[petsc-users] Zero entries of a sparse matrix in a block-wise style

[Mingchang Ding]

> On Jul 26, 2019, at 3:21 PM, Jed Brown <jed at jedbrown.org> wrote:
> 
> Please always use "reply-all" so that your messages go to the list.
> This is standard mailing list etiquette.  It is important to preserve
> threading for people who find this discussion later and so that we do
> not waste our time re-answering the same questions that have already
> been answered in private side-conversations.  You'll likely get an
> answer faster that way too.

  Sorry for this.

> Mingchang Ding <dmcvamos at udel.edu> writes:
> 
>> Thank you for your quick reply. The code I have now is attached. The sparse matrix obtained from LDG discretization to u_xx is assembled in SetupMat routine of setup.h. When I choose my polynomial degree nk =1 (DOF = 2), only the first and last row of the output sparse matrix is as what I want. Again, I want to impose essential (Dirichlet) boundary condition to the problem.
>> 
>> Let me know if any explanation of the question is needed.
> It appears that you are calling MatZeroRows with rows = {0, info.mx-1},
> which will only affect the first and last rows.  The other entries are
> how you have assembled them (with the matrix-matrix product).

Yes. I think so. Is there anyway I can fill in the rows array with all the first and last DOF rows index efficiently instead of only filling in one row index at one time?

>      PetscInt        nrows = 0;
>      PetscInt        *rows;
> 
>      ierr = DMDAGetLocalInfo(da, &info);CHKERRQ(ierr);
> 
>      PetscMalloc1(info.mx, &rows);
> 
>      for (j=info.xs; j<info.xs+info.xm; j++) {
> 
>          if (j == 0) {
>              rows[nrows++] = j;
> 
> [...]
> 
>          }
>          else if (j == info.mx - 1) {
>              rows[nrows++] = j;
> 
> [...]
> 
> 
>          }
>          else {
> 
> [...]
> 
> 
>          }
> 
>          /* Mass_inv */
>          ierr = MatSetValuesBlocked(appctx->Mass_inv, 1, &j, 1, &j, &inv[0][0], INSERT_VALUES);CHKERRQ(ierr);
> 
>      }
> 
> 
> [...]
> 
> 
>      MatZeroRows(appctx->J_dif, nrows, rows, 1.0, NULL, NULL);