[R] diff, POSIXct, POSIXlt, POSIXt

[Gabor Grothendieck]

Just one more comment. It is possible to define length.POSIXlt yourself
in which case diff works with POSIXlt objects.

> length.POSIXlt <- function(x) length(x[[1]])
> diff(dts)
Time differences of  91,  92, 183,  91,  92, 182,  91,  92, 182 days


On 7/23/06, Gabor Grothendieck <ggrothendieck at gmail.com> wrote:
> 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 at 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 at univ-fcomte.fr
> > > <mailto:patrick.giraudoux at 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 at stat.math.ethz.ch <mailto:R-help at 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]]
> >
> >
> >
> > ______________________________________________
> > R-help at 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.
> >
> >
> >
>