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

[Matthew Knepley]

On Wed, Jun 10, 2020 at 9:08 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.
>

I would do

  MatGetSize(&N);
  MatGetLocalSize(&m);

https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Mat/MatDenseGetArray.html
  <create vector of local size m*N>

https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Vec/VecPlaceArray.html

https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Vec/VecScatterCreateToAll.html
  <do scatter>

https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Vec/VecResetArray.html#VecResetArray

https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Mat/MatCreateDense.html
  <use it>
  <destroy matrix>
  <destroy vector from CreateToAll>

  Thanks,

     Matt


> 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/
>


-- 
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/>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20200610/1eb87e9f/attachment.html>