[Rd] Floating Point with POSIXct

[John Muschelli]

That’s a good point. I think that’s fair and why rounding may not be the
appropriate default.  Oddly enough, I think 1:59:60 may be more appropriate
though wrong.  The way the seconds are separated in POSIXlt however, I
don’t think that would ever happen, but the big downside would be if that
would round to 1:59:59.00

On Thu, Mar 3, 2022 at 2:38 PM Duncan Murdoch <murdoch.duncan using gmail.com>
wrote:

> On 03/03/2022 11:52 a.m., Martin Maechler wrote:
> >>>>>> John Muschelli
> >>>>>>      on Thu, 3 Mar 2022 11:04:05 -0500 writes:
> >>>>>> John Muschelli
> >>>>>>      on Thu, 3 Mar 2022 11:04:05 -0500 writes:
> >
> >      > I see in ?POSIXct and I'm trying to understand the note:
> >      >> Classes "POSIXct" and "POSIXlt" are able to express fractions of
> a second. (Conversion of fractions between the two forms may not be exact,
> but will have better than microsecond accuracy.)
> >
> >      > Mainly, I'm trying to understand printing of POSIXct with
> fractional
> >      > seconds.  I see print.POSIXct calls format.POSIXct and eventually
> >      > calls format.POSIXlt, which then takes into account `digits.secs`
> for
> >      > printing. The format uses %OS3, which strptime indicates (*
> added):
> >
> >      >> Specific to R is %OSn, which for output gives the seconds
> *truncated* to 0 <= n <= 6 decimal places (and if %OS is not followed by a
> digit, it uses the setting of getOption("digits.secs"), or if that is
> unset, n = 0).
> >
> >      > So I'm seeing it truncates the seconds to 3 digits, so I think
> that is
> >      > why the below is printing 0.024.
> >
> >      > I think this is especially relevant even if you set
> >      > `options(digits.secs = 6)`, then the code in
> >      > format.POSIXlt would still return np=3 as the following condition
> >      > would break at i = 3
> >
> >      > for (i in seq_len(np) - 1L)
> >      >   if (all(abs(secs - round(secs, > i)) < 1e-06)) {
> >      >     np <- i
> >      >     break
> >      > }
> >
> >      > as sub_seconds - round(sub_seconds,3) < 1e-06.   This seems to be
> >      > expected behavior given the docs, but would any consider this a
> bug?
> >
> >
> >      > Example:
> >
> >      > options(digits.secs = 4)
> >      > x = structure(947016000.025, class = c("POSIXct", "POSIXt"),
> tzone = "UTC")
> >
> > I think you've fallen into the R FAQ 7.31 trap :
> >
> >> ct <- 947016000.025
> >> ct %% 1
> > [1] 0.02499998
> >>
> >
> > Of course, the issue may still be somewhat interesting, ...
> >
> > Yes, POSIXct is of limited precision and I think the help page
> > you mentioned did document that that's one reason for using
> > POSIXlt instead, as there, sub second accuracy can be much better.
> >
> > But FAQ 7.31 and the fact that all numbers are base 2 and in
> > base 2,  no decimal   <n>.025   can be represented in full accuracy.
> >
> > Also, as you've noticed the R POSIX[cl]t  code just truncates,
> > i.e. rounds towards 0 unconditionally, and I tend to agree that it
> > should rather round than truncate.
>
> If you print the hour and minute at 01:59:59, you get 1 and 59, not 2
> and 0.  That may be the motivation for doing the same for fractional
> seconds.  Should 1:59:59.9 really print as 2:00:00?
>
> Duncan Murdoch
> >
> > But we should carefully separate the issues here, from the
> > underlying inherent FAQ 7.31 truth that most decimal numbers in
> > a computer are not quite what they look like ...
> >
> > Martin Maechler
> > ETH Zurich and  R Core Team (also author of the CRAN package 'round')
> >
> >
> >      > summary(x, digits = 20)
> >      > #>                      Min.                   1st Qu.
>         Median
> >      > #> "2000-01-04 20:00:00.024" "2000-01-04 20:00:00.024"
> "2000-01-04 20:00:00.024"
> >      > #>                      Mean                   3rd Qu.
>           Max.
> >      > #> "2000-01-04 20:00:00.024" "2000-01-04 20:00:00.024"
> "2000-01-04 20:00:00.024"
> >      > x
> >      > #> [1] "2000-01-04 20:00:00.024 UTC"
> >      > format.POSIXct(x, format = "%Y-%m-%d %H:%M:%OS3")
> >      > #> [1] "2000-01-04 20:00:00.024"
> >      > format.POSIXct(x, format = "%Y-%m-%d %H:%M:%OS4")
> >      > #> [1] "2000-01-04 20:00:00.0249"
> >      > sub_seconds = as.numeric(x) %% 1
> >      > sub_seconds
> >      > #> [1] 0.02499998
> >      > round(sub_seconds, 3)
> >      > #> [1] 0.025
> >
> >      > rounded = as.POSIXct(
> >      > floor(as.numeric(x)) +
> >      > round(as.numeric(x) %% 1, 3),
> >      > origin = "1970-01-01")
> >      > rounded
> >      > #> [1] "2000-01-04 20:00:00.024 UTC"
> >      > as.numeric(rounded) %% 1
> >      > #> [1] 0.02499998
> >
> >      > R.version
> >      > _
> >      > platform       x86_64-pc-linux-gnu
> >      > arch           x86_64
> >      > os             linux-gnu
> >      > system         x86_64, linux-gnu
> >      > status
> >      > major          4
> >      > minor          1.2
> >      > year           2021
> >      > month          11
> >      > day            01
> >      > svn rev        81115
> >      > language       R
> >      > version.string R version 4.1.2 (2021-11-01)
> >      > nickname       Bird Hippie
> >
> >
> >
> >      > Best,
> >      > John
> >
> >      > ______________________________________________
> >      > R-devel using r-project.org mailing list
> >      > https://stat.ethz.ch/mailman/listinfo/r-devel
> >
> > ______________________________________________
> > R-devel using r-project.org mailing list
> > https://stat.ethz.ch/mailman/listinfo/r-devel
>
> --
Best,
John

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