min does work for POSIXct and Date too: > ct <- ISOdatetime(2008, 1, 1:10, 0, 0, 0)
> min(ct) [1] "2008-01-01 EST" > min(as.Date(ct)) [1] "2008-01-01" On Mon, Sep 8, 2008 at 6:57 PM, Adam D. I. Kramer <[EMAIL PROTECTED]> wrote: > Hi Erich, > > Since min() is defined for numbers and not dates, the problem is in the > min() function. min() is converting from date format to number format. > > Your best bet is to make this conversion explicit...such that it is > reversable. So, convert the date into UTC, then UTC to seconds since epoch, > then take the minimum, then convert back to UTC time. > > This sounds like a pain...but that's basically what a version of min() > designed to work with dates would do. The reason this is a pain is basically > due to timezones: > > Consider a comparison between x = 3:54 PM September 8 in California (right > now where I am) and y = 12:54 AM September 9 in Zurich (right now where you > are). Is it earlier here than there? Yes, because it's Sept 8 to your Sept > 9. Is it earlier there than here? Yes, because your day started 56 minutes > ago, mine over 15 hours ago. Is it the same time here than there? Yes, > because our UTC times are equal. > > So it's not clear what min should return, so min is not defined for dates. > However, min is defined for numbers, and dates can be converted to > numbers...but what those numbers actually mean is not necessarily clear. > > --Adam > > On Mon, 8 Sep 2008, Erich Studerus wrote: > >> Hi >> >> I have a dataframe in which some subjects appear in more than one row. I >> want to extract the subject-rows which have the minimum date per subject. >> I >> tried the following aggregate function. >> >> attach(dataframe.xy) >> >> aggregate(Date,list(SubjectID),min) >> >> Unfortunately, the format of the Date-column changes to numeric, when I'm >> applying this function. How can I preserve the date format? >> >> Thanks >> >> Erich >> >> ______________________________________________ >> R-help@r-project.org mailing list >> https://stat.ethz.ch/mailman/listinfo/r-help >> PLEASE do read the posting guide >> http://www.R-project.org/posting-guide.html >> and provide commented, minimal, self-contained, reproducible code. >> > > ______________________________________________ > R-help@r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. > ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.