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.
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