On Jul 9, 5:16 am, kcrisman <kcris...@gmail.com> wrote:
> On Jul 8, 2:38 pm, David Sanders <dpsand...@gmail.com> wrote:
>
>
>
> > Hi,
>
> > I am trying to extract part of a symbolic expression.
> > The expression -- an eigenvalue of a matrix -- has the form
>
> > A + B*sqrt(C)
>
> > where A, B and C are themselves complicated symbolic expressions.
>
> > I wish to extract the subexpression C from this to test where the
> > eigenvalues change type (where C==0).
>
> > By using introspection and the help (both excellent features!), I
> > stumbled across one possible solution, using iterator. But it's very
> > fussy: I have to do something like:
>
> > var('A B C')
> > eigval = A + B*sqrt(C)
> > terms = list( eigval.iterator() )
> > first = terms[0]
> > terms2 = list(first.iterator())
> > desired = list(terms2[1].iterator())[0]
>
> > to extract the part I want into the variable "desired"
>
> I don't know if this would be better, but...
>
> sage: eigval
> B*sqrt(C) + A
> sage: eigval.operands()[0].operands()[1].operands()[0]
> C
>
> At least it is using the things specific to symbolic expressions.Thanks, this is certainly better than the previous option! It still feels a bit clumsy to me though. > > > To me it would seem more intuitive to use indexing directly on the > > expression, to be able to do something like > > > eigval[0][1][0] > > > which is similar to what is available in Mathematica, for example, but > > this doesn't work, since apparently indexing is not defined for > > symbolic expressions. (Couldn't it be defined to have exactly this > > functionality?) > > This seems intriguing, but I have no idea if it's possible. Me neither, but it would seem to me (with no deep understanding of either Python or Sage...) that it's a question of defining the __getitem__ method to return the corresponding element by iterating. Perhaps this is just hopelessly naive though, I'm afraid I have no understanding of the undoubted complexity of the symbolic objects. David. > > - kcrisman -- To post to this group, send email to sage-support@googlegroups.com To unsubscribe from this group, send email to sage-support+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/sage-support URL: http://www.sagemath.org
