[petsc-users] MatSetValues vs MatSetValuesBlocked

Nidish nb25 at rice.edu
Mon Aug 10 17:26:01 CDT 2020

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 ;)

Thank You,

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.

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