How to zero all values on a particular row?

[Ben Tay]

Hi,

I tried to do it in 2 ways:

A:

1.Do initial sweep:

do j=1,size_y

 do i=1,size_x

  matsetvalue ....

 end do

end do"

2.matssemblybegin/end

3 do a loop using matzerorows to zeros the particular rows

4. do loop and use matsetvalues to insert values into those rows which I had
zeroed previously

5 matssemblybegin/end

6 solve



B:

1. Same as A

2 use matsetvalues to put zeros into those location where I earlier insert
in the 1st full i,j sweep
3. insert correct value using matsetvalues

4. finally matassemblybegin/end - just once.

5. solve

l realised that B is about  3 times faster than A. May I know why this is
so? In A, I only call matssemblybegin/end once more. Is it so serious to the
performance?



Thanks

On 6/11/07, Barry Smith <bsmith at mcs.anl.gov> wrote:

>
>
> On Mon, 11 Jun 2007, Ben Tay wrote:
>
> > Thanks Barry,
> >
> > I managed to get it working. I realise that I had to call
> > matssemblybegin/end each time after I zero a particular row. I also
> tried
> > MatSetOption(mat,MAT_KEEP_ZEROED_ROWS) as what you 've suggested.
> >
> > So do I have to callMatSetOption(mat,MAT_KEEP_ZEROED_ROWS) everytime
> just
> > before I need to zero it or just once ie after the 1st
> matassemblybegin/end?
>
> Call it once when  the matrix is created is fine.
> >
> > Calling matassemblybegin/end each time before matzerorows gives me the
> > impression that it is very inefficient. Is that so?
>
> Yes, if you do this a bunch of imes
>
> >
> > In that case, I wonder if it is better for me to :
> >
> > 1 use matsetvalues to put zeros into those location where I earlier
> insert
> > in the 1st full i,j sweep
> > 2. insert correct value using matsetvalues
> > 3. finally matassemblybegin/end - just once.
>
> You can do this
>
> >
> > Lastly, is MatZeroRowsIS exactly the same as matzerorows, except that it
> is
> > meant for just 1 row instead of consecutive no. of rows?
>
> They are the same but one takes an array of integers while the other
> takes an IS. Neither require "consecutive no. of rows" you can zero
> whatever rows you want together.
>
>   Barry
>
> >
> > Thanks again
> >
> >
> > On 6/11/07, Barry Smith <bsmith at mcs.anl.gov> wrote:
> > >
> > >
> > >
> > > On Sun, 10 Jun 2007, Ben Tay wrote:
> > >
> > > > Hi,
> > > >
> > > > I need to insert values into a matrix and then solve this poisson
> eqn as
> > > > part of my cfd code. For efficiency sake, I  simply use
> > > >
> > > > "do j=1,size_y
> > > >
> > > > do i=1,size_x
> > > >
> > > >  matsetvalue ....
> > > >
> > > > end do
> > > >
> > > > end do"
> > > >
> > > > to insert values into all the cells
> > > >
> > > > Then for specific cells, I need to enter some other values. In other
> > > words,
> > > > for specific rows of the matrix, I need to zero the whole row and
> insert
> > > new
> > > > values. How can I do that?
> > > >
> > > > I tried to use MatZeroRows but it says it's not for unassembled
> matrix.
> > > But
> > > > I need to zero the particular row, insert values, and then finally
> > > assembly
> > > > it. Is there a command to do this?
> > >
> > >   matassemblybegin/end
> > >   matzerorows
> > >   matsetvalues, matsetvalues, matsetvalue
> > >   matssemblybegin/end
> > >
> > >   For improved efficiency you will want to call
> > >   MatSetOption(mat,MAT_KEEP_ZEROED_ROWS); before the matzerorows()
> > >
> > > >
> > > > Thanks
> > > >
> > >
> > >
> >
>
>
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