Unelegant decorate-count-undecorate on data.table cumulative sum

问题

I wish to keep a counter by "description". Can the following code be 1-liner:

dt[, dummy:=1]
dt[, count:=lapply(.SD,cumsum), by = "description", .SDcols=("dummy")]
dt[, dummy:=NULL]

回答1:

If I understand correctly, you just want:

dt[ , count := rowid(description)]

来源：https://stackoverflow.com/questions/34730544/unelegant-decorate-count-undecorate-on-data-table-cumulative-sum