[R] aggregating dates

[Gabor Grothendieck]

Paul Sorenson <Paul.Sorenson <at> vision-bio.com> writes:

: 
: I have a frame which contains 3 columns:
: 
: "date" "defectnum" "state"
: 
: And I want to get the most recent state change for a given defect number.  
date is POSIXct.
: 
: I have tried:
: 	aggregate(ev$date, by=list(ev$defectnum), max)
: 
: Which appears to be working except that the dates seem to come back as 
integers (presumably the internal
: representation of POSIXct).
: 
: When I execute max(ev$date) the result remains POSIXct.
: 
: I have been dredging through the help among DateTimeClasses and haven't 
found a function that converts
: these integers to some kind of date class.  Or a method for using aggregate 

RNews 4/1 has an article with this and other date conversions.  

: which doesn't perform the
: conversion in the first place.

You could sort ev by date and use aggregate on an index vector (untested):

ev <- ev[order(ev$date),]
ev[aggregate(1:nrow(ev), list(ev$defectnum), max),]