Unelegant decorate-count-undecorate on data.table cumulative sum
Question
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]
1 answers
solution1
4 ACCPTED 2016-01-11 21:08:03
如果我理解正确，则只需要：
dt[ , count := rowid(description)]
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