[petsc-users] How to get Matrix from MatGetArray()

[Matthew Knepley]

On Sat, Oct 1, 2011 at 11:07 AM, behzad baghapour <
behzad.baghapour at gmail.com> wrote:

> Thanks, then i should change my idea abut matrix evaluation in Petsc. I
> will try to keep matrix in the same structure during successive iterations.
>

No, that is not the points at all. You should use the interface to alter the
matrix as you want. More directly,

  Give up on a[i[row]+offset[col]] = value;

and use instead

  MatSetValues(A, 1, &row, 1, &col, &value, INSERT_VALUES);

I strongly suggest you read

  http://dl.acm.org/citation.cfm?id=245883

  Thanks,

      Matt


> On Sat, Oct 1, 2011 at 7:26 PM, Matthew Knepley <knepley at gmail.com> wrote:
>
>> On Sat, Oct 1, 2011 at 10:44 AM, behzad baghapour <
>> behzad.baghapour at gmail.com> wrote:
>>
>>> OK. All I am concering is:
>>>
>>> 1- define matrix A in sparse format AIJ.
>>> 2- Get the content of A.
>>> 3- change the content of A for some elements
>>> 4- Restore the changed matrix back into origin.
>>>
>>> I test MatGetArray( Mat A, PetscScalar* a ) but I couldnt find the
>>> correct content of matrix by using a[i*n+j]. I need to know how to access
>>> the elements of an assembled sparse matrix.
>>>
>>> Am I clear now?
>>>
>>
>> No, definitely do not do that. The whole point of PETSc matrices is that
>> you do not do this. This is what
>> is wrong with 90% of numerical software. You want *interfaces*, not data
>> structures. That allows you
>> to seamlessly
>>
>>   - work in parallel
>>   - work on GPUs
>>   - optimize for different architectures
>>
>> What you want to do is use MatSetValues() (or MatZeroRows, etc.) to change
>> your entries.
>>
>>     Matt
>>
>>
>>>
>>> Thanks
>>> Behzad
>>>
>>> On Sat, Oct 1, 2011 at 6:56 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:
>>>
>>>>  On Sat, Oct 1, 2011 at 10:23, behzad baghapour <
>>>> behzad.baghapour at gmail.com> wrote:
>>>>
>>>>> I meant by MatAssembly, the global indices of matrix elements (related
>>>>> to a dense format) which is changed into sparse one.
>>>>
>>>>
>>>> I don't understand what you mean. It doesn't make sense to convert
>>>> "dense" indices into sparse ones.
>>>>
>>>>
>>>>> So what would be right pattern to read the content of the Matrix from
>>>>> MatGetArray if I know that the original matrix is created by AIJ?
>>>>
>>>>
>>>> Use MatGetRow() if you need access to this. (The internal data
>>>> structures are different in parallel and serial, so accessing them directly
>>>> is bad.)
>>>>
>>>
>>>
>>>
>>> --
>>> ==================================
>>> Behzad Baghapour
>>> Ph.D. Candidate, Mechecanical Engineering
>>> University of Tehran, Tehran, Iran
>>> https://sites.google.com/site/behzadbaghapour
>>> Fax: 0098-21-88020741
>>> ==================================
>>>
>>>
>>
>>
>> --
>> 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
>>
>
>
>
> --
> ==================================
> Behzad Baghapour
> Ph.D. Candidate, Mechecanical Engineering
> University of Tehran, Tehran, Iran
> https://sites.google.com/site/behzadbaghapour
> Fax: 0098-21-88020741
> ==================================
>
>


-- 
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
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