how do you access the nonzero elements in SeqAIJ matrices

[Ahmed El Zein]

On Mon, 2008-08-04 at 23:13 +0800, Zi-Hao Wei wrote:
> On Mon, Aug 4, 2008 at 10:53 PM, Ahmed El Zein <ahmed at azein.com> wrote:
> > my code looks like:
> >    ierr = MatCreate(PETSC_COMM_WORLD,&A); CHKERRQ(ierr);
> >    ierr = MatSetSizes(A,M,N,M,N); CHKERRQ(ierr);
> >    ierr = MatSetType(A, MATSEQAIJ); CHKERRQ(ierr);
> >    ierr =  MatAssemblyBegin(A, MAT_FINAL_ASSEMBLY); CHKERRQ(ierr);
> >    [...lots of MatSetValue() calls...]
> >    ierr = MatAssemblyEnd(A, MAT_FINAL_ASSEMBLY); CHKERRQ(ierr);
> 
> I think that the code should be
>   ierr = MatCreate(PETSC_COMM_WORLD,&A); CHKERRQ(ierr);
>   ierr = MatSetSizes(A,M,N,M,N); CHKERRQ(ierr);
>   ierr = MatSetType(A, MATSEQAIJ); CHKERRQ(ierr);
>   [...lots of MatSetValue() calls...]
>   ierr = MatAssemblyBegin(A, MAT_FINAL_ASSEMBLY); CHKERRQ(ierr);
>   ierr = MatAssemblyEnd(A, MAT_FINAL_ASSEMBLY); CHKERRQ(ierr);
> 
> The routines MatAssemblyBegin and MatAssemblyEnd should be called
> after completing all calls to MatSetValues().
You are right of course. But if these 2 calls are meant to be right
after each other, why isn't there just one MatAssembly() call versus a
Begin and End call?

btw fixing this did not affect the problem I had.

Ahmed