[petsc-users] MatGetRow for global rows of a parallel matrix

[Eda Oktay]

Thank you so much!

Eda

Zhang, Hong <hongzhang at anl.gov>, 2 Tem 2020 Per, 07:29 tarihinde şunu yazdı:
>
> Dense matrix is stored in column-major order, so you might want to use MatGetColumnVector() to access columns of the matrix instead of the rows. For instance, you can get the n-th column out of the matrix and place it into a vector that shares the same parallel layout as the matrix with the following:
>
>       MatDenseGetColumn(J,n,&xarr);
>       VecPlaceArray(vec,xarr);
>       ...
>       VecResetArray(vec);
>       MatDenseRestoreColumn(J,&xarr);
>
> Hong (Mr.)
>
> On Jun 10, 2020, at 8:08 AM, Eda Oktay <eda.oktay at metu.edu.tr> wrote:
>
> Dear Matt,
>
> Matthew Knepley <knepley at gmail.com>, 10 Haz 2020 Çar, 16:03 tarihinde
> şunu yazdı:
>
>
> On Wed, Jun 10, 2020 at 8:56 AM Eda Oktay <eda.oktay at metu.edu.tr> wrote:
>
>
> Hi all,
>
> I am trying to get all the rows of a parallel matrix as individual
> vectors. For instance, if I have 72*4 matrix, I want to get 72
> different vectors having size 4.
>
> As far as I understood, MatGetRow is only for local rows, so
> MatGetOwnershipRange is used, however, when I tried this one, I
> couldn't get the whole and desired row vectors.
>
> In MatGetRow explanation, it is written that I should use
> MatCreateSubMatrices first, then use MatGetRow. But I couldn't
> understand to which extent I should create submatrices. I just need to
> have all 72 rows as 72 different vectors each having 4 elements.
>
>
>
> 1) For sparse matrices, the storage is always divided by row, so that values can only be retrieved for local rows with MatGetRow()
>
> 2) Is this matrix sparse? It sounds like it is dense.
>
>
> Matrix is dense.
>
>
> 3) Are you asking to get all matrix values on all processes? If so, I think the easiest thing to do is first wrap a Vec around the
>    values, then use VecScatterToAll(), then wrap each one in a MatDense again.
>
>
> Yes, I want all row vectors on all processes. In a dense matrix,
> should I still wrap a Vec around the values? I know I should use
> scatter but I couldn't even wrap a Vec around them.
>
> Thanks so much!
>
> Eda
>
>
>  Thanks,
>
>     Matt
>
>
> Thanks!
>
> Eda
>
>
>
>
> --
> 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/
>
>