how do you access the nonzero elements in SeqAIJ matrices

[Ahmed El Zein]

On Mon, 2008-08-04 at 04:17 -0500, Matthew Knepley wrote:
> On Mon, Aug 4, 2008 at 3:14 AM, Ahmed El Zein <ahmed at azein.com> wrote:
> > I am working on a project where I would like to copy a sparse matrix in
> > CSR format.
> >
> > I have tried MatGetRow() which works OK but I would really like to get
> > pointers to the 3 arrays directly.
> >
> > I also tried MatGetRowIJ() which allows me to get the i and j arrays but
> > I can't see how to access the nonzero elements.
> 
> You can use MatGetArray().
Thanks. The man page states that:
The result of this routine is dependent on the underlying matrix data
structure, and may not even work for certain matrix types.

How do I find out which matrix types support it? and is there a method
that works across all matrix types?

Thanks,
Ahmed
> 
>   Matt
> 
> > and finally I attempted to access the arrays directly like this:
> > Mat_SeqAIJ  *a = (Mat_SeqAIJ*)A->data;
> > MatScalar *val = a->a;
> > PetscInt  *ptr = a->i;
> > PetscInt  *ind = a->j;
> >
> > However when accessing directly I get different values for ptr and
> > SIGSEGV when accessing val or ind.
> >
> > also I get a bogus number for a->nz (134630032 instead of 21)
What is the correct way to get the number of nonzeros?

> >
> > Can someone please explain when I am doing wrong?
> >
> > Ahmed
> >
> >
> 
> 
>