dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Sun, Sep 05, 2010 at 08:36:22AM -0700, Johan Hake wrote:
> On Sunday September 5 2010 07:26:19 Anders Logg wrote:
> > On Sat, Sep 04, 2010 at 08:16:43PM +0200, Marie Rognes wrote:
> > > On 04. sep. 2010 18:26, Johan Hake wrote:
> > > > On Saturday September 4 2010 03:54:35 Marie Rognes wrote:
> > > >> On 04. sep. 2010 01:36, Johan Hake wrote:
> > > >>> Hello!
> > > >>>
> > > >>> Garth found an ambiguity in the Expression doc string regarding how a
> > > >>> user should initiate a user defined Expression (not a compiled one)
> > > >>> in Python. If the Expression is vector or tensor valued the user
> > > >>> need to provide information about this.
> > > >>>
> > > >>> This can be done in two ways. Either overload the dim method or
> > > >>> provide a ufl FiniteElement while instantiating the Expression.
> > > >>> Neither of these methods are documented in the docstring.
> > > >>>
> > > >>> In the mixed poisson demo both dim and rank are overloaded. I think
> > > >>> rank is not needed (works fine without). However dim is just a
> > > >>> method introduced to be able to automatically select a
> > > >>> FiniteElement.
> > > >>
> > > >> Thanks for bringing this up -- evidently I'm confused.
> > > >
> > > > Well, me too. Ever since Anders introduced the automatic selection of
> > > > elements for the Expressions, I have been some what confused.
> > >
> > > Well, that part made perfect sense to me --- I think ;)
> > >
> > > >>> First I am not sure dim is the correct name. We could use value_size
> > > >>> as this is present in the GenericFunction interface for just this
> > > >>> purpose. Second I am not sure overloading a method is the best and
> > > >>> most clear way to provide this information. Maybee he could do it
> > > >>> while instantiating the Expression instead?
> > > >>>
> > > >>>   class BoundarySource(Expression):
> > > >>>       def eval_data(self, values, data):
> > > >>>           g = sin(5*data.x()[0])
> > > >>>           values[0] = g*data.normal()[0]
> > > >>>           values[1] = g*data.normal()[1]
> > > >>>
> > > >>>   s = BoundarySource(value_size=2)
> > > >>
> > > >> I don't really like the name value_size, but this looks natural to me.
> > > >> What would it be for matrix-valued functions?
> > > >
> > > > As I have understand it, value_size is the size of the values array
> > > > passed to
> > > >
> > > > the Expression. So that would then be:
> > > >   value_size = m x n
> > > >
> > > > I am not totally happy with the above code as the class should contain
> > > > everything it needs to be instantiated correctly.
> > > >
> > > > Would it make more sense to change the name of dim to value_size?
> > >
> > > Exactly what do you mean here? Keep needing to supply a method "dim",
> > > but renaming it to "value_size" ? (I still don't find the name
> > > value_size very intuitive.)
> >
> > We have value_rank and value_dimension in the UFC interface. Those are
> > well defined and well documented. value_size is not something the user
> > should be worried about, but it is something that comes up in the code
> > again and again when allocating arrays:
> >
> >   int value_size = 1;
> >   for (int i = 0; i < value_rank; i++)
> >     value_size *= value_dimension(i)
> >
> > Another option would be to set value_shape which is
> >
> >   int value_shape[value_rank];
> >   for (int i = 0; i < value_rank; i++)
> >     value_shape[i] = value_dimension(i)
>
> What with:
>
>   class MyExpression(Expression):
>       def eval(value, x):
>            some_tensor_expression
>       def value_shape(self):
>           return (2,2)
>
> If not given we assume, as today, that the value shape is ().
>
> One should be able to instantiate it using a not automatically chosen
> FiniteElement. We add check so the value shape agree with the passed Element.
>
> We can add deprecation warnings for dim and rank for next release.

I think that looks good. It is natural to have

  value_rank
  value_dimension

in C++ and

  value_shape

in Python (since tuples are problematic in C++).

--
Anders