[petsc-users] create a sparse symmetric matrix

[Matthew Knepley]

On Thu, Aug 9, 2012 at 9:53 PM, Yifei Li <yifli82 at gmail.com> wrote:

> Matt,
>
> Thanks.  I have another two related questions:
> 1) So just because I did not get the correct value doesn't mean the
> symmetric matrix is not setup correctly?
>

Right.


> 2) Which function should I use to retrieve the value then? I can't get the
> correct value even though I disabled MPI when  building PETSC?
>

Since it would require communication, there is no easy way to do this. Why
would you need this?

   Matt


> Yifei
>
>
>
> On Thu, Aug 9, 2012 at 8:45 PM, Matthew Knepley <knepley at gmail.com> wrote:
>
>> On Thu, Aug 9, 2012 at 5:20 PM, Yifei Li <yifli82 at gmail.com> wrote:
>>
>>> Hi all,
>>>
>>> I only filled the entries in the upper triangular part of the matrix,
>>> however, I got  0 when I tried to get the value of some entry in the lower
>>> triangular part.
>>>
>>> Here's how I do it:
>>>
>>>
>>> MatCreate(PETSC_COMM_WORLD, &A);
>>>
>>> MatSetSizes(A, PETSC_DECIDE, PETSC_DECIDE, n, n);
>>>
>>> MatSetType(A, MATSEQSBAIJ);
>>>
>>> MatSetUp(A);
>>>
>>> // I set the value for the entry at (0, 2)
>>>
>>> int i = 2, j = 0;
>>>
>>> MatGetValues(A,1,&i,1,&j,values);
>>>
>> MatGetValues() is not synchronous, so it would not be able to communicate
>> to the
>> correct process to retrieve the values. Thus, you cannot retrieve the
>> lower triangle.
>> This is analogous to only having access to local values with
>> VecGetArray().
>>
>>    Matt
>>
>>
>>>  Any help is highly appreicated.
>>>
>>>
>>>
>>> Yifei
>>>
>>
>>
>>
>> --
>> 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
>>
>
>


-- 
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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20120809/245a33d3/attachment-0001.html>