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

[Therneau, Terry M., Ph.D.]

I prefer the duplicated() function, since the final code will be clear to a future reader. 
  (Particularly when I am that future reader).

last <- !duplicated(mydata$ID, fromLast=TRUE)  # point to the last ID for each subject
mydata$data3[last] <- NA

Terry T.

(I read the list once a day in digest form, so am always a late reply.)

On 9/10/21 5:00 AM, r-help-request using r-project.org wrote:
> 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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