[petsc-users] Correct Usage of MatDiagonalSet

[Edoardo alinovi]

Dear Guys,

I have quick question for you. Can anyone tell me if MatDiagonalSet needs
also MatAssemblyBegin/MatAssemblyEnd to be called afterwards or not?

Basically, I am trying to compute: sum (a_nb*\phi_nb), where anb are the
off diagonal coeffs of the matrix and phi_nb the corresponding field
values.

I was thinking to set the matrix diagonal to zero using MatDiagonalSet and
then simply use MatMult(A, x) to accomplish my task. Do we have a better
way in PETSc?  Any suggestion in welcome :)

Thank you very much
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