[R] how to find "first" or "last" record after sort in R

[Kai Yang]

Hello List,
Please look at the sample data frame below:

ID         date1              date2             date3
1    2015-10-08    2015-12-17    2015-07-23

2    2016-01-16    NA                 2015-10-08
3    2016-08-01    NA                 2017-01-10
3    2017-01-10    NA                 2016-01-16
4    2016-01-19    2016-02-24   2016-08-01
5    2016-03-01    2016-03-10   2016-01-19
This data frame was sorted by ID and date1. I need to set the column date3 as missing for the "last" record for each ID. In the sample data set, the ID 1, 2, 4 and 5 has one row only, so they can be consider as first and last records. the data3 can be set as missing. But the ID 3 has 2 rows. Since I sorted the data by ID and date1, the ID=3 and date1=2017-01-10 should be the last record only. I need to set date3=NA for this row only.

the question is, how can I identify the "last" record and set it as NA in date3 column.
Thank you,
Kai
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