On Sat, Apr 03, 2010 at 10:54:52AM -0700, Robert Dodier wrote:
> On Apr 2, 12:14 pm, Jason Grout <jason-s...@creativetrax.com> wrote:
> > On 04/02/2010 12:47 PM, Franco Saliola wrote:
>
> > > Here is a very simple implementation of SymbolicVariables.
> >
> > > class SymbolicVariables(SageObject):
> > > def __init__(self, prefix='x'):
> > > self._prefix = prefix
> > > def __getitem__(self, i):
> > > return var("%s%s"%(self._prefix, i))
>
> > This has come up several times before on sage-devel, and I've always
> > liked it. For example, Nathann Cohen brought this up when he was first
> > integrating the linear programming stuff. I believe Robert Bradshaw
> > suggested this "indexing creates a variable" approach a while ago too.
> >
> > When/if we wrap pynac vectors/matrices, though, or have "symbolic vector
> > variables", we might have to be careful about how we treat the
> > __getitem__ function. I think in your case, where you explicitly are
> > creating x with the understanding that indexing x creates new variables,
> > things would be fine, though.
>
> Not that it matters, but creating new symbols for indexed
> variables seems to have some potential for unexpected behavior,
> aside from cluttering the namespace.
>
> Another approach is to represent an indexed variable as a
> symbolic expression the same as an unevaluated function call,
> but with an extra flag which essentially just shows it's
> a subscripted expression instead of a function call.
> (There isn't any real conceptual difference, right?
> A subscripted variable is just a function on some countable
> set.)
+1
Just: why countable? Well, yeah, there are only countably objects that
can be represented on a computer. I guess all I want to point out is
that such subscripted variables would be useful with all sorts of
subscripts (trees, graphs, groups, ...), and not just integers.
> There is an existing program which uses this scheme,
> although it doesn't matter which one it is, so I won't
> bother to mention it.
More than one :-)
Cheers,
Nicolas
--
Nicolas M. Thiéry "Isil" <nthi...@users.sf.net>
http://Nicolas.Thiery.name/
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