[R] how to assign a group mean to individual cases?

[Casey Klofstad]

I need advice on how to create a variable that is the group mean of
another variable.

For example, I have a variable called x for which each row in the data
set has a value. I also have a nominal variable called g that
indicates which of 100 different groups each row belongs to.

So, I want to create a new variable called w, which is the group mean
of x for which ever group the row belongs to (as indicated by variable
g). Ideally, I'd also like to take out each row's value of x before
calculating the group mean assigned to that row.

I've already tried the aggregate command. That gives me the group
means, but does not allow me to assign them to each row in the data
set.

THANKS!
-- 
Casey A. Klofstad
University of Miami
Department of Political Science
Coral Gables, FL

klofstad at gmail.com
http://moya.bus.miami.edu/~cklofstad