
Ublas : 
From: Michael Stevens (mail_at_[hidden])
Date: 20050426 09:18:43
Hello Gunter,
On Monday 25 April 2005 14:29, Gunter Winkler wrote:
> Hello,
>
> I was looking for an ublas expression, to compute
>
> y += D * A * x;
>
> where D is a diagonal matrix (stored in a vector of the diagonal elements
> of D) and A is a sparse matrix.
I have been wishing a vector to diagonal matrix adaptor too. It is something I
need quite regularly.
> In banded.hpp I found the (still to document) diagonal_matrix and
> diagonal_adaptor types which are special banded matrices (with lower and
> upper bandwidth equal to zero).
I never noticed they were undocumented. Definitely something to be fixed.
> Using these types with prod(D,A) is not optimal, since it uses the most
> complicated available prod: (sparse = sparse * sparse).
Are you sure prod dispatches to sparse*sparse. It should be dispatching to
a 'packed' prod?
> Should we introduce a new matrix proxy? Something like
> DA := scaled_matrix(D, A)
> where D is a vector and A a matrix. DA(i,j) would evaluate to D(i)*A(i,j)
> and could reuse all iterators of A. The proxy should be usable (and
> optimizeable) by axpy_prod, too.
Interesting solution. I'm wondering if it is not possible to define prod so
prod (diagonal_adaptor(vector), matrix) result in such a scaled_matrix object
automatically.
Michael
 ___________________________________ Michael Stevens Systems Engineering 34128 Kassel, Germany Phone/Fax: +49 561 5218038 Navigation Systems, Estimation and Bayesian Filtering http://bayesclasses.sf.net ___________________________________