Hi Florent,
On Tue, 15 Nov 2011 17:29:38 +0100
Florent Hivert <florent.hiv...@lri.fr> wrote:
<moved indexed variable discussion below>
> So I'm trying to inherits from Expression. Here is my code:
>
> from sage.symbolic.expression import Expression
> class IndexedVarExpr(Expression):
> def index(self):
> return self._index
>
> class IndexedVar(SageObject):
> def __init__(self, name, parent = SR):
> self._name = name
> self._parent = parent
>
> @cached_method
> def __getitem__(self, i):
> res = IndexedVarExpr(self._parent,
> self._parent.symbol("%s[%s]"%(self._name,
> i))) res._index = i
> return res
>
> This seems to work as a first glance:
>
> sage: Y = IndexedVar("Y")
> sage: v = Y[1,2,3]
> sage: v
> Y[(1, 2, 3)]
> sage: v^2+1
> Y[(1, 2, 3)]^2 + 1
> sage: vbis = (v^2+1).variables()[0]
> sage: vbis
> Y[(1, 2, 3)]
> sage: vbis == v
> Y[(1, 2, 3)] == Y[(1, 2, 3)]
> sage: bool(vbis == v)
> True
>
> However, vbis doesn't return my exact object:
>
> sage: type(v), type(vbis)
> (<class '__main__.IndexedVarExpr'>, <type
> 'sage.symbolic.expression.Expression'>)
The Expression class hard codes the type of the resulting object after
arithmetic, like many other element classes in Sage. This is done for
speed of course. There have been many requests of this kind lately, I'm
starting to think it is worth the speed penalty.
See the discussion here for a patch that should fix the error and some
timings:
http://groups.google.com/group/sage-support/t/f749ba3cd079c6f4
We would also need this for #9556:
http://trac.sagemath.org/sage_trac/ticket/9556
> I'm trying to make sage aware of indexed variables. For the
> information, this is a long standing Sage-Combinat wish. more
> precisely, basically I need a sage object Y such that, for any object
> (say hashable) o: 1 - the call Y[o] returns a variable which is legal
> in symbolic expressions. 2 - the variable v = Y[o] has a method index
> such that v.index() return o Since Expression doesn't have any
> __dict__ I can't use the quick hack consisting in inserting the
> attribute index in it.
Pynac supports indexed expressions already. It just isn't wrapped in
Sage yet. Again, there is an experimental patch available:
http://sage.math.washington.edu/home/burcin/indexed_expression-20110727.patch
Since you can wrap any hashable Python object in a symbolic expression
with SR._force_pyobject(), this could replace your IndexedVarExpr
class.
I would appreciate any help with cleaning up these patches, adding
doctests etc. so they can be merged. :)
Cheers,
Burcin
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