Using "by"

["David Parkhurst" <parkhurs@indiana.edu>]

I have a data frame, df say, having four columns, the first of which is
a factor variable called, say, fv.

I want to us "by" to evaluate a function for each set of rows in df defined
by
the various values of fv.  The function needs to know what the current
value of fv is, at each call.  How do I arrange that?

Thanks.

Dave Parkhurst