Thanks, I've already tried that. The problem is, that the original date is not restored when I change the numeric back to date. I get a totally different date. Maybe it has something to do with the original date format. My data are directly imported from a SQL-database. The date column to which I want to apply the aggregate function has the two classes POSIXt and POSIXct. Changing the column to class "Date" before applying the aggregate function did not help. I still get a different date, when I transform it back to class "Date". I would be glad, if someone knew a more elegant way to extract the rows with minimum/earliest date per subject.
Erich -----Ursprüngliche Nachricht----- Von: jim holtman [mailto:[EMAIL PROTECTED] Gesendet: Montag, 8. September 2008 14:24 An: Erich Studerus Cc: r-help@r-project.org Betreff: Re: [R] How to preserve date format while aggregating Try changing the 'class' of the numeric result back to Date: > x <- as.Date('2008-09-08') > x [1] "2008-09-08" > y <- as.numeric(x) > y [1] 14130 > str(y) num 14130 > class(y) <- "Date" > y [1] "2008-09-08" > str(y) Class 'Date' num 14130 > On Mon, Sep 8, 2008 at 6:38 AM, Erich Studerus <[EMAIL PROTECTED]> 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. > -- Jim Holtman Cincinnati, OH +1 513 646 9390 What is the problem that you are trying to solve? ______________________________________________ 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.