dolfin team mailing list archive

["Garth N. Wells" <gnw20@xxxxxxxxx>]

Anders Logg wrote:
> On Tue, Oct 21, 2008 at 06:01:53PM +0100, Garth N. Wells wrote:
> > Anders Logg wrote:
> > > On Tue, Oct 21, 2008 at 04:45:01PM +0100, Garth N. Wells wrote:
> > > > I have a few questions and thoughts regarding the new Function design
> > > >
> > > > * It's not clear to me what the intention is with user-defined 
> > > > functions. The functions Function::interpolate(...) never call eval(..), 
> > > > so they can't pick up user-defined values. Should Function::interpolate 
> > > > test for the presence of a GenericVector to decide whether or not the 
> > > > Function is discrete or user-defined?
> > > Yes, sorry. I've missed this. I'll fix it.
> > >
> > > > * It would be useful to declare user-defined functions without 
> > > > associating a FunctionSpace. If we want to interpolate the function, a 
> > > > FunctionSpace must then be provided. Anyone see any problems with this?
> > > The reasoning here is that all Functions must always be associated
> > > with a FunctionSpace so that they may be correctly interpreted in
> > > forms and correctly plotted. When a Function is created in PyDOLFIN,
> > > it must always be associated with a certain FiniteElement (and in a
> > > while FunctionSpace). It would simplify the handling of Functions if
> > > they are always associated with a FunctionSpace.
> > I agree that is makes life simple if every function has a space, but it 
> > is a bit clunky for declaring user-defined functions. The forms must be 
> > declared first to extract the finite element to create the function 
> > space. Could look nasty when a lot of functions are involved.
> >
> > We have a function Function::interpolate which takes a function space V 
> > as an argument and it interpolates the function u in V. What if we 
> > permit undefined function spaces (which perhaps only have a domain)? We 
> > would then interpolate the user defined function u in the provided space V.
> >
> > Garth
>
> Are user-defined functions ever used without being related to a
> particular element/function space?

No, but one user-defined function can be used with different elements.

> It don't think it will be very clumsy. The clumsy thing will be to (in
> C++) get from something compiled by a form compiler to a FunctionSpace.
>
> If we can make that operation smooth, then creating (user-defined)
> functions will be very simple and convenient. One just needs to supply
> the variable V holding the function space.
>
> The current way of extracting function space data from the form is not
> very nice (in C++).

Agree.

 What would be the optimal way to initialize a
> FunctionSpace in C++? We could think of extending the code generation
> to generate code that makes this convenient.

Yes, FFC could generate some simple code to help with the creation of a 
FunctionSpace.

In the old Function design there was an a function

    void interpolate(double* coefficients,
                     const ufc::cell& ufc_cell,
                     const FiniteElement& finite_element,
                     Cell& cell, int facet=-1);

which says: given a Function u defined on its own element (or perhaps no 
element), interpolate u on finite_element. Do we still need this style 
of interface to check the finite element expected by the form during 
assembly matches the finite element of the provided Function? Also this 
way we deal with user-defined functions without a basis. What I don't 
know is how to attach a finite element to a FunctionSpace after the 
FunctionSpace has been created, which would be need for a user-defined 
function without a finite element to become a discrete function.

Garth

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