dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Sun, Sep 05, 2010 at 11:02:08AM +0100, Garth N. Wells wrote:
>
>
> On 05/09/10 10:29, Marie Rognes wrote:
> >On 05. sep. 2010 00:00, Johan Hake wrote:
> >>On Sep 4, 2010, at 11:16, Marie Rognes<meg@xxxxxxxxx>  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.)
> >>>
> >>Something like that. The point is that a user need to provide this
> >>information so the Expression can be properly evaluated.
> >>
> >
> >
> >How about
> >
> >s = Expression(value_shape=I)
> >
> >where I is a multiindex such as (m,) or (m, n) or (m, n, q, ...)
> >etc. Maybe with the options of
> >
> >Expression(value_shape=m)<=>  Expression(value_shape=(m,))
> >
> >And no overloading of dim or rank?
> >
>
> Looks very Python centric. The issue of the shape isn't Python
> specific - it needs to be refined/sorted out in the C++ interface
> too.

Is there a need for that? The C++ Expression class has these two
virtual functions:

  /// Return value rank
  virtual uint value_rank() const;

  /// Return value dimension for given axis
  virtual uint value_dimension(uint i) const;

They make perfect sense and have a 1-1 mapping to the UFC interface.

--
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.
>
> What do you mean by never used in a form? What's it used for if not
> in a form?
>
> >>>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.
> >>>
> >>He he no need to take that disc one more time.
> >
> >
> >Ok :)al po
> >
> >>The point is that a user can optionally provide a FiniteElement. In
> >>this case the expression will get the correct value_size.
> >>
> >
> >
> >My point was simply that it should not be required.
> >
>
> My original point from looking at the doc string  was less subtle
> than these. It was that doc string doesn't make clear that a user
> can provide a finite element to a non-JIT Expression subclass,
> thereby prescribing the space in which the expression should be
> interpolated rather than letting FFC decide.
>
> Garth
>
> >
> >-- Marie
> >
> >
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--
Anders