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

Avi Gross @v|gro@@ @end|ng |rom ver|zon@net
Fri Sep 10 20:45:32 CEST 2021

Excellent function to use, Terry.


I note when I used it on a vector (in this case the  first column of a data.frame, it accepted last=TRUE as well a fromlast=TRUE, which I did not see documented. Used on a data.frame, that change fails as function duplicated.data.frame only passes along the fromlast keyword value. 😉


When given a problem, we sometimes use a hammer when existing functions are already there to help.


-----Original Message-----
From: R-help <r-help-bounces using r-project.org> On Behalf Of Therneau, Terry M., Ph.D. via R-help
Sent: Friday, September 10, 2021 8:14 AM
To: yangkai9999 using yahoo.com; R-help <r-help using R-project.org>
Subject: Re: [R] how to find "first" or "last" record after sort in R


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,  <mailto:r-help-request using r-project.org> 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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