obtaining first and last record for rows with same identifier

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

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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