dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Fri, Nov 27, 2009 at 11:59:52PM -0800, 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.

I think a dim method makes more sense than adding a variable since it
is consistent with eval: Then all data a subclassed Expression needs
to give is defined by a member function.

If we need value_shape and value_dimension (which is good to have),
can't we just store those values as _value_shape, _value_dimension
after we have figured out the shape and provide member functions that
return those values?

Also note the clever/simplified interface for dim() allowing (1) not
to overload it for scalars and (2) to return the just the vector
dimension for vectors: n instead of (n,)).

--
Anders


> 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.
>
> Johan
>
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