dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Wed, Oct 22, 2008 at 04:33:18PM +0100, Garth N. Wells wrote:
> 
> 
> Anders Logg wrote:
> > On Tue, Oct 21, 2008 at 08:35:45PM +0100, Garth N. Wells wrote:
> >>
> >> 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.
> > 
> > That would be accomplished by sending different function spaces to the constructor:
> > 
> >   f = MyFunction(V)
> >   g = MyFunction(W)
> > 
> 
> Of course, so that will be easy.
> 
> >>> 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
> > 
> > This is not needed anymore since a function is always interpolated
> > (locally) to its own space which is always defined. 
> 
> How do we check that it's the right space?
> 
> Garth

Sorry, wrong answer. Correct answer: not yet implemented. We do need
to check.

-- 
Anders

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