get several submatrices from several matrices and combine submatrices into another matrix

[Yujie]

Hi, PETSc Developers

In parallel mode, I have 3 MPIDense matrices, A1, A2, A3. Now, I need to
exact submatrices B1, B2, B3 in parallel mode from A1, A2, A3 respectively.
I know I should use MatGetSubMatrix(). My problem is in

MatGetSubMatrix(Mat mat,IS isrow,IS iscol,PetscInt csize,MatReuse
cll,Mat *newmat)

, the parameter "isrow" means "rows this processor should obtain". if Bi in
some nodes (cpus) of the cluster (Ai have rows in them) don't have rows, is
it work?

After getting Bi, I will create MPIDense-based C and combine them into C
like

     B1
C= B2
     B3
MatGetRow();
MatSetValues() should work, right?

Could you give me some comments about these operations? thanks a lot. Happy
new year!

Regards,
Yujie
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