[petsc-users] Getting access to matrix rows without and setting values simultaneously

[Barry Smith]

   I am afraid that Jed gave you some misinformation on this response that led you guys off the trail a bit.  This is the first mistake I've seen Jed make in the three years I've known him (as opposed to Matt and I who make mistakes everyday :-)) so don't  be too hard on him.

   MatGetArray() on dense matrices returns a pointer to the entire local matrix stored in column major order (Fortran style), thus you can call MatGetArray() and then muck with all the local entries very efficiently since you are just doing direct array access.

   Hope I am right (and this is not my daily mistake) and this helps you,


    Barry

Dang proposal issues most of the day so not able to read email until now.


On Jun 16, 2011, at 6:10 AM, Jed Brown wrote:

> On Thu, Jun 16, 2011 at 12:59, Alexander Grayver <agrayver at gfz-potsdam.de> wrote:
>    ! store transformed elements
>    call MatSetValues(A,1,rowidx,nvals,cols,transformedvals,INSERT_VALUES,ierr);CHKERRQ(ierr);
>    call MatRestoreRow(A,i,nvals,cols,vals,ierr);CHKERRQ(ierr); 
> 
> You can't mix these because MatGetRow() needs an assembled matrix, and MatSetValues() makes the matrix be unassembled.
> 
> What do you want the parallel semantics to be during this transformation?
> 
> The MatGetArray() for MPIDense just gives you access to the "diagonal" block.
> 
> What is the high-level description of the operation you're trying to do? Is this "matrix" actually being used as a linear operator, or is it a grid of values?