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

[behzad baghapour]

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.

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
==================================
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