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

[Eda Oktay]

Dear Matt,

I have one last question I believe. Up to creating a dense matrix I
did what you've suggested. Thank you so much for that.

I created a new dense matrix. Now, how should I wrap each vector in a
MatDense again? I mean, what is wrapping vectors in a matrix? To put
each of them again as rows?

Thanks!

Eda

Matthew Knepley <knepley at gmail.com>, 10 Haz 2020 Çar, 16:16 tarihinde
şunu yazdı:
>
> 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/