Question on Index Sets and VecScatters

[Randall Mackie]

I have a situation where I've put a model vector m(i,j,k) into a parallel
PETSc vector for use in my modeling code. However, I'm now adding a bit of code
where I want to do some calculations based on the 1D average of the model.
In other words, for each k, I want to average m(i,j), and so produce a new
model vector m_avg(k).

So, to do this, it would seem that I need to create a VecScatter that will,
for each layer, scatter all the m(i,j) into a 2D vector, then I can take
the average. It would seem that I need to create an Index Set to do this,
but I'm a bit confused as to how to go about it actually, since I've never
used Index Sets.

Can someone outline the basic steps given my description above?

Thanks, Randy

-- 
Randall Mackie
GSY-USA, Inc.
PMB# 643
2261 Market St.,
San Francisco, CA 94114-1600
Tel (415) 469-8649
Fax (415) 469-5044

California Registered Geophysicist
License No. GP 1034