problem with anonymous variables

hi, i have a problem with counting non-anonymous variables in a list of
variables. the straitforward way i come up with is to remove all
anonymous variables from the list and then count the length of the
resulted list. Anybody has a better idea or how to remove anonymous
variables? thx in advance.

ouyang.jie@gmail.com wrote:
> hi, i have a problem with counting non-anonymous variables in a list of
> variables.

What is an "anonymous" variable in a list ?
Is it a variable that occurs only once in the list ?

Cheers

Bart Demoen

an anonymous variable is representated by an underscore "_". for
instance , f(_,A,_) has 2 anonymous variables. given a list [_,A,B,_],
i want to have the list [A,B] returned or 2 as the number of
non-anonymous variables returned. any idea?

ouyang.jie@gmail.com wrote:
> an anonymous variable is representated by an underscore "_". for
> instance , f(_,A,_) has 2 anonymous variables. given a list [_,A,B,_],
> i want to have the list [A,B] returned or 2 as the number of
> non-anonymous variables returned. any idea?

As far as Prolog terms are concerned there is no difference between
[_,A,B,_] and [X,Y,Z,W] since in all cases you have 4 variables all
distinct from each other.

Ie the anonymous variable is a convenience only (so you don't have to
think up names for variables you don't care about and so that you can
get the singleton-variable warning when a non-anonymous variable is
only used once), and although implementations can make use of the fact
that an anonymous variable has been used to improve performance, the
language itself has no way to distinguish an anonymous variable from
any other variable.

A list doesnt contain variables, only constants and functors.
When you unify a list with a predicate term, that predicate can refer to tho
se
constants and functors in an anonymous way, and _that_ is by the use of vari
ables.

if you do this query: pred([1,2,3],c)

that will unify with the predicate

pred([A|B],c) :- foo(A)

so that foo(1) will remain in the resolvent. Note that the variables A and B
 are only having the life length of the unification
process.


The same query would also match this predicate:

pred([_|_],c).


But here we see that the predicate doesnt save the unification results in va
riables like A and B.
The unification will result in a fact, because we doesnt demand that a foo/1
 predicate must be true
as in the example before.


/Anders

<ouyang.jie@gmail.com> wrote in message news:1117218983.597268.155000@g44g2000cwa.googlegro
ups.com...
> hi, i have a problem with counting non-anonymous variables in a list of
> variables. the straitforward way i come up with is to remove all
> anonymous variables from the list and then count the length of the
> resulted list. Anybody has a better idea or how to remove anonymous
> variables? thx in advance.
>

"Anders Lindén" <xxxx@xxx.xx> writes:

> A list doesnt contain variables, only constants and functors.

The Herbrand universe doesn't contain variables, but lists, like
other Prolog structures, can be incomplete and contain unbound
variables.

yeah, thats actually right, I understood what he meant after I had
replied to his posting :)

Sorry!

/Anders


"Torkel Franzen" <torkel@sm.luth.se> wrote in message news:vcbmzqe52zx.fsf@beta19.sm.ltu.se
..
> "Anders Lindén" <xxxx@xxx.xx> writes:
> 
>
>   The Herbrand universe doesn't contain variables, but lists, like
> other Prolog structures, can be incomplete and contain unbound
> variables.

Ok, here is an attempt

shrink([],[]).
shrink([A|B],C):- var(A),!, shrink(B,C).
shrink([A|B],[A|C]):- nonvar(A), shrink(B,C).


| ?-  shrink([4,5,B,d(r,e)],C).
C = [4,5,d(r,e)] ? ;
no

thank you for all your guys' reply. actually i wanted to distinguish a
non-anonymous variable, eg. A,B,..., from an anonymous variable,eg, _.
However i realized that it is unable to make it according to how prolog
works. i am trying to walk around this problem

Anders Lind=E9n wrote:
> Ok, here is an attempt
>
> shrink([],[]).
> shrink([A|B],C):- var(A),!, shrink(B,C).
> shrink([A|B],[A|C]):- nonvar(A), shrink(B,C).
>=20
>=20
> | ?-  shrink([4,5,B,d(r,e)],C).
> C =3D [4,5,d(r,e)] ? ;
> no

<ouyang.jie@gmail.com> wrote in message
news:1117555181.352717.206360@g49g2000cwa.googlegroups.com...
thank you for all your guys' reply. actually i wanted to distinguish a
non-anonymous variable, eg. A,B,..., from an anonymous variable,eg, _.
However i realized that it is unable to make it according to how prolog
works. i am trying to walk around this problem

<snipped>

Prolog can distinguish anonymous variables when reading terms.

Look up read_term/[2,3] in your manual, specifically the 'variable_names'
option.

Rgds