how to extract a subvector?

[Dave May]

Your ix array looks fine.

If you are happy ordering the sub vector as you've indicated, just pass in
PETSC_NULL for iy when you define the VecScatter. This will indicate to just
insert/add the entries for the sub vector in order from lowest global index
largest global index.

Cheers,
  Dave


On Sat, Jan 3, 2009 at 3:06 PM, Yujie <recrusader at gmail.com> wrote:

> Dear Matthew:
>
> Assuming the parent vector is 0-9 and there are 3 processes (0,1,2; 3,4,5;
> 6,7,8,9),
> Now the subvector contains 2; 4; 8, 9.
> I should create "ix" in 3 processores with MPI_COMM using arraies {2}, {4},
> {8,9} respectively?
> create "iy" using arraies {0},{1},{2,3} respectively?
> If it is, I think it is a little difficult to get "iy". Since the user
> needs to communicate between processors to confirm the global position of
> the local subvector. thanks  a lot.
>
> Regards,
> Yujie
>
>
> On Fri, Jan 2, 2009 at 6:19 PM, Matthew Knepley <knepley at gmail.com> wrote:
>
>> No, you use the global indices ONLY for the values needed on this
>> processor. I would
>> also note that there is an extensive discussion of this IN the manual.
>>
>>    Matt
>>
>>
>> On Fri, Jan 2, 2009 at 7:31 PM, Yujie <recrusader at gmail.com> wrote:
>>
>>> You mean I need to let all processes have a copy of the whole global
>>> index and to use this function? thanks
>>>
>>>
>>> On Fri, Jan 2, 2009 at 5:15 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
>>>
>>>>
>>>>   The ix and iy are always global indices based on where the vector
>>>> lives.
>>>>
>>>>   If you are going from a parallel to parallel vector then both indices
>>>> are "global"
>>>>
>>>>  Barry
>>>>
>>>>
>>>> On Jan 2, 2009, at 2:21 PM, Yujie wrote:
>>>>
>>>>  Dear Barry:
>>>>>
>>>>> I have a new question about the parameter "iy"(new index set of
>>>>> subvector). To parallel vector, how to provide "ix" and "iy"?
>>>>>
>>>>> Just providing the local index subset for "ix" and "iy"? if it is, it
>>>>> is a little difficult to let the local vector know its global position in
>>>>> the new subvector? If the user needs to provide the global position of local
>>>>> vector for "iy". some MPI communication should be needed, it looks like not
>>>>> a good method. In exacting submatrix, the function hides this problem. could
>>>>> you give me any comments? thanks a lot.
>>>>>
>>>>> Regards,
>>>>>
>>>>> Yujie
>>>>>
>>>>>
>>>>> On Fri, Jan 2, 2009 at 11:46 AM, Barry Smith <bsmith at mcs.anl.gov>
>>>>> wrote:
>>>>>
>>>>>  VecScatter is for this purpose.
>>>>>
>>>>>  Rational: extracting subparts of vectors for ghost points etc takes
>>>>> place many times in a simulation; maybe millions.
>>>>> Thus separating it into a set-up followed by many uses is a  worthwhile
>>>>> optimization. Extracting submatrices occur
>>>>> much less often in a simulation, maybe tens, hundreds or thousands of
>>>>> times so it is not worth the extra complexity
>>>>> of having separate set-up followed by many uses. One could argue that
>>>>> uniformity of design means we should have
>>>>> handled matrices with a MatScatter concept to parallel the Vec
>>>>> approach, but it is too late now :-).
>>>>>
>>>>>  Barry
>>>>>
>>>>>
>>>>> On Jan 2, 2009, at 1:41 PM, Yujie wrote:
>>>>>
>>>>> Like MatGetSubMatrix(), whether is there a function to get a subvector
>>>>> in parallel mode? I have checked some scatter functios, they don't likely
>>>>> work. thanks a lot.
>>>>>
>>>>> Regards,
>>>>>
>>>>> Yujie
>>>>>
>>>>>
>>>>>
>>>>>
>>>>
>>>
>>
>>
>> --
>> 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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