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

[Boyce Griffith]

On 9/27/10 5:28 PM, Shri wrote:
> Barry,
>        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);
>   }

I don't understand why this is the right thing to do.  It seems like 
this either sets nnz in the new matrix to be nzx or nzy, whichever is 
smaller.  Am I misreading this code snippet?

-- Boyce