[EMAIL PROTECTED] wrote:
> Full_Name: Rene Locher
> Version: 2.7.2 Patched (2008-09-12 r46541)
> OS: XP
> Submission from: (NULL) (160.85.231.40)
>
>
> dat <- data.frame(event=factor(c("A","A","B")),
> time=as.POSIXct(c("2008-01-10","2008-01-01","2008-01-04")))
>
> min(dat$time)
> ## "2008-01-01 CET"
> ## as expected
>
> aggregate(dat$time,by=list(event=dat$event),min)
> ## results in
> ## event x
> ## 1 A 1199142000
> ## 2 B 1199401200
>
> ## I expected:
> ## event x
> ## 1 A "2008-01-01 CET"
> ## 2 B "2008-01-04 CET"
>
This is as documented, possibly annoying, but not a bug.
aggregate() is documented to build on tapply() for which the discarding
of class is documented on ?tapply. The root cause is unlist():
> tapply(dat[["time"]],dat$event,min,simplify=FALSE)
$A
[1] "2008-01-01 CET"
$B
[1] "2008-01-04 CET"
> unlist(tapply(dat[["time"]],dat$event,min,simplify=FALSE))
A B
1199142000 1199401200
and a partial rationale is that unlist() wouldn't know what to do if the
arguments had different classes.
The workaround is, of course, just to stick the class back on.
--
O__ ---- Peter Dalgaard Øster Farimagsgade 5, Entr.B
c/ /'_ --- Dept. of Biostatistics PO Box 2099, 1014 Cph. K
(*) \(*) -- University of Copenhagen Denmark Ph: (+45) 35327918
~~~~~~~~~~ - ([EMAIL PROTECTED]) FAX: (+45) 35327907
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