How to preserve date format while aggregating

Here's how the dates look like after the sql-query:

Oav$Date[1:3]
[1] "1991-11-22 00:45:00 CET" "1991-12-13 00:01:00 CET" "1992-02-06 00:45:00
CET"

class(oav$Date[1:3])

[1] "POSIXt"  "POSIXct"

x<-as.numeric(oav$Date[1:3])
x

[1] 690767100 692578860 697333500

class(x)<-"Date"
x

[1] "3226-01-31" "8186-07-07" "1204-04-11"

I wanteded to apply the aggregate function in R instead of in the SQL-query,
because R is much more flexible and faster in manipulating large dataframes.

Erich



-----Urspr?ngliche Nachricht-----
Von: Dr Eberhard Lisse [mailto:el at lisse.NA]
Gesendet: Montag, 8. September 2008 15:43
An: Erich Studerus
Cc: r-help at r-project.org
Betreff: Re: [R] How to preserve date format while aggregating

Erich,

how does the data look, when it comes from SQL?

And why not extract the data with SQL directly, so you don't
have this issue in the first place?

el

on 9/8/08 3:15 PM Erich Studerus said the following:

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


-----Urspr?ngliche 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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