dolfin team mailing list archive

[Jake Ostien <tostien@xxxxxxxxx>]

Dag Lindbo wrote:
> Hello all,
>
> I saw that there is a convenient way to evaluate a DiscreteFunction at 
> all vertices of its mesh, i.e. u.evaluate( val ). Is there a convenient 
> way to evaluate a function on each cell (e.g. on the mid-point of each 
> triangle)?
>
> If not, I would really appreciate some suggestions how to go about. Is 
> the more general function
>
> interpolate(real* coefficients,const ufc::cell& cell,
> 	    const ufc::finite_element& finite_element)
>
> the way to go? How do I evaluate the sum of basis functions in a point 
> if I know the coefficients?
>   
I have been playing with a similar functionality in my own project, 
where I need to evaluate some field on a cell.  What I ended up doing is 
adding two methods to the Function class, specifically for 
DiscreteFunctions, that return the local data, uint* dofs (which are the 
coefficients) and the DofMap* dof_map.  Then I can iterate over cells 
and evaluate the coefficients on each cell however I want.  So my loop 
looks something like:

 (say field is a DiscreteFunction) 

 for (; !cell.end(); ++cell) {
   ufc_cell.update(*cell);
   dof_map->tabulate_dofs(dofs, ufc_cell);
  
   field.vector().get(array, dof_map->local_dimension(), dofs);

   ...do something with array...
 }

There is probably a better way to do this, and I'd like to hear it, 
too.  Otherwise I can submit my own changes.

Jake
> Thanks!
> /Dag Lindbo
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