# [R] diff, POSIXct, POSIXlt, POSIXt

Gabor Grothendieck ggrothendieck at gmail.com
Sun Jul 23 15:02:35 CEST 2006

```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
> >     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?
>
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>
>
>
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