Create a sequential number (counter) for rows within each group of a dataframe [duplicate]

Answer 1

I think there's a canned command for this, but I can't remember it. So here's one way:

> test <- sample(letters[1:3],10,replace=TRUE)
> cumsum(duplicated(test))
 [1] 0 0 1 1 2 3 4 5 6 7
> cumsum(duplicated(test))+1
 [1] 1 1 2 2 3 4 5 6 7 8

This works because duplicated returns a logical vector. cumsum evalues numeric vectors, so the logical gets coerced to numeric.

You can store the result to your data.frame as a new column if you want:

dat$id <- cumsum(duplicated(test))+1