[petsc-users] adding mat rows

[Carles Bona]

I have a system of equations:

a11*x1 + a12*x2 + ... + a1n*xn = b1
a21*x1 + a22*x2 + ... + a2n*xn = b2
.
.
.
an1*x1 + an2*x2 + ... + ann*xn = bn

Let's say I want to modify the first equation, but I don't want to lose the
information there, so I will add first the first and second equations,
store the result in the second equation and then modify the first equation.
Like this:

x1 - x2 = 0
(a11+a21)*x1 + (a12+a22)*x2 + ... + (a1n+a2n)*xn = b1+b2
.
.
.
an1*x1 + an2*x2 + ... + ann*xn = bn

And I want to do this for a few rows of my matrix. I have already built the
nonzero structure so that these additions can be done without hitting a non
preallocated location (for example a case where a21 was never allocated
because it was meant to be zero always and now, with a11 present, it's
different than zero).

Any hints?

Thanks!

Carles
El dia 19/11/2015 19:55, "Barry Smith" <bsmith at mcs.anl.gov> va escriure:

>
> > On Nov 19, 2015, at 11:25 AM, Carles Bona <carlesbona at gmail.com> wrote:
> >
> > Dear all,
> >
> > I would like to add some of my equations before I modify them.
>
>    Please explain what you mean by this. Algebraically exactly what do you
> want to do?
>
>
> > I haven't found any high level function that would allow me to add rows
> of a matrix (I am working with a parallel BAIJ). Is there any nice way of
> doing this?
> >
> > I have tried with MatGetRow/MatRestoreRow, but I am struggling a bit to
> retain the cols and vals, as only one processor can call MatGetRow but if
> only that processor tries to allocate memory then one gets a segmentation
> fault. I guess I should allocate enough memory on all processors...
> >
> > If I refrain from storing the cols and vals I need to call MatSetValues
> before returning the pointer, with a subsequent call to assemblybegin/end
> for each row, which slows down the code.
> >
> > The other option would be to forget about these row additions after the
> matrix has been filled and try to fill it while taking into account these
> row additions at the same time. I guess I need to be constantly checking
> for indices then.
> >
> > So, which option (not necessarily mentioned here) would you reccomend?
> >
> > Thanks a lot,
> >
> > Carles
>
>
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