[petsc-users] Finding off diagonal blocks of matrix

[Smith, Barry F.]

  If you are using a MPIAIJ matrix that has already been created you can use MatMPIAIJGetSeqAIJ(). This gives you back on each process the "diagonal" block and the "off-diagonal" block.



> On Jun 17, 2019, at 7:44 AM, Eda Oktay via petsc-users <petsc-users at mcs.anl.gov> wrote:
> 
> Hello everyone,
> 
> I am trying to find the off diagonal blocks of a matrix. For example, I partitioned a 10*10 matrix into 4*4 and 6*6 block sub matrices. I need to find the other blocks in order to find the number of nonzero elements in them.
> 
> I tried to use MatGetLocalSubMatrix but I couldn't figure out how to give iscol and isrow. Also, from the manual, ı understood that MatSetLocalToGlobalMapping should be used before this routine but I couldn't used it properly because before MatGetLocalSuMatrix, the submatrix doesn't exist. I tried to give zero to all elements so that I can use Mapping routine, but I got an error saying that it is invalid.
> 
> How can I fix this probelm?
> 
> Thank you,
> 
> Eda