[petsc-users] create a sparse symmetric matrix

[Yifei Li]

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?

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?

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
>
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