[petsc-users] Question about MatGetRow

[Matthew Knepley]

On Fri, Feb 3, 2023 at 8:52 AM 김성익 <ksi2443 at gmail.com> wrote:

> I want to extract same row values of global matrix in all processes.
> Then how can I do this??
>

Create the same IS on each process.

  THanks,

    Matt


> The case of same problem of vector, I just use vecscattertoall.
> However, I can't find same function for matrix.
>
> Hyung Kim
>
> 2023년 2월 3일 (금) 오후 10:47, Matthew Knepley <knepley at gmail.com>님이 작성:
>
>> On Fri, Feb 3, 2023 at 8:45 AM 김성익 <ksi2443 at gmail.com> wrote:
>>
>>> Following your comments,
>>> If I extract first row of below matrix.
>>> [image: image.png]
>>> IS isrow;
>>> PetscInt *indices;
>>> PetscMalloc1(1, *indices);
>>>
>>
>> That should be &indices.
>>
>>
>>> Indices[0] = 0;
>>> ISCreateGenreral(PETSC_COMM_WORLD, 1, indices, PETSC_COPY_VALUES,
>>> &isrow);
>>>
>>
>> You should use PETSC_OWN_POINTER.
>>
>>
>>> MatCreateSubMatrix(mat,isrow,NULL,MAT_INITIAL_MATRIX,&newmat);
>>>
>>> Then can I get the array about first row of global matrix in all
>>> processes?
>>>
>>
>> No, just on the process which gives 0. If you do that on every process,
>> every rank with get row 0.
>>
>>   Thanks,
>>
>>      Matt
>>
>>
>>> Hyung Kim
>>>
>>> 2023년 2월 3일 (금) 오후 10:26, Matthew Knepley <knepley at gmail.com>님이 작성:
>>>
>>>> On Fri, Feb 3, 2023 at 8:06 AM 김성익 <ksi2443 at gmail.com> wrote:
>>>>
>>>>> Following your comments,
>>>>> I want to check below things.
>>>>> For example, the global dense matrix are as below.
>>>>> [image: image.png]
>>>>> If I want to get first row ('1 2 0 0 3 0 0 4') in Proc 1.
>>>>> Then I should put 'MatCreateSubMatrix
>>>>> <https://petsc.org/main/docs/manualpages/Mat/MatCreateSubMatrix/>(mat,
>>>>> isrow, NULL, MAT_INITIAL_MATRIX, *&*newmat)'
>>>>> and isrow will be [0 1 2 3 4 5 6 7].
>>>>>
>>>>> In this case, How can I make isrow?
>>>>> Actually I can't understand the procedure of handling isrow.
>>>>>
>>>>
>>>> You create an IS object of type ISGENERAL and give it the array of
>>>> global indices that you want to extract.
>>>>
>>>>   Thanks,
>>>>
>>>>      Matt
>>>>
>>>>
>>>>> Hyung Kim
>>>>>
>>>>> 2023년 2월 3일 (금) 오후 9:03, Mark Adams <mfadams at lbl.gov>님이 작성:
>>>>>
>>>>>> https://petsc.org/main/docs/manualpages/Mat/MatCreateSubMatrix/
>>>>>>
>>>>>> Note, PETSc lets you give NULL arguments if there is a reasonable
>>>>>> default.
>>>>>> In this case give NULL for the column IS and you will get the whole
>>>>>> columns.
>>>>>>
>>>>>> Mark
>>>>>>
>>>>>> On Fri, Feb 3, 2023 at 4:05 AM 김성익 <ksi2443 at gmail.com> wrote:
>>>>>>
>>>>>>> Hello,
>>>>>>>
>>>>>>>
>>>>>>> By using MatGetRow, user can get vectors from local matrix (at each
>>>>>>> process).
>>>>>>>
>>>>>>> However, I need other process's row values.
>>>>>>> So I have 2 questions.
>>>>>>>
>>>>>>> 1. Is there any function for getting arrays from other process's??
>>>>>>>
>>>>>>> 2. Or is there any function like matrix version of vecscattertoall??
>>>>>>>
>>>>>>> Thanks,
>>>>>>> Hyung Kim
>>>>>>>
>>>>>>
>>>>
>>>> --
>>>> 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/>
>>>>
>>>
>>
>> --
>> 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/>
>>
>

-- 
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/>
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