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[Martin Sandve Alnæs <martinal@xxxxxxxxx>]

Exactly, and also SubFunction to SubFunction:

w[0] = v[1];

Then I could extract a row from a tensor field:

v[0] = A[3];
v[1] = A[4];
v[2] = A[5];

Martin



2009/3/21 Anders Logg <logg@xxxxxxxxx>:
> On Sat, Mar 21, 2009 at 08:53:55PM +0100, Martin Sandve Alnæs wrote:
>> I want to do the opposite, assign to a subfunction.
>
> You mean things like
>
>  w[0] = u;
>  w[1] = p
>
>  w[0][0] = ux;
>  w[0][1] = uy;
>
> ?
>
> That can be implemented. The indexing operator [] returns an object of
> class SubFunction which is a temporary object holding a reference to
> the Function (here w) and the component (stored as std::vector<uint>).
>
> The constructor for Function is overloaded so that one may
> assign a SubFunction to a Function:
>
>  u = w[0];
>
> We could also overload the constructor for SubFunction to
> allow the opposite:
>
>  w[0] = u;
>
> While we're at it, we should also move the implementation to the
> assignment operator in both cases and call the assignment operator in
> the copy constructor to avoid duplication of code.
>
> --
> Anders
>
>
>> 2009/3/21, Anders Logg <logg@xxxxxxxxx>:
>> > On Sat, Mar 21, 2009 at 05:05:53PM +0100, Martin Sandve Alnæs wrote:
>> >> Hi,
>> >> now that we can use tensor elements in dolfin through UFL,
>> >> what would be the easiest way to extract a row of a tensor
>> >> field (Function over a TensorElement) as a vector field?
>> >> I think scalar subcomponents can easily be extracted,
>> >> but is it possible to piece these together to a vector field?
>> >> Something like:
>> >>
>> >> Function v(V);
>> >> Function a = A[0];
>> >> Function b = A[1];
>> >> v[0] = a;
>> >> v[0] = b;
>> >
>> > I don't understand. What is A and what dimension does v have?
>> >
>> > We already have some support for this. For any mixed space one may
>> > extract sub spaces and sub functions. Here's the favorite Stokes
>> > example:
>> >
>> > (W = V + Q)
>> >
>> > Function w(W);
>> >
>> > Function u = w[0]; // u vector-valued with d components
>> > Function p = w[1]; // p scalar
>> >
>> > Function ux = u[0]; // ux scalar
>> > Function uy = u[1]; // uy scalar
>> >
>> >
>>
>
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