dolfin team mailing list archive

[Harish Narayanan <harish.mlists@xxxxxxxxx>]

Johan Hake wrote:

> You can always define a complex JIT-compiled function in PyDOLFIN. Using this 
> functionality, everything should just work ;)
> 
>   >>> cppcode = '''
>   class MyFunc : public Function
>   {
>   public:
>      MyFunc(FunctionSpace& V) : Function(V) {}
>      void eval(double* values, const Data& data) const{
>        double time = data.t;
>        double first_normal = data.normal()[0];
>        ...
>      }
>   };
>   '''
>   >>> f = Function(V,cppcode) 

I did not know this, I will try to use this way of defining functions
instead.


>> I've been told time ought to "just work," but will the Python gurus be
>> adding support for other special functions like facet normals soon?
> 
> Well, sadly this is not true (for now). The time, 't', is passed with the Data 
> argument to eval using the eval(double *value, const Data data) function of 
> Function. When produce the code for the JIT-compiled functions using the 
> simple c++-expressions, we overload the the simpler eval(double *value, const 
> double *x) function. The descision for this was, that if a user want more 
> complex expressions in its JIT compiled functions it should use the above 
> explained JIT compilation.

Is the reasoning for this that overloading eval is a lot slower than
JIT-compiling the function?

> We could add access to some of the common attributes of Data in the simpler 
> c++-expression JIT compiled Functions too. By overloading eval(double *value, 
> const Data data) and then map all use of:
>   
>   't' to 'data.t' 
>   'n' to 'data.normal()'
>   'h' to 'data.Cell().diameter()'
> 
> Doing this we should be able to support:
> 
>   >>> f = Function(V,"t*n[0]*h*sin(x[0])")
> 
> I desided not to implement this with reason mentioned above.

Your definition of 'f' above is along the lines of what I want to do,
but if it is going to be inefficient to wrap these common variables and
overload eval, I will use the cppcode-style definition directly.

Harish