[petsc-users] create a sparse symmetric matrix

[Yifei Li]

On Thu, Aug 9, 2012 at 10:59 PM, Matthew Knepley <knepley at gmail.com> wrote:

> 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?
>
>
I just want to make sure the lower triangular part gets the same values as
the upper triangular part.

>
>
>    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
>
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