Unexpected behaviour of the as.Date (was: Error as.Date on Invalid Dates)

Thu, Jan 22, 2009 11:16 AM

I believe the original thread was about whether the function returns NA or stops with an error when given an invalid date (such as Feb 29 in a non-leap year).  Your question was about how as.Date returned something different from what you expected.  Related, but different enough that it probably would have been better to start a new thread.

For your question, the help page for as.Date includes:

  "format: A character string.  The default is '"%Y-%m-%d"'.  For
          details see 'strftime'."

And 

"Character strings are processed as far as
     necessary for the format specified: any trailing characters are
     ignored."

I don't see anything in your examples that runs counter to the above.

Remember that computers do exactly what you tell them to do, not what you think that they should do.

Gregory (Greg) L. Snow Ph.D.
Statistical Data Center
Intermountain Healthcare
greg.snow at imail.org
801.408.8111


> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of Marie Sivertsen
> Sent: Thursday, January 22, 2009 11:55 AM
> To: Brian D Ripley; R-help at stat.math.ethz.ch
> Subject: [R] Unexpected behaviour of the as.Date (was: Error as.Date on
> Invalid Dates)
> 
> Dear Brian,
> 
> I dont understand what you mean.  The thread was about the as.Date
> which you
> suggested to be used instead of the as.date.  Following your advice I
> tried
> the as.Date and have questions about the observed behaviour, which was
> surprising to me.  Is this what you call hijacking?  Do you mean I
> ought
> start a new thread instead?  I thought my question were relevant to the
> threads' subject.  I am sorry if it were not.
> 
> So here is the questions once again:  why do the as.Date behave as in
> my
> examples below, is this intended?
> 
> 
> 
> On Thu, Jan 22, 2009 at 3:55 PM, Brian D Ripley
> <ripley at stats.ox.ac.uk>wrote:
> 
> > You've hijacked a thread here.
> >
> >
> > On Thu, 22 Jan 2009, Marie Sivertsen wrote:
> >
> >
> >> I am relatively new to R, so maybe I am miss something, but I now
> tried
> >> the a
> >> s.Date now and have problems understanding how it works (or don't
> work as
> >> it seem).
> >>
> >> Brian D Ripley wrote:
> >> > On Thu, 22 Jan 2009, Terry Therneau wrote:
> >> >>
> >> >> One idea is to use the as.date function, for the older (and less
> >> capable) 'date'
> >> >> class.  This is currently loaded by default with
> library(survival).  It
> >> re
> >> turns
> >>
> >> >> NA for an invalid date rather than dying.
> >> >
> >> > So does as.Date *if you specify the format* (as you have to with
> your
> >> as.da
> >> te:
> >> > it has a default one):
> >
> >
> 
> My examples:
> 
> 
> 
> >
> >> as.Date("2001/1/1")
> >> Works fine
> >> as.Date("1/1/2001")
> >> Prints "1-01-20" ???
> >> as.Date("13/1/2001")
> >> Prints "13-01-20" ???
> >>
> >> as.Date("1/13/2001")
> >> Prints error: not in standard unambigous format
> >> It seems that as if both "1/1/2001" and "13/1/2001" were considered
> by R
> >> to b
> >> e in a standard unambiguous format (or otherwise an error be
> reported?)
> >> and yet they
> >>
> >>
> >> are parsed incorrectly according to what one could think is obvious.
> It
> >> is a
> >> lso surprizing that not only "13/1/2001" but also "1/2/2001" and
> >> "2/1/2001" are successful but incorrect parsed as if they are
> unambiguous,
> >> and yet
> >> "13/1/2001" is ambiguous, though there is really just one way to
> parse it
> >> meaningfully.
> >> I think the strings that are incorrectly parsed should raise errors,
> and
> >> the last example should be succesful parsed.  What is the reason for
> the
> >> observed
> >> ?
> >>
> >>
> >>
> 
> 
> Mvh.
> Marie
> 
> 
> 
> >
> >>
> > --
> > Brian D. Ripley,                  ripley at stats.ox.ac.uk
> > Professor of Applied Statistics,
> http://www.stats.ox.ac.uk/~ripley/<http://www.stats.ox.ac.uk/%7Eripley/
> >
> > 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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> 
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Unexpected behaviour of the as.Date (was: Error as.Date on Invalid Dates)

Thread (8 messages)