aggregate() does not return POSIXct object correctly (PR#12887)

rene.locher at zhaw.ch 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.