numbering by groups [duplicate]

Answer 1

> dat$number.in.group <- ave(dat$ID,dat$ID, FUN=seq_along)
> dat
   ID Shoot hit number.in.group
1   1    10   2               1
2   1     9   3               2
3   1     8   1               3
4   2    10   8               1
5   2     8   8               2
6   2    11  10               3
7   2     7   2               4
8   3     9   2               1
9   4     6   6               1
10  4     6   5               2

Answer 2

If you want a one-liner, something like

df$number.in.group = unlist(lapply(table(df$ID),seq.int))