# How to fill a matrix with a vector parallelly

ming zhu foolishzhu at yahoo.com.cn
Sun Nov 29 21:49:52 CST 2009

```Thank you I know that you are trying to let me know to match the vector element with the matrix.But it seems, the total number of rows (suppose m) has to satisfy    m = N * r,where N is the number of process and r is the local range.Am I right?

--- 09年11月30日，周一, Barry Smith <bsmith at mcs.anl.gov> 写道：

On Nov 29, 2009, at 9:21 PM, ming zhu wrote:

> OK.
> may I ask , I think Petsc matrix is row -based. That means,
> MatGetArray(Mat mat,PetscScalar *v[])
> v[0] is the pointer to the first row. v[0][0] is the value of mat[0][0].
> Am I right?

No. It is column based and there is just one array for the who thing so v[0:m-1] is the first column v[m:2m-1] is the next column etc
> And do you mean that, each time I got a vector, I create a sub matrix to store it.

No. Just create the full matrix and copy over each vector as you get it.

Barry

> Later on, the merging will be a problem.
>
> --- 09年11月30日，周一, Barry Smith <bsmith at mcs.anl.gov> 写道：
>
> 发件人: Barry Smith <bsmith at mcs.anl.gov>
> 主题: Re: How to fill a matrix with a vector parallelly
> 收件人: "PETSc users list" <petsc-users at mcs.anl.gov>
> 日期: 2009年11月30日,周一,上午11:12
>
>
> On Nov 29, 2009, at 9:09 PM, ming zhu wrote:
>
> > Two questions:
> > one,     how do I know which part belongs to the present processor?
>
>   Call VecGetOwnershipRange() to know what rows of the vector belong to the process. Then create the MPIDense matrix with the same number of local rows.
>
> > second, is there any examples for this kind of question?
> >
> > Thank you
> >
> > --- 09年11月30日，周一, Barry Smith <bsmith at mcs.anl.gov> 写道：
> >
> > 发件人: Barry Smith <bsmith at mcs.anl.gov>
> > 主题: Re: How to fill a matrix with a vector parallelly
> > 收件人: "PETSc users list" <petsc-users at mcs.anl.gov>
> > 日期: 2009年11月30日,周一,上午9:53
> >
> >
> >   Since evenvectors will (almost always) be dense vectors you will want to use a MPIDENSE matrix to store them. So create a MPIDENSE matrix with the same row layout as the eigenvector then on each process use VecGetArray() to access that processes part of the vector and MatGetArray() to access that part of the matrix and copy the values over from the vector array to the matrix array.
> >
> >    Barry
> >
> > On Nov 29, 2009, at 7:41 PM, ming zhu wrote:
> >
> > > HI
> > > I am trying to solve an eigenvalue problem. A = U * LAMBDA*U'.  While SLEPC proivdes a solver to get an eigenvector and the related eigenvalue, I have to fill the eigenvector to form the eigen matrix. However, there seems no such function to do this directly. I was trying to use VecGetValues and filled the matrix one by one. Howevevr, it can not do that in parallel.
> > >
> > >    Is there anyone can help me ?
> > >
> > > Thank you
> > >
> > >
> > > 好玩贺卡等你发，邮箱贺卡全新上线！
> >
> >
> > 好玩贺卡等你发，邮箱贺卡全新上线！
>
>
> 好玩贺卡等你发，邮箱贺卡全新上线！

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