[petsc-users] reordering the dmda dof

[Jed Brown]

Bishesh Khanal <bisheshkh at gmail.com> writes:
> Thanks Matt, but I was trying to check if there is any problem in a matrix
> I created. I had my old prototype code in MATLAB where I created a matrix
> by having a block of rows for each dof. So as a quick test, I wanted to
> write the matrix I created in Petsc to a file and read it with matlab and
> compare the two matrices, taking a difference would help me spot errors
> quickly.

Just permute the matrix, either using ISCreateGeneral and MatPermute or
in Matlab by creating P = [1, m+1, 2*m+1, 2, m+2, 2*m+2, ...] and then
looking at A(P,P).

> Please let me know if there is a quick way to do that. And if there are
> better ways to track down possible errors in the matrix creation by having
> better visualization of elements of the matrix created in petsc (without
> going to matlab), I'd be happy to learn that too.

If this is the Jacobian of something, use -snes_type test to compare
what you assembled to the matrix obtained by finite differencing the
residual function.
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