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

[Smith, Barry F.]

   How about something like,

   MatMPIAIJGetSeqAIJ(A,NULL,&Ao,NULL);

> MatGetOwnershipRange(A, &rS, &rE);
> for (r = 0; r < rE-rS; ++r) {
>   sum = 0.0;
>   MatGetRow(Ao, r, &ncols, NULL, &vals);
>   for (c = 0; c < ncols; ++c) sum += PetscAbsScalar(vals[c]);
   // do what you need with sum
> }


   Barry



> On Mar 4, 2019, at 10:38 AM, Matthew Knepley via petsc-users <petsc-users at mcs.anl.gov> wrote:
> 
> 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/