dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Sat, Nov 28, 2009 at 12:47:40AM -0800, Johan Hake wrote:
> On Friday 27 November 2009 23:59:52 Johan Hake wrote:
> > Hello!
> >
> > I have not pushed anything, but Expressions should be up to shape now, and
> >  I am working on simplifying the logic in other places of expression.py
> >
> > I do not like the implementation of the dim method. It is used to read a
> >  user defined value dimension and then discarded. I suggest that we add a
> >  value_shape attribute to the dolfin.Expression class. If not set by the
> >  user it is by default (), i.e., a scalar Expression.
> >
> >   class MyExpression(Expression):
> >       value_shape = (2,)
> >       def eval(self, values, x):
> >          values[0] = 1
> >          values[1] = 1
> >
> > The dim name is also only making sense for Vector Expressions, where I
> >  think a value_shape attribute makes sense for all occasions. A value_shape
> >  for scalar Expression will still be the default.
> >
> > After creation and instantiation we then have an object that have both a
> >  shape method (from ufl.Function) and a value_shape attribute from the
> >  subclassing definition.
> >
> > What you think.
>
> Hmm, more trouble I think. How should we be able to specify the degree, and or
> element when we have derived python Expression? Should we add more
> attributes?

I think it works to do

  f = MyExpression(element=FiniteElement("Lagrange", triangle, 3))

Or is that broken?

--
Anders


> One way is to adapt the CompiledExpression solution, and to make
> dolfin.Expression a pure and nice subclass of ufl.Function and cpp.Expression.
> a user defined python Expression would then look like:
>
>    class MyExpression(Expression):
>        def __init__(self):
>            Expression.__init__(self, element, degree, value_shape)
>        def eval(self, values, x):
>            values[0] = 1
>            values[1] = 1
>
> Where element, degree, and value_shapes are all optional. All the logic of
> what element, should be used is concealed in dolfin.Expression.__init__
>
> With this solution we split all the logic that goes into a compiled Expression
> from what goes into a user defined Python Expression. One of the main problems
> with the present design is that they are too intermingled.
>
> Johan
>
> > Johan
> >
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