dolfin team mailing list archive

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

Shawn Walker wrote:
> I may be missing part of the argument here, but for the case when a user 
> defined function is something like exp(x^2 + y^2), could we just pass a 
> function pointer (for this particular function) to the interpolation 
> routine?  In MATLAB, this type of thing is easy because of function 
> handles.  I know we don't have that luxury, but a function pointer kind 
> of does this.
>
> Sorry, if I am not understanding the issue.

The difficulty is that all the functions in a form come from a finite 
element space, and the space determines where we need to evaluate the 
functions. Before, DOLFIN figured out the appropriate space, and 
user-defined function were interpolated in this space,
but it made the design complex when handling different function types. 
What we aim for now is to have all Functions fully-defined (but we'd 
like to keep it simple for users too).

Garth

> - Shawn
>
> On Tue, 21 Oct 2008, 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
> >
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