[petsc-users] better way of setting dirichlet boundary conditions

[Bishesh Khanal]

Hi all,
In solving problems such as laplacian/poisson equations with dirichlet
boundary conditions with finite difference methods, I set explicity the
required values to the diagonal of the boundary rows of the system matrix,
and the corresponding rhs vector.
i.e.  typically my matrix building loop would be like:

e.g. in 2d problems, using DMDA:

FOR (i=0 to xn-1, j = 0 to yn-1)
     set row.i = i, row. j = j
     IF (i = 0 or xn-1) or (j = 0 or yn-1)
            set diagonal value of matrix A to 1 in current row.
     ELSE
            normal interior points: set the values accordingly
     ENDIF
ENDFOR

Is there another preferred method instead of doing this ? I saw functions
such as MatZeroRows()
when following the answer in the FAQ regarding this at:
http://www.mcs.anl.gov/petsc/documentation/faq.html#redistribute

but I did not understand what it is trying to say in the last sentence of
the answer "An alternative approach is ... into the load"

Thanks,
Bishesh
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