dolfin team mailing list archive

[Marie Rognes <meg@xxxxxxxxx>]

On 05. sep. 2010 17:36, Johan Hake wrote:
> On Sunday September 5 2010 07:26:19 Anders Logg wrote:
>   
>> On Sat, Sep 04, 2010 at 08:16:43PM +0200, Marie Rognes wrote:
>>     
>>> 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.)
>>>       
>> We have value_rank and value_dimension in the UFC interface. Those are
>> well defined and well documented. value_size is not something the user
>> should be worried about, but it is something that comes up in the code
>> again and again when allocating arrays:
>>
>>   int value_size = 1;
>>   for (int i = 0; i < value_rank; i++)
>>     value_size *= value_dimension(i)
>>
>> Another option would be to set value_shape which is
>>
>>   int value_shape[value_rank];
>>   for (int i = 0; i < value_rank; i++)
>>     value_shape[i] = value_dimension(i)
>>     
> What with:
>
>   class MyExpression(Expression):
>       def eval(value, x):
>            some_tensor_expression
>       def value_shape(self):
>           return (2,2)
>
> If not given we assume, as today, that the value shape is ().
>
> One should be able to instantiate it using a not automatically chosen 
> FiniteElement. We add check so the value shape agree with the passed Element.
>


Side note: At the moment, we allow both specifying a finite element
and/or a element function space. Should we really allow specifying
element? Using element (instead of function space) from dolfin seems a
bit odd to me.

--
Marie