[petsc-users] MatAYPX for dense and sparse matrices

[Alexander Grayver]

On 21.03.2011 12:37, Jed Brown wrote:
> On Mon, Mar 21, 2011 at 12:31, Alexander Grayver 
> <agrayver at gfz-potsdam.de <mailto:agrayver at gfz-potsdam.de>> wrote:
>
>     Would I have to rebuild PETSc from scratch to make this wrapper
>     working?
>
>
> You would put it in your project and link it with your Fortran code.
>
>
>
>>     Alternatively, the code for this is really simple, just iterate
>>     through the rows of L and insert them into J.
>     Do you mean using MatGetOwnershipRange+MatGetSubMatrices+MatGetRow?
>
>
> Just translate the following (taken from MatAXPY_Basic) to Fortran:
>
>   ierr = MatGetSize(X,&m,&n);CHKERRQ(ierr);
>   ierr = MatGetOwnershipRange(X,&start,&end);CHKERRQ(ierr);
>   for (i = start; i < end; i++) {
>     ierr = MatGetRow(X,i,&ncols,&row,&vals);CHKERRQ(ierr);
>     ierr = MatSetValues(Y,1,&i,ncols,row,vals,ADD_VALUES);CHKERRQ(ierr);
>     ierr = MatRestoreRow(X,i,&ncols,&row,&vals);CHKERRQ(ierr);
>   }
>   ierr = MatAssemblyBegin(Y,MAT_FINAL_ASSEMBLY);CHKERRQ(ierr);
>   ierr = MatAssemblyEnd(Y,MAT_FINAL_ASSEMBLY);CHKERRQ(ierr);
>
Thanks Jed, it looks simple enough for me (PETSc is great).

One more question. I get matrix J out of MatMatMult operation and then I 
have to add L matrix up, wouldn't there be a conflict using 
MatAssemblyBegin/MatAssemblyEnd again? Generally, is it allowed to set 
new values into the matrix after MatAssemblyBegin/MatAssemblyEnd already 
has been done once?

Thanks a lot.
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