[petsc-dev] [petsc-users] matrix-matrix addition

[Jed Brown]

On Mon, Sep 27, 2010 at 23:28, Shri <abhyshr at mcs.anl.gov> wrote:
>      I've modified MatAXPY_SeqAIJ as follows when DIFFERENT_NONZERO_STRUCTURE flag is true (i) create a new matrix and do preallocation for it. For determining the number of nonzeros per row in the new matrix, i've used the number of nonzeros in the original two matrices. This is the piece of code which computes the nnz per row in the new matrix
>  for(i = 0;i < m;i++) {
>    nzx = xi[i+1] - xi[i]; nzy = yi[i+1] - yi[i];
>    if (nzx > nzy) nnz[i] = nzy + (nzx - nzy);
>    else nnz[i] = nzx + (nzy - nzx);
>  }

What's this supposed to do?  You might as well write

  nnz[i] = PetscMax(nzx,nzy);

But this is not correct because each matrix may have disjoint or
overlapping patterns.  You have to loop through the column indices
(which are sorted) and count the number of unique entries.

Jed