[petsc-users] MatSetValues vs MatSetValuesBlocked

[Matthew Knepley]

On Mon, Aug 10, 2020 at 6:26 PM Nidish <nb25 at rice.edu> wrote:

> Ah I get it now, MatSetBlocked has to be set node-wise. I tried this and
> it works, thank you.
>
> The other question I had was why are the arguments for MatSetValues()
> and MatSetValuesBlocked() set to const PetscInt* and const PetscScalar*
> instead of just PetscInt* and PetscScalar* ? I have the typecast there
> so my flycheck doesn't keep throwing me warnings on emacs ;)
>

Jed is correct that this cast is implicit. The idea here is to tell the
caller that we will not change the contents of the arrays that you pass in.

  Thanks,

     Matt


> Thank You,
> Nidish
>
> On 8/10/20 5:16 PM, Jed Brown wrote:
> > Nidish <nb25 at rice.edu> writes:
> >
> >> It's a 1D model with displacements and rotations as DoFs at each node.
> >>
> >> I couldn't find much in the manual on MatSetBlockSize - could you
> >> provide some more information on its use?
> >>
> >> I thought since I've setup the system using DMDACreate1d (I've given
> >> 2dofs per node and a stencil width of 1 there), the matrix object should
> >> have the nonzero elements preallocated. Here's the call to DMDACreate1d:
> >>
> >>     DMDACreate1d(PETSC_COMM_WORLD, DM_BOUNDARY_NONE, N, 2, 1, NULL,
> &mshdm);
> > Ah, that will set the block size, but then it'll expect elstiff to be an
> 8x8 matrix where you've only passed 4x4.
> >
> >      idx[0] = 2*e; idx[1] = 2*e+1; idx[2] = 2*e+2; idx[3] = 2*e+3;
> >
> >      MatSetValuesBlocked(jac, 4, (const PetscInt*)idx, 4, (const
> PetscInt*)idx,
> >                       (const PetscScalar*)elstiff, ADD_VALUES);
> >
> > You don't need the casts in either case, BTW.  You probably want
> something like this.
> >
> >      idx[0] = e; idx[1] = e + 1;
> >
> >      MatSetValuesBlocked(jac, 2, idx, 2, idx, elstiff, ADD_VALUES);
> >
> > Also, it might be more convenient to call MatSetValuesBlockedStencil(),
> especially if you move to a multi-dimensional problem at some point.
> --
> Nidish
>


-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener

https://www.cse.buffalo.edu/~knepley/ <http://www.cse.buffalo.edu/~knepley/>
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