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

[Jed Brown]

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.

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).


      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);