dolfin team mailing list archive

[Johan Hake <johan.hake@xxxxxxxxx>]

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. 

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)

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?

Johan