[petsc-users] Compute the sum of the absolute values of the off-block diagonal entries of each row

[Matthew Knepley]

On Mon, Mar 4, 2019 at 11:28 AM Cyrill Vonplanta via petsc-users <
petsc-users at mcs.anl.gov> wrote:

> Dear Petsc Users,
>
> I am trying to implement a variant of the $l^1$-Gauss-Seidel smoother from
> https://doi.org/10.1137/100798806 (eq. 6.1 and below). One of the main
> issues is that I need to compute the sum  $\sum_j |a_{i_j}|$ of the matrix
> entries that are not part of the local diagonal block. I was looking for
> something like MatGetRowSumAbs but it looks like it hasn't been made yet.
>
> I guess i have to come up with something myself, but would you know of
> some workaround for this without going too deep into PETCs?
>

MatGetOwnershipRange(A, &rS, &rE);
for (r = rS; r < rE; ++r) {
  sum = 0.0;
  MatGetRow(A, r, &ncols, &cols, &vals);
  for (c = 0; c < ncols; ++c) if ((cols[c] < rS) || (cols[c] >= rE)) sum +=
PetscAbsScalar(vals[c]);
}

  Thanks,

     Matt


> Best Cyrill
>
-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener

https://www.cse.buffalo.edu/~knepley/ <http://www.cse.buffalo.edu/~knepley/>
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