ufl team mailing list archive

["Martin Sandve Alnæs" <martinal@xxxxxxxxx>]

2008/10/21 Anders Logg <logg@xxxxxxxxx>:
> On Tue, Oct 21, 2008 at 06:14:00PM +0200, Martin Sandve Alnæs wrote:
>> 2008/10/21 Anders Logg <logg@xxxxxxxxx>:
>> > On Tue, Oct 21, 2008 at 06:03:27PM +0200, Martin Sandve Alnæs wrote:
>> >> 2008/10/21 Anders Logg <logg@xxxxxxxxx>:
>> >> > On Tue, Oct 21, 2008 at 05:41:15PM +0200, Martin Sandve Alnæs wrote:
>> >> >> 2008/10/21 Anders Logg <logg@xxxxxxxxx>:
>> >> >> > On Tue, Oct 21, 2008 at 05:27:55PM +0200, Anders Logg wrote:
>> >> >> >> Is there a simple way to evaluate tensor expressions? For example, if
>> >> >> >>
>> >> >> >>  a = dot(v, u)*dx
>> >> >> >>
>> >> >> >> is there then a way to convert this to
>> >> >> >>
>> >> >> >>  v[0]*u[0] + v[1]*u[1] + ...
>> >> >> >>
>> >> >> >> ?
>> >> >> >
>> >> >> > I found the following in Dot:
>> >> >> >
>> >> >> >  def as_basic(self, dim, a, b):
>> >> >> >      ii = Index()
>> >> >> >      aa = a[ii] if (a.rank() == 1) else a[...,ii]
>> >> >> >      bb = b[ii] if (b.rank() == 1) else b[ii,...]
>> >> >> >      return aa*bb
>> >> >> >
>> >> >> > Why are the arguments dim, a and b required? A Dot already knows its
>> >> >> > operands.
>> >> >> >
>> >> >> >> On Tue, Oct 21, 2008 at 05:27:55PM +0200, Anders Logg wrote:
>> >> >> >> Is there a simple way to evaluate tensor expressions? For example, if
>> >> >> >>
>> >> >> >>  a = dot(v, u)*dx
>> >> >> >>
>> >> >> >> is there then a way to convert this to
>> >> >> >>
>> >> >> >>  v[0]*u[0] + v[1]*u[1] + ...
>> >> >> >>
>> >> >> >> ?
>> >> >> >
>> >> >> > I found the following in Dot:
>> >> >> >
>> >> >> >  def as_basic(self, dim, a, b):
>> >> >> >      ii = Index()
>> >> >> >      aa = a[ii] if (a.rank() == 1) else a[...,ii]
>> >> >> >      bb = b[ii] if (b.rank() == 1) else b[ii,...]
>> >> >> >      return aa*bb
>> >> >> >
>> >> >> > Why are the arguments dim, a and b required? A Dot already knows its
>> >> >> > operands.
>> >> >> >
>> >> >>
>> >> >> Because it is used in an algorithm that first expands its operands,
>> >> >> so a and b here are not necessarily its original operands.
>> >> >> As you see, dim is not used here, but it's used in some other Compounds.
>> >> >>
>> >> >> These functions could of course also be placed in the algorithm
>> >> >> in question instead of in each Compound object, like with several
>> >> >> other algorithms. But I think that's a detail we can discuss later,
>> >> >> in context with other similar issues.
>> >> >
>> >> > Yes, if as_basic does not work like a member function, it seems
>> >> > strange to make it a member function. Maybe it could be made static if
>> >> > necessary.
>> >> >
>> >>
>> >> I'm fine with moving it into the algorithm, I can do that later.
>> >>
>> >> Keeping it (static) in the class simplifies adding new Compound
>> >> objects, that's the only reason I can think of now. But this is not
>> >> really necessary since they would have to be handled in e.g. the
>> >> latex compiler and maybe other places for that to make any sense.
>> >>
>> >> This probably makes no sense to you right now, we should set
>> >> up a session where I show how these things fit together.
>> >
>> > There are a lot of things that don't make sense to me right now... :-)
>> >
>> > So is there any way right now to handle compound expressions, or do I
>> > need to implement it?
>> >
>> > Say that I have a = dot(v, u)*dx, how can I expand it into simpler
>> > operations (including only +, *, and [])?
>> >
>>
>> As I just mentioned, the function expand_compounds will expand
>> dot, inner, grad, div, and several others into index-notation.
>> Not sure what it is you want beyond that?
>>
>> Expanding implicit sums and products into explicit sums and
>> products is not implemented in UFL, since so far I've opted
>> to do that while converting to swiginac in SFC. It would be nice
>> to have it in UFL though, but I didn't think you needed that.
>
> What is the dim argument to expand_compounds, and why is it needed?
>
> Should I look at the operands and find out the appropriate dimension,
> and how is that best done?
>
> --
> Anders

As you've discovered, you don't need to use that.

But anyway, dim is the "default dimension", i.e. the spatial dimension.
It's used by as_basic in at least the inverse and cofactor, which
are simply hardcoded with expressions I obtained from ginac...

There are some issues with this dimension / shape thing,
but I'll leave that for another day.

--
Martin