dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Sun, Sep 05, 2010 at 05:33:45PM +0100, Garth N. Wells wrote:
>
>
> On 05/09/10 17:22, Johan Hake wrote:
> >On Sep 5, 2010, at 8:47, Marie Rognes<meg@xxxxxxxxx>  wrote:
> >
> >>On 05. sep. 2010 17:36, 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.
> >>>
> >>
> >>
> >>Side note: At the moment, we allow both specifying a finite element
> >>and/or a element function space. Should we really allow specifying
> >>element? Using element (instead of function space) from dolfin seems a
> >>bit odd to me.
> >
> >We do not accept a FunctionSpace today. We just silently take it and do nothing with it... Will fix.
> >
> >I think we landed on just the element as an Expression should not be tied to a mesh.
> >
>
> That's right - an expression can be evaluated for any x, which is
> why we use an element (if we use a function space, it's just a lazy
> way of passing the element). The optional element in the constructor
> tells the form compiler how to deal with the Expression, rather than
> having the form compiler choose the element.

Agree. It is a technical thing that allows advanced users to specify
how the coefficient should be treated in a form. Therefore we should
not have a lazy FunctionSpace option that extracts the element since
that would encourage lazy usage.

--
Anders