How to select max time stamp for each group?

Question

How to select max timestamp for each group in R.

     df <- read.table(text = " ID   obj MR_time
        1599    1   20:05:22
        1599    1   20:06:38
        1599    1   20:07:22
        1599    2   20:08:38
        1599    2   20:09:28", header = TRUE)

dt <- data.table(df)

I required output to be

1599    1   20:07:22
1599    2   20:09:28

Answer 1

我们将其转换为DateTime ，然后按组（假设组为ID，obj）获取索引，以将.SD （ .SD子集）作为子集

dt[, .SD[which.max(as.POSIXct(MR_time, format = "%H:%M:%S"))], by = .(ID, obj)]