dolfin team mailing list archive

["Martin Sandve Alnæs" <martinal@xxxxxxxxx>]

2008/11/29 Anders Logg <logg@xxxxxxxxx>:
> On Sat, Nov 29, 2008 at 03:09:53PM +0100, Martin Sandve Alnæs wrote:
>> How do you intend to handle (sub)functions in a mixed space?
>> Currently I don't see how the function space stuff generalizes
>> to even simple mixed spaces.
>>
>> Take this simple example in a pure form language (FFC or UFL):
>>     element1 = VectorElement("CG", "triangle", 2)
>>     element2 = FiniteElement("CG", "triangle", 1)
>>     mixed_element = element1 + element2
>>     f, g = Functions(mixed_element)
>>
>> Now in PyDOLFIN, would a function space be defined by (reusing the elements):
>>     space1 = FunctionSpace(mesh, element1)
>>     space2 = FunctionSpace(mesh, element2)
>>     mixed_space1 = space1 + space2
>>
>> or:
>>     mixed_space2 = FunctionSpace(mesh, mixed_element)
>>
>> and would the functions in this space be
>>     f, g = Functions(mixed_space)
>> or
>>     fg = Function(mixed_space)
>>     f, g = fg.sub(0), fg.sub(1)
>> ?
>> Neither of these two definitions of (f, g) works out.
>
> It should be just as in your first example, but replace Element with
> FunctionSpace:
>
>  V1 = VectorFunctionSpace(mesh, "CG", 2)
>  V2 = FunctionSpace(mesh, "CG", 1)
>  mixed_space = V1 + V2
>  f, g = Functions(mixed_space)
>
>> The syntax
>>     f, g = Functions(mixed_space)
>> doesn't work, since ufl.Functions does not return ufl.Function objects,
>> but ufl.Indexed objects that refer to subexpressions of ufl.Function objects.
>>
>> The syntax
>>     fg = Function(mixed_space)
>>     f, g = fg.sub(0), fg.sub(1)
>> would require that sub returns something that is a subclass of what
>> ufl.Functions returns (ufl.Indexed) and at the same time is a dolfin.Function.
>
> It should be enough to just define the following function:
>
> def Functions(V):
>    v = Function(V)
>    return tuple(v[i] for i in range(V.num_subspaces()))

In pure UFL, given
    v = ufl.Function(mixed_element)
then
   v[i]
is not subfunction i, but value component i of v.

You can't override v[i] in dolfin.Function, then indexing of vector
functions won't work.

--
Martin