Re: [R] diff, POSIXct, POSIXlt, POSIXt

From: Gabor Grothendieck <ggrothendieck_at_gmail.com>
Date: Sun 23 Jul 2006 - 23:02:35 EST

Moving this to r-devel.

Looking at the diff.POSIXt code we see the problem is that it takes the length of the input using length which is wrong since in the case of POSIXlt the length is always 9 (or maybe length should be defined differently for POSIXlt?). Try this which gives the same problem:

   dts[-1] - dts[-length(dts)]

We get a more sensible answer if length is calculated correctly:

  dts[-1] - dts[-length(dts[[1]])]

On 7/23/06, Patrick Giraudoux <patrick.giraudoux@univ-fcomte.fr> wrote:
> > Try converting to POSIXct:
> That's what I did finally (see the previous e-mail).
>
> dts<-c("15/4/2003","15/7/2003","15/10/2003","15/04/2004","15/07/2004","15/10/2004","15/4/2005","15/07/2005","15/10/2005","15/4/2006")
>
> dts <- as.POSIXct(strptime(dts, "%d/%m/%Y"))
> diff(dts)
>
> Time differences of 91, 92, 183, 91, 92, 182, 91, 92, 182 days
>
> > What is the problem you are trying to solve?
> Actually, I don't understand why using diff() and POSIXct provides the
> expected result and not using POSIXlt. Both POSIXct and POSIXlt are of
> class POSIXt. The doc of diff() stresses that <'diff' is a generic
> function with a default method and ones for classes '"ts"', '"POSIXt"'
> and '"Date"'>. It does not mention differences between POSIXct and POSIXlt.
>
> Moreover, using diff() with POSIXlt has provided (wrong) numbers... and
> not an error. This may be difficult to detect sometimes along programme
> lines. Must one keep in mind that diff() is reliably applicable only on
> POSIXct? In this case, should not it bve mentionned in the documentation?
>
> All the best,
>
> Patrick
>
>
>
>
>
>
>
> jim holtman a écrit :
> > Try converting to POSIXct:
> >
> > > str(dts)
> > 'POSIXlt', format: chr [1:10] "2003-04-15" "2003-07-15" "2003-10-15"
> > "2004-04-15" "2004-07-15" "2004-10-15" "2005-04-15" ...
> > > dts
> > [1] "2003-04-15" "2003-07-15" "2003-10-15" "2004-04-15" "2004-07-15"
> > "2004-10-15" "2005-04-15" "2005-07-15"
> > [9] "2005-10-15" "2006-04-15"
> > > dts <- as.POSIXct(dts)
> > > dts
> > [1] "2003-04-15 EDT" "2003-07-15 EDT" "2003-10-15 EDT" "2004-04-15
> > EDT" "2004-07-15 EDT" "2004-10-15 EDT"
> > [7] "2005-04-15 EDT" "2005-07-15 EDT" "2005-10-15 EDT" "2006-04-15 EDT"
> > > diff(dts)
> > Time differences of 91, 92, 183, 91, 92, 182, 91, 92, 182 days
> > >
> >
> >
> >
> > On 7/23/06, *Patrick Giraudoux* <patrick.giraudoux@univ-fcomte.fr
> > <mailto:patrick.giraudoux@univ-fcomte.fr>> wrote:
> >
> > Dear Listers,
> >
> > I have encountered a strange problem using diff() and POSIXt:
> >
> > dts<-c("15/4/2003","15/7/2003","15/10/2003","15/04/2004","15/07/2004","15/10/2004","15/4/2005","15/07/2005","15/10/2005","15/4/2006")
> >
> > dts <- strptime(dts, "%d/%m/%Y")
> > class(dts)
> >
> > [1] "POSIXt" "POSIXlt"
> >
> > diff(dts)
> >
> > Time differences of 7862400, 7948800, 15811200, 7862400, 7948800,
> > 15724800, 7862400, 7948800, 0 secs
> >
> > In this case the result is not the one expected: expressed in seconds
> > and not in days, and the difference between the two last dates is
> > not 0.
> >
> > Now, if one use a vector of 9 dates only (whatever the date removed),
> > things come well:
> >
> > diff(dts[-1])
> >
> > Time differences of 92, 183, 91, 92, 182, 91, 92, 182 days
> >
> > Also if one contrains dts to POSIXct
> >
> > dts<-c("15/4/2003","15/7/2003","15/10/2003","15/04/2004","15/07/2004","15/10/2004","15/4/2005","15/07/2005","15/10/2005","15/4/2006")
> >
> > dts <- as.POSIXct(strptime(dts, "%d/%m/%Y"))
> > diff(dts)
> >
> > Time differences of 91, 92, 183, 91, 92, 182, 91, 92, 182 days
> >
> > Any rational in that?
> >
> > Patrick
> >
> > ______________________________________________
> > R-help@stat.math.ethz.ch <mailto:R-help@stat.math.ethz.ch> 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 you are trying to solve?
>
> [[alternative HTML version deleted]]
>
>
>
> ______________________________________________
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> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>
>
>



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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. Received on Mon Jul 24 00:18:40 2006

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