Within Group Index in R

There are many occasions in my research when I want to create a within group index for a data frame. For example, with demographic data for siblings one might want to create a birth order index.

The below illustrates a simple example of how one can create such an index in R.

# two families/groups 1 and 2
# with random ages
data = data.frame(group = c(rep(1,5),rep(2,5)), age = rpois(10,10))

# birth order
# use rank function with negative age for descending order
data$bo = unlist(by(data, data$group, 
                    function(x) rank(-x$age, ties.method = "first")))