dolfin team mailing list archive

[Marie Rognes <meg@xxxxxxxxx>]

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.)


>>> 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. 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.

--
Marie


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