Observation number by group [duplicate]

Question

In R I have a data frame with observations described by several values one of which is a factor. I have sorted the dataset by this factor and would like to add a column in which I would get a number of observation on each level of the factor e.g.

factor   obsnum
a        1
a        2
a        3
b        1
b        2
b        3
b        4
c        1
c        2
...

In SAS I do it with something like:

data logs.full;
    set logs.full;
    count + 1;
    by cookie;
    if first.cookie then count = 1;
run;

How can I achieve that in R?

Thanks,

Answer 1

Use rle (run length encoding) and sequence:

x <- c("a", "a", "a", "b", "b", "b", "b", "c", "c")

data.frame(
    x=x,
    obsnum = sequence(rle(x)$lengths) 
)

  x obsnum
1 a      1
2 a      2
3 a      3
4 b      1
5 b      2
6 b      3
7 b      4
8 c      1
9 c      2