dolfin team mailing list archive

[Johan Hake <hake@xxxxxxxxx>]

On Tuesday 16 December 2008 23:30:17 Bartosz Sawicki wrote:
> On 16/12/08 03:04 PM, Garth N. Wells wrote:
> > Bartosz Sawicki wrote:
> >> Before the function revolution there was method cell() inside the
> >> Function. I used it to determine index of current cell during assembly
> >> procedures, in eval() method. I realized that it disappeared now.
> >> How similar functionality can be achieved today?
> >
> > You'll need to use the eval variant
> >
> >    void eval(double* values, const Data& data) const;
> >
> > and you can then access the cell via
> >
> >   const Cell& cell = data.cell();
>
> Thanks. That's clear now.
> Remember that corresponding section of the manual needs to be updated.

That is probably true, together with a lot of other stuff... :)

> Do you plan to extend python interface to follow this changes?

The pure python interface only support values and x as arguments to the 
callback function eval(). It should probably be possible to expose the data 
structure to python too, but this wont be a priority right now. 

You can always define your own C++ function and compile it in python.

   cppcode = '''
    class MyFunc : public Function
    {
    public:

      MyFunc(FunctionSpace& V) : Function(V) {}

      void eval(double* values, const Data& data)
      {
         // write your C++ code here!
      }

    };'''
 
   my_func = Function(V,cppcode)
   
my_func can then be used in form formulation, and then automaticly used in the 
assembly too. Unfortunatly you cannot use this function directly from the 
python prompt, i.e.
  
   my_func.eval(v,x)

wont work, as the data structure is created duing assemble.

Btw, with the new Function interface we have made it much easier to use 
compiled c++ function from python. Have a look in the docstring of Function. 
I strongly encourage you to use compiled function instead of pure python 
functions as the performance boost is significant during assemble.
 

Johan