[R] How to preserve date format while aggregating

[Erich Studerus]

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:jholtman at gmail.com] 
Gesendet: Montag, 8. September 2008 14:24
An: Erich Studerus
Cc: r-help at 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
<erich.studerus at bli.uzh.ch> 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
>
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-- 
Jim Holtman
Cincinnati, OH
+1 513 646 9390

What is the problem that you are trying to solve?