ffc team mailing list archive

[Marie Rognes <meg@xxxxxxxxxxx>]

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.

--
Marie E. Rognes
Ph.D Fellow, 
Centre of Mathematics for Applications, 
University of Oslo
http://folk.uio.no/meg