[petsc-users] Changing the diagonal of a matrix via a vector

[Jed Brown]

On Wed, Apr 27, 2011 at 17:57, Xiangdong Liang <xdliang at gmail.com> wrote:

> I am a novice to Petsc and parallel computing. I have created a mpi sparse
> matrix A (size n-by-n) and a parallel vector b (size n-by-1). Now I want to
> modify the diagonal of A by adding the values of vector b. Namely, A(i,i) =
> A(i,i) + b(i) and all the off-diagonal elements remains the same. I am
> worrying that when I use MatSetValue or MatSetValues, b(i) may not be
> accessed by some particular processor since VecGetValues can only get values
> on the same processor. One possible solution I am thinking  is converting
> vector b to a diagonal matrix B and then do the MatAXPY operation. However,
> using MatSetValue to set diagonal elements of B,  B(i,i) = b(i),  still
> faces the similar problem. Can anyone give me some suggestion? Thanks.
>

MatDiagonalSet(A,b,ADD_VALUES)

http://www.mcs.anl.gov/petsc/petsc-as/snapshots/petsc-dev/docs/manualpages/Mat/MatDiagonalSet.html


> P.S. When I compiled my program, I get warnings like that: warning: return
> makes pointer from integer without a cast. Actually, these lines are
> standard Petsc functions like that:
>
> ierr = VecCreate(PETSC_COMM_WORLD,&x);CHKERRQ(ierr);
> ierr = VecDestroy(x); CHKERRQ(ierr);
>
> How can I get rid of these warnings?
>

Either make your function return int (or PetscErrorCode), passing "return
values" back through arguments or use CHKERRV (worse because errors won't
propagate up correctly).

http://www.mcs.anl.gov/petsc/petsc-as/snapshots/petsc-dev/docs/manualpages/Sys/CHKERRQ.html
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