Create an new column in data frame : index in group (not unique between groups)

Question

I have a data frame with two columns: the first column contains the group to which each individual belongs, and the second the individual's ID. See below:

df <- data.frame( group=c('G1','G1','G1','G1','G2','G2','G2','G2'), 
      indiv=c('indiv1','indiv1','indiv2','indiv2','indiv3',
              'indiv3','indiv4','indiv4'))

   group   indiv
1     G1  indiv1
2     G1  indiv1
3     G1  indiv2
4     G1  indiv2
5     G2  indiv3
6     G2  indiv3
7     G2  indiv4
8     G2  indiv4

I would like to create a new column in my data frame (retaining the long format) with the index of each individual in the group, that is:

   group   indiv  Ineed
1     G1  indiv1      1
2     G1  indiv1      1
3     G1  indiv2      2
4     G1  indiv2      2
5     G2  indiv3      1
6     G2  indiv3      1
7     G2  indiv4      2
8     G2  indiv4      2

I have tried with the data.table .N or .GRP methods, without success (nice work on data.table by the way!).

Any help much appreciated!

Answer 1

You could use the new rleid function here (from the development version v >= 1.9.5)

setDT(df)[, Ineed := rleid(indiv), group][]
#    group  indiv Ineed
# 1:    G1 indiv1     1
# 2:    G1 indiv1     1
# 3:    G1 indiv2     2
# 4:    G1 indiv2     2
# 5:    G2 indiv3     1
# 6:    G2 indiv3     1
# 7:    G2 indiv4     2
# 8:    G2 indiv4     2

Or you could convert to factors (in order to create unique groups) and then convert them back to numeric (if you using the CRAN stable version v <= 1.9.4)

setDT(df)[, Ineed := as.numeric(factor(indiv)), group][]
#    group  indiv Ineed
# 1:    G1 indiv1     1
# 2:    G1 indiv1     1
# 3:    G1 indiv2     2
# 4:    G1 indiv2     2
# 5:    G2 indiv3     1
# 6:    G2 indiv3     1
# 7:    G2 indiv4     2
# 8:    G2 indiv4     2