dolfin team mailing list archive

[Martin Sandve Alnæs <martinal@xxxxxxxxx>]

On Thu, Mar 12, 2009 at 10:47 AM, Anders Logg <logg@xxxxxxxxx> wrote:
> 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

With my suggestions (see other email), no flag is necessary.
A Function is (potentially) discrete if it is initialized with a function
space, and the first call to f.vector() gives either a zero vector
if no eval is implemented, or interpolates the user-defined
eval to create the initial vector. I'm sure  this can be hacked
to work somehow with ugly but completely hidden code.

Martin