Hi there,
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.
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'>)
I'd like to get some input or advice before pushing this further. Do you this
this way of working is robust ?
Cheers,
Florent
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