问题 I have a data frame like as shown below test_df <- data.frame("subject_id" = c(1, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3), "date_1" = c("01/01/2003", "12/31/2007", "12/30/2008", "12/31/2005", "01/01/2007", "01/01/2013", "12/31/2008", "03/04/2006", "12/31/2009", "01/01/2015", "01/01/2009")) What I would like to do is Arrange the dates in ascending order for each subject (sort asc within groups) Remove date records for each subject based on below criteria (year doesn't matter): 2a. remove only Dec 31st