ffc team mailing list archive

["Robert Kirby" <robert.c.kirby@xxxxxxxxx>]

Thanks for all the feedback.  I will try the sum + list comprehension.

Rob

On Wed, Apr 16, 2008 at 3:14 PM, Anders Logg <logg@xxxxxxxxx> wrote:

> On Wed, Apr 16, 2008 at 09:41:38PM +0200, Marie Rognes wrote:
> > Robert Kirby wrote:
> > >
> > > It seems that if you put a vector element into a mixed element and
> > > then take it back out, you can no longer index it.
> > >
> > > On the other hand, if I do:
> > > # start new .form file here
> > > vel_element = VectorElement("Lagrange","triangle",2)
> > > pre_element = FiniteElement("Lagrange","triangle",1)
> > > element = vel_element + pre_element
> > > (u,p) = TrialFunctions( element )
> > > (v,q) = TestFunctions( element )
> > > u0 = Function( vel_element )
> > >
> > > a = D(v[i],j)*D(u[i],j)*dx + v[i]*u0[j]*D(u[i],j)*dx
> > > # end new .form file
> > >
> >
> > What happens is that TestFunction (and TrialFunction) returns an object
> > (BasisFunction) that has a well-defined operator for taking components
> > with arbitrary indices.
> >
> > TestFunctions (and TrialFunctions), however, returns "all the
> > components" structured as a tuple of lists. Taking "an arbitrary index"
> > of a python list doesn't work that well.
> >
> > Will think some more about whether this is
> > trivial/easy/difficult/impossible to change.
> >
> > PS: If you want something like this to work *right* now try iterating
> > over i and j manually ;) Simplify will contract it back for you.
>
> Yes. In some more detail, when you do
>
>  element = vel_element + pre_element
>
> FFC first creates a vector-valued element (mixed element) with three
> components, the first two for u and the last for p. Then when you do
>
>  (u, p) = TrialFunctions(element)
>
> FFC picks out the three components of the vector-valued function
> w = (u, p), indexes w and returns ([w[0], w[1]], w[2]).
>
> This breaks the index notation since Python lists obviously can't
> be index by an FFC Index.
>
> I'd recommend doing something like
>
>  a = sum([D(v[i],j)*D(u[i],j)*dx + v[i]*u0[j]*D(u[i],j)*dx for i in
> range(2) for j in range(2)])
>
> Does that work for you?
>
> I'm pretty sure we won't invest much time in improving the FFC form
> language before switching to UFL. We (mostly Martin) have done some
> work on it. You can follow the progress here:
>
>  http://www.fenics.org/dev/ufl
>
> --
> Anders
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