dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Thu, Mar 12, 2009 at 10:41:28AM +0100, Johan Hake wrote:
> On Thursday 12 March 2009 10:32:10 Anders Logg wrote:
> > On Thu, Mar 12, 2009 at 10:25:36AM +0100, Martin Sandve Alnæs wrote:
> > > I have serious problems with the idea of letting user-defined
> > > functions change nature to discrete functions as a side effect
> > > of other operations, effectively hiding the user-defined implementation
> > > of eval.
> > >
> > > (I know this concept wasn't introduced in this discussion, but it's
> > > related).
> >
> > You mean by calling u.vector()? Yes, I agree it's problematic.
> >
> > But I don't know how to handle it otherwise. Consider the following
> > example:
> >
> >   u = Function(V)
> 
> FYI:
> u is a discrete function here. This is a difference between the python and c++ 
> interface. Please shout if this is not what we want. The c++ base class of u 
> is the cpp.DiscreteFunction defined SpecialFunctions.h. 
> 
> I did this mostly for avoiding the director wrapper attached to all python 
> functions that inherits the cpp.Function, and with the assumption that when a 
> Function is instantiated as:
> 
>   u = Function(V) 
> 
> you also want a discrete function. The only thing the DiscreteFunction does is 
> instantiating the vector and defining some constructors that is used else 
> where in the python interface.
> 
> Johan

This looks good to me. I didn't know.

Too bad we can't do the same thing in C++. It's a bit odd now. A
Function is by default discrete in Python, but not in C++ where it is
by default user-defined.

Should we add a flag to the constructor(s) of Function so everything
is by default discrete?

  Function(const FunctionSpace& V, user_defined=false);

or

  Function(const FunctionSpace& V, has_eval=true);

If a Function is user-defined, the call to vector() would give an
error and suggest that the Function be interpolated first:

  u.interpolate();
  u.vector();

This would require adding a flag to all user-defined functions in C++.
It would also silently break all user code and make functions without
the flag correctly set just return zero (from the initialized zero
vector). But it might be worth it.

-- 
Anders

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