dolfin team mailing list archive

[Martin Sandve Alnæs <martinal@xxxxxxxxx>]

On 8 June 2011 13:55, Martin Sandve Alnæs <martinal@xxxxxxxxx> wrote:
> On 8 June 2011 13:46, Kristian Ølgaard <k.b.oelgaard@xxxxxxxxx> wrote:
>> On 8 June 2011 13:31, Martin Sandve Alnæs <martinal@xxxxxxxxx> wrote:
>>> On 8 June 2011 13:11, Kristian Ølgaard <k.b.oelgaard@xxxxxxxxx> wrote:
>>>> On 8 June 2011 12:11, Martin Sandve Alnæs <martinal@xxxxxxxxx> wrote:
>>>>> Done and checked in. If someone updates FFC to support this, we can
>>>>> hopefully close this bug.
>>>>
>>>> I'm not sure this is enough to handle the bug. If you look at the
>>>> output of printing M in the example code I posted you'll see that the
>>>> list tensor contains component '7'. This is what you'll get from
>>>> calling self.component(), but the
>>>> TT.symmetry() only contains:
>>>> {(2,): (1,), (6,): (5,)}
>>>>
>>>> Is there some function we need to call first to map the component '7'
>>>> --> '6', before looking at symmetries to map '6' --> '5'?
>>>> Doing so will get us into trouble with mapping '3' --> '2' since
>>>> symmetry will map that to '1'.
>>>> The TT element has 2 x 3 sub elements due to symmetry.
>>>
>>> The 7 is an index into the value index space of the coefficient and is
>>> correct. It has nothing (directly) to do with subelement indexing. I
>>> think you're assuming a closer relation between them than there is?
>>> Let me try to clear it up...
>>>
>>> The value index space is contiguous from the point of view of UFL
>>> expressions, but has holes when symmetries are considered. The
>>> noncontiguous value index space will then need to be mapped to a
>>> contiguous subelement space by associating each value index that is
>>> not in the symmetry mapping with a subelement index.
>>>
>>> 1) We have a component/value index
>>> 2) We map that value index to another value index using a symmetry
>>> mapping (e.g. 6->5 and 7->7 in your example)
>>> 3) We map from the noncontiguous value index space to the contiguous
>>> subelement index space
>>>
>>> Clear as mud? :)
>>
>> Yes, but since we only deal with the (sub)elements that are actually
>> present in FFC, it's really inconvenient that we can't get a mapping
>> from the component to the subelement.
>> I somehow suspected the FiniteElement.extract_component() to do this,
>> but it turns out not to be the case.
>>
>>> UFL handles (2) for you only when you apply expand_indices.
>>>
>>> FFC will have to handle (3) when generating code, it doesn't touch
>>> anything UFL needs to know about. I'll see if I can whip up a quick
>>> utility function for it though.
>>
>> That would really be nice.
>
> Done :) The latest patch contains code and test showing usage.
>
> But maybe it should be integrated into extract_component, I'll take a
> look at that now that I'm into this.

I applied the symmetry mapping inside extract_component for tensor elements,
that way you shouldn't have to do the symmetry mapping in addition to
calling extract_component. The numbering utility I checked in could still
come in handy though, so I'll let it stay.

Note that when I pushed now I pushed the float formatting fix as well,
so some FFC references will probably need regeneration again :)

Martin