).
>
> This works, but it seems that there must be a better way.
>
> Similarly to test :
>
> For no X in S foo(X).
>
> I use findall and check if it returns an empty set.
>
>     fornoX() :- findall(X,foo(X),[]).
>
> Again this seems like a hack.
>
> What is the best way to make these tests?

Take a look at the forall/2 built-in predicate.

Cheers,

Paulo


-----------------------------------------------------------------
Paulo Jorge Lopes de Moura, PhD
Assistant Professor
Dep. of Computer Science, University of Beira Interior
6201-001 Covilhã, Portugal

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Email: <mailto:pmoura@...>

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[Markus Triska-2]

Ed Keith <e_d_k@...> writes:

> For all X in S foo(X).

In other words, there is no X in S such that NOT foo(X):

   \+ ( x_in_s(X, S), \+ foo(X) )

or:

   forall(x_in_s(X, S), foo(X))


> For no X in S foo(X).

   \+ ( x_in_s(X, S), foo(X) )
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[Ed Keith-2]

I'd like to thank everyone who helped me.

I wish a list like this had been available in 1985 when I was first trying to learn prolog. It would have made it a LOT easier!

Thanks a lot,

   -EdK

Ed Keith
e_d_k@...

Blog: edkeith.blogspot.com


--- On Sun, 11/1/09, Markus Triska <markus.triska@...> wrote:

> From: Markus Triska <markus.triska@...>
> Subject: Re: [SWIPL] There must be an easier way!
> To: "Ed Keith" <e_d_k@...>
> Cc: swi-prolog@...
> Date: Sunday, November 1, 2009, 10:26 AM
> Ed Keith <e_d_k@...>
> writes:
>
> > For all X in S foo(X).
>
> In other words, there is no X in S such that NOT foo(X):
>
>    \+ ( x_in_s(X, S), \+ foo(X) )
>
> or:
>
>    forall(x_in_s(X, S), foo(X))
>
>
> > For no X in S foo(X).
>
>    \+ ( x_in_s(X, S), foo(X) )
> _______________________________________________
> SWI-Prolog mailing list
> SWI-Prolog@...
> https://mailbox.iai.uni-bonn.de/mailman/listinfo.cgi/swi-prolog
>

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