Re: [eigen] Feature idea: coeffRef() ---> setCoeff()

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2010/7/30 Manoj Rajagopalan <rmanoj@xxxxxxxxx>:
> On Friday 30 July 2010 12:08:42 am Benoit Jacob wrote:
>> oh... i see. I guess that you omitted setCoeff in the above line, by
>> the way, so you meant:
>> A.selfadjointView<Lower>().setCoeff(j, j+1, value); // note: write to
>> upper triangle
>> so this would write 2 coeffs at once in A ? Why not. Indeed that cant
>> be done with coeffRef. But I don't adhere to your rather complex plan
>> of generally allowing to write to Conjugate expressions, this is not a
>> prerequisite for that, we could just implement setCoeff in
>> selfadjointView, where it is useful, without implementing it in stuff
>> like conjugate() which IMO just don't want to be writable expressions.
>> This would be much simpler --- the patch could be just a few kB's.
>> But again, coeffRef() is definitely not going away.
>> Benoit
> In my implementation of LLT on self-adjoint matrices with compact triangular
> storage, which involves partitioning the matrix into 2x2 block form, one step
> leads to an Eigen expression of the form,
>    L11.solveInPlace(L21.adjoint()); // L11 is triangular-view

Oh, if that's what you wanted to do, why don't you just apply complex
conjugation here, so this is equivalent to doing:


Now you're good to go, because transpose() returns a writable
expression which can perfectly be used in solve-in-place.


> This expression compiles but I'm not sure it will work the way I want it.
> Basically, I want the adjoint of the computed solution to be written back to
> memory because, the mathematical expression I end up solving looks like:
>      L21 L11^H = A21;  // L11 triangular, known, A21 known, superimposed L21
> => L21^H = L11^{-1}  A21^H
>    If I required transpose() instead of adjoint(), the transpose of the
> solution gets written back as expected because Transpose<> allows
> write-through. But the conjugate component of the adjoint functor is declared
> const so I am not sure the solutions is transposed AND conjugated before
> write-to-memory for adjoint().
> This got me thinking about having a Conjugate<> wrapper like Transpose<> that
> writes the reverse-transformed value into memory so that we get what we
> expect during read-back.
> I would find this feature useful. I thought some others might too.
> Implementing Complex<> should be simpler than Transpose<>. What you are
> referring to as hard might be propagating this through the library and
> testing it. But sure, setCoeff could co-exist with coeffRef().
> thanks,
> Manoj

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