Re: [eigen] Returning an expression unifying two types of matrices

[Jens Mueller <jens.k.mueller@xxxxxx>]

Rhys Ulerich wrote:
> On Nov 24, 2011 8:19 AM, "Jens Mueller" <jens.k.mueller@xxxxxx> wrote:
> >
> > Hi,
> >
> > I'm facing a problem and don't know how to approach it.
> > In my code I have many expressions of the form y.transpose() * getG() *
> > y, where y is a column vector of length n and getG() returns a reference
> > to a nxn matrix.
> > Sometimes I know that G is diagonal (at run-time) and I'd like to use
> > this information when evaluating the expressions (storing a diagonal is
> > also much cheaper).
> > I need to store G somewhere to return it. But G is either a full matrix
> > or a diagonal matrix. So what do I store?
> 
> Store neither. Pass vector y into a method that, like getG, knows how to
> compute either inner product expression (using G diagonal or full
> efficiently)*.* Return the scalar result.

Currently, the matrix is stored in a class A and another class B uses
class A for its computations. Moving the computation to A breaks this
separation. And the expressions using the matrix may become more diverse
in future. So I should add that there are different expressions (all
similar to the one above) but may have more terms. And all need to
access the matrix G.
Maybe my current design is just broken.

Jens