[ublas] diagonal matrix and scaled axpy

From: Gunter Winkler (guwi17_at_[hidden])
Date: 2005-04-25 07:29:26

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.

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). 
Using these types with prod(D,A) is not optimal, since it uses the most 
complicated available prod: (sparse = sparse * sparse).

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.

mfg
Gunter