As requested in the last line to every message to r-help a cutdown version of the data should be posted with the question.
Suppose such a cut down version is this: DF <- data.frame(Date = ISOdatetime(2008, 1, c(1, 2, 5, 3, 4), 0, 0, 0), Subject = c(1, 1, 2, 2, 2)) # Then sort the data and take first one in each group: DFsort <- DF[order(DF$Subject, DF$Date), ] DFsort[!duplicated(DFsort$Subject), ] If its already sorted then omit first line and use DF in place of DFsort in second line. On Mon, Sep 8, 2008 at 9:15 AM, Erich Studerus <[EMAIL PROTECTED]> wrote: > 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. > ______________________________________________ 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.