adding a record counter to a data.table

Question

I have an existing data.table that I am wanting to add an 'identity' column to.

I know how to add a column e.g:

DT$newCol <- x

However I am struggling to make the new value an auto-incrementing counter from 1 to n.

I would have asked as a comment to this question (data.table "key indices" or "group counter") as it is related, however I don't have enough reputation to comment.

Thanks in advance.

On testing it is found that .I produces serial numbers while .N produces nrow in each row of new column. '.I' is the solution. — rnso, Sep 25 '14 at 07:38
Even if not asked, this can become handy in the future also: numbering rows by group `library(data.table) agroup <- c(rep("A", times = 2), rep("B", times = 7)) dt <- data.table(agroup) dt[, anothercounter := seq_along(.I), by = "agroup"] dt` — Tom Martens, Sep 25 '14 at 09:12
Thanks so much to everyone for your help. As I all the answers have come in commments I can't select an answer to credit. But thanks all. — Dan, Sep 25 '14 at 22:28

score 1 · Accepted Answer · answered Sep 25 '14 at 22:33

1

In the spirit of completeness in case someone stumbles across this.

DT[, newcol := 1:.N]

is how I solved the problem. Thanks to go @thelatemail and @Simon

answered Sep 25 '14 at 22:33

Dan

1 Answers1