Re: [ublas] size of a matrix when concerned about "real" (non-zero) values only

From: Gunter Winkler (guwi17_at_[hidden])
Date: 2007-04-26 17:01:15

Am Donnerstag, 26. April 2007 06:45 schrieb Pallav G:
> say i declare a 3 x 3 mapped_matrix. all of the
> elements are initialized to 0. say i set the value of
> 2 elements to 1 so that the matrix is like this
>
> 0 0 0
> 0 1 0
> 0 0 1

... compute effective size of a matrix ...

> also it would be nice to know how to many non-zero
> elements are there in the matrix at any given time.
>
> maybe these facilities already exist and i'm not aware
> of them? please let me know.

we have nnz(), nnz_capacity() which return the actual and maximal (until 
internal reallocation) number of structural nonzeros. If you frequently 
have to count nonzeros per row you may consider different sparse types:

generalized_vector_of_vector< T, row_major, vector<compressed_vector<T> 
> > A;

here you can use the (undocumented?) access to the internal array of 
compressed vectors:

A.data()[i].nnz(); // nnz of row i

mfg
Gunter