[Rd] seq improperly increments dates (PR#9120)

[Gabor Grothendieck]

That's, in fact, the way seq.dates works in the chron package:

library(chron)
x <- chron("01/31/2006")
seq(x, by = "month", length = 2)   #  01/31/06 02/28/06

See the help desk article in R News 4/1 for more about the main
date classes.

On 8/3/06, Ponzio, Stephen [CIB-LAVA] <stephen.ponzio at citigroup.com> wrote:
> You're right, it's tricky.
>
> I guess I would expect Jan. 30 + 1 month = Feb. 28.
>
> Of couse, then  Jan. 30 + 1 month = Jan. 28 + 1 month;
> I understand.
>
> Being that "1 month" is imprecise in terms of number of days,
> this anomaly is preferable to skipping months (Jan. 30 + 1 month = March 2),
> in my opinion.
>
> That is what I expected, anyway.
>
> Thanks,
> Stephen
>
>
>
> -----Original Message-----
> From: Duncan Murdoch [mailto:murdoch at stats.uwo.ca]
> Sent: Thursday, August 03, 2006 11:24 AM
> To: Ponzio, Stephen [CIB-LAVA]
> Cc: r-devel at stat.math.ethz.ch; R-bugs at biostat.ku.dk
> Subject: Re: [Rd] seq improperly increments dates (PR#9120)
>
>
> On 8/3/2006 10:26 AM, stephen.ponzio at citigroup.com wrote:
> > Full_Name: Stephen Ponzio
> > Version: 2.3.1
> > OS: Windows
> > Submission from: (NULL) (199.67.138.42)
> >
> >
> > With the option by="1 month" and a date that is the 31st,
> > the function seq doesn't give the last day of successive months,
> > as I would expect it should:
>
> Why would you expect that?  I'd expect it to give the day 30 days after
> the 1st, and that appears to be what it does.  This doesn't look like a
> bug to me.
>
> I don't understand what logic you would like it to use.  If I ask to
> increment Jan 30 in a non-leap-year by one month, should I get March 2
> (29 days after the 1st of January, as now), February 27 (the day before
> the end of the month), or February 28 (the closest you can get in February)?
>
> Whatever you choose, you'll get anomalies.  Currently it skips months in
> sequences like you show below.  If you try for the same distance from
> the end of the month each time, or truncate to the end of a short month,
> you lose important arithmetic properties:  x - y is not equal to
> (x+1month) - (y+1month).
>
> Duncan Murdoch
>
> >
> >> Sys.Date()
> > [1] "2006-08-03"
> >
> >> Sys.Date()-3
> > [1] "2006-07-31"
> >
> > # WORKS OK on Aug. 1:
> >> seq(Sys.Date()-2, len=5, by="1 month")
> > [1] "2006-08-01" "2006-09-01" "2006-10-01" "2006-11-01" "2006-12-01"
> >
> > # DOESN'T WORK on July 31 (list has no date in Sept):
> >> seq(Sys.Date()-3, len=5, by="1 month")
> > [1] "2006-07-31" "2006-08-31" "2006-10-01" "2006-10-31" "2006-12-01"
> >
> > ______________________________________________
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>
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