dolfin team mailing list archive

[Johan Hake <johan.hake@xxxxxxxxx>]

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.

Johan

> --
> Anders
> 
> > >>> Or just use the FiniteElement which is a litle bit over kill but
> > >>> already possible as it is.
> > >>> 
> > >>>   s = BoundarySource(element=BDM.ufl_element())
> > >>> 
> > >>> What you say?
> > >> 
> > >> When prescribing an element (or a function space) to anything that
> > >> resembles a function, I assume that the function (in this case, the
> > >> Expression) is interpolated into the function space defined by that
> > >> element. Is that correct?
> > > 
> > > Yes, that is correct. We need a FiniteElement to be able to initiate
> > > the ufl.Coefficient. If not provided we automatically choose one based
> > > on (for now the dim method) what the user provides.
> > 
> > But no element space is needed in this case. If the "s" is never used in
> > a form (as is the case in the mixed Poisson example), then it only needs
> > to be evaluate-able. Hence, we should not require an element or function
> > space to be set for Expressions. I seem to remember this being one of
> > the main points with the Function vs Expression design.
> 
> --
> Anders