[R] seq.POSIXt() with short time intervals

[ripley@stats.ox.ac.uk]

On Tue, 26 Mar 2002, Don MacQueen wrote:

> I was surprised when seq.POSIXt() returned a single value rather than
> a vector, from inputs that I thought were reasonable. Here's an
> example to illustrate:
>
> >  t0 <- ISOdatetime(2002,6,24,0,0,10)
>
> ## expected a sequence of 16 times 1 second apart
> >  seq.POSIXt(from=t0,to=t0+15,by='1 sec')
> [1] "2002-06-24 00:00:10 PDT"
>
> ## traces to this call
> >  seq.default(1024902010,1024902025,by=1)
> [1] 1024902010
> >
>
> ## which is due to the following expressions in seq.default()
>       del <- to - from
>       ## a few lines omitted
>       dd <- abs(del)/max(abs(to), abs(from))
>       if (dd < sqrt(.Machine$double.eps)) return(from)
>
> This looks like a trap to catch bad input, and return something
> reasonable (or at least not crash).
>
> It's a given that seq.default() pre-dates seq.POSIXt() by many years,
> and the test for relative range undoubtedly has good reasons, so it's
> probably not a good idea to change seq.default().

Oh, I think it needs to be changed.  That is far too generous a test for
numerical inaccuracies, by say 5 orders of magnitude.  It is also
undocumented, so I'd post a bug report on seq.default.

> There's nothing inherently wrong with such a time sequence, and in
> fact, if t0 = ISOdatetime(1970,6,24,0,0,10) instead, seq.POSIXt()
> does return a vector.
>
> I'd like to see seq.POSIXt() return a full sequence in such cases,
> regardless of the starting point, so what I'll do is think about it,
> and see if I can't come up with a good way to modify seq.POSIXt() to
> do that. In the meantime, I'm interested in any comments or
> suggestions anyone might have.

Easy: subtract the starting value before the call and add it back on
afterwards.

> Turns out that with the base time, t0, as above, t0+15 is right at
> the threshold of the relative range test:
> ## compare with t0=t0+16
> ## this call returns what was expected
> >  seq.POSIXt(from=t0, to=t0+16, by='1 sec')
>   [1] "2002-06-24 00:00:10 PDT" "2002-06-24 00:00:11 PDT" "2002-06-24
> 00:00:12 PDT" "2002-06-24 00:00:13 PDT"
>   [5] "2002-06-24 00:00:14 PDT" "2002-06-24 00:00:15 PDT" "2002-06-24
> 00:00:16 PDT" "2002-06-24 00:00:17 PDT"
>   [9] "2002-06-24 00:00:18 PDT" "2002-06-24 00:00:19 PDT" "2002-06-24
> 00:00:20 PDT" "2002-06-24 00:00:21 PDT"
> [13] "2002-06-24 00:00:22 PDT" "2002-06-24 00:00:23 PDT" "2002-06-24
> 00:00:24 PDT" "2002-06-24 00:00:25 PDT"
> [17] "2002-06-24 00:00:26 PDT"
>
>
> I've found this on two platforms:
> >  version
>           _
> platform sparc-sun-solaris2.7
> arch     sparc
> os       solaris2.7
> system   sparc, solaris2.7
> status
> major    1
> minor    4.1
> year     2002
> month    01
> day      30
> language R
>
> platform powerpc-apple-darwin5.3
> arch     powerpc
> os       darwin5.3
> system   powerpc, darwin5.3
> status
> major    1
> minor    4.1     (also 4.0)
> year     2001
> month    12
> day      19
> language R
>
> --
> --------------------------------------
> Don MacQueen
> Environmental Protection Department
> Lawrence Livermore National Laboratory
> Livermore, CA, USA
> --------------------------------------
> -.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-
> r-help mailing list -- Read http://www.ci.tuwien.ac.at/~hornik/R/R-FAQ.html
> Send "info", "help", or "[un]subscribe"
> (in the "body", not the subject !)  To: r-help-request at stat.math.ethz.ch
> _._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._
>

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-
r-help mailing list -- Read http://www.ci.tuwien.ac.at/~hornik/R/R-FAQ.html
Send "info", "help", or "[un]subscribe"
(in the "body", not the subject !)  To: r-help-request at stat.math.ethz.ch
_._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._