[R] the first and last case

[Mauricio Cardeal]

Hi all

Sometime ago I asked for a solution about how to aggregate data and the 
help was wonderful. Now, I´d like to know how to extract for each 
individual case below the first and the last observation to obtain this:

ind  y
1    8
1    9
2    7
2   11
3    9
3   10
4   8
4   5

# Below the example:

ind <- c(1,1,1,2,2,3,3,3,4,4,4,4)
y <- c(8,10,9,7,11,9,9,10,8,7,6,5)
dat <- as.data.frame(cbind(ind,y))
dat
attach(dat)
mean.ind <- aggregate(dat$y, by=list(dat$ind), mean)
mean.ind

Thanks
Mauricio