# [R] obtaining first and last record for rows with same identifier

Sean Davis sdavis2 at mail.nih.gov
Tue May 24 20:37:31 CEST 2005

```If you have your data.frame ordered by the patid, you can use the
function rle in combination with cumsum.  As a vector example:

> a <- rep(c('a','b','c'),10)
> a
[1] "a" "b" "c" "a" "b" "c" "a" "b" "c" "a" "b" "c" "a" "b" "c" "a"
"b" "c" "a"
[20] "b" "c" "a" "b" "c" "a" "b" "c" "a" "b" "c"
> b <- a[order(a)]
> b
[1] "a" "a" "a" "a" "a" "a" "a" "a" "a" "a" "b" "b" "b" "b" "b" "b"
"b" "b" "b"
[20] "b" "c" "c" "c" "c" "c" "c" "c" "c" "c" "c"
> l <- rle(b)\$length
> cbind(l,cumsum(l),cumsum(l)-l+1)
l
[1,] 10 10  1
[2,] 10 20 11
[3,] 10 30 21

# use the line below to get the length of the block of the dataframe,
the start, and then end indices
> cbind(l,cumsum(l)-l+1,cumsum(l))
l
[1,] 10  1 10
[2,] 10 11 20
[3,] 10 21 30
>

Sean

On May 24, 2005, at 2:27 PM, sms13+ at pitt.edu wrote:

> I have a dataframe that contains fields such as patid, labdate,
> labvalue.
> The same patid may show up in multiple rows because of lab
> measurements on multiple days.  Is there a simple way to obtain just
> the first and last record for each patient, or do I need to write some
> code that performs that.
>
> Thanks,
> Steven
>
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```