R – data.table “key indices” or “group counter”

data.tabler

After creating a key on a data.table:

set.seed(12345)
DT <- data.table(x = sample(LETTERS[1:3], 10, replace = TRUE),
                 y = sample(LETTERS[1:3], 10, replace = TRUE))
setkey(DT, x, y)
DT
#       x y
#  [1,] A B
#  [2,] A B
#  [3,] B B
#  [4,] B B
#  [5,] C A
#  [6,] C A
#  [7,] C A
#  [8,] C A
#  [9,] C C
# [10,] C C

I would like to get an integer vector giving for each row the corresponding "key index". I hope the expected output (column i) below will help clarify what I mean:

#       x y i
#  [1,] A B 1
#  [2,] A B 1
#  [3,] B B 2
#  [4,] B B 2
#  [5,] C A 3
#  [6,] C A 3
#  [7,] C A 3
#  [8,] C A 3
#  [9,] C C 4
# [10,] C C 4

I thought about using something like cumsum(!duplicated(DT[, key(DT), with = FALSE])) but am hoping there is a better solution. I feel this vector could be part of the table's internal representation, and maybe there is a way to access it? Even if it is not the case, what would you suggest?

Best Answer

Update: From v1.8.3, you can simply use the inbuilt special .GRP:

DT[ , i := .GRP, by = key(DT)]

See history for older answers.

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