[R] First. Last. Data row selection

[wookie1976]

I am in the process of switching from SAS over to R.  I am working on very
large CSV datasets that contain vehicle information.  As I am processing the
data, I need to select the first (or sometimes the second) record (by date)
for any records that have the same license plate number.  In SAS, there is a
function called 'first.' that can be used on sorted datasets to pull out
those first entries for each occurrence of a particular variable  (in this
case the variable is 'license plate') found in the data.  I have spent some
time looking around and cannot seem to find an equivalent function in R. 
Can anyone recommend an efficient technique that would pull this off?  I
assume the database must first be sorted by vehicle plate and date, and then
apply the filter or function.  Any help would be greatly appreciated.  

Thanks, Joe
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