[R] the first. from SAS in R

[Nutter, Benjamin]

My apologies for coming to the party so late.

I'm sure this question has been answered a couple of times.  The
attached function is one I pulled from the help archives, but I can't
seem to duplicate the search that led me to it.

In any case, I've attached the function I found, and an .Rd file I use
as part of a local package.  I've also attached a pair of accompanying
records to retrieve the last record and the nth record.  These have the
advantage of not requiring data frames to be sorted prior to
extraction--the function will sort them for you.

Benjamin  

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
On Behalf Of David Katz
Sent: Wednesday, November 24, 2010 10:17 AM
To: r-help at r-project.org
Subject: Re: [R] the first. from SAS in R


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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