[R] the first. from SAS in R

[David Katz]

Often the purpose of first/last in sas is to facilitate grouping of
observations in a sequential algorithm. This purpose is better served in R
by using vectorized methods like those in package plyr.

Also, note that first/last has different meanings in the context of "by x;"
versus "by x notsorted;". R "duplicated" does not address the latter, which
splits noncontiguous records with equal x.

Regards,
David
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