dolfin team mailing list archive

[Marie Rognes <meg@xxxxxxxxx>]

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.


> 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?

> 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? If no, then the behaviour is confusing. If
yes, then that is not what I wanted to do above.


--
Marie