[R] How to preserve date format while aggregating
Prof Brian Ripley
ripley at stats.ox.ac.uk
Tue Sep 9 08:57:43 CEST 2008
This is completely wrong: min _is_ defined for date-times:
 "1972-07-01 01:00:00 BST"
Please do study the posting guide and do your homework before posting: you
seem unaware of what the POSIXct class is, so ?DateTimeClasses is one
place you need to start. And
 Summary.data.frame Summary.Date Summary.difftime
 Summary.factor Summary.numeric_version Summary.POSIXct
so ?Summary is another.
On Mon, 8 Sep 2008, Adam D. I. Kramer 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.
> On Mon, 8 Sep 2008, Erich Studerus wrote:
>> 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.
>> Unfortunately, the format of the Date-column changes to numeric, when I'm
>> applying this function. How can I preserve the date format?
>> R-help at r-project.org mailing list
>> PLEASE do read the posting guide
>> and provide commented, minimal, self-contained, reproducible code.
> R-help at r-project.org mailing list
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
Brian D. Ripley, ripley at stats.ox.ac.uk
Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
University of Oxford, Tel: +44 1865 272861 (self)
1 South Parks Road, +44 1865 272866 (PA)
Oxford OX1 3TG, UK Fax: +44 1865 272595
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