# how to extract a subvector?

Fri Jan 2 19:31:32 CST 2009

```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
>>
>>
>>
>>
>
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