[Rd] Floating Point with POSIXct

[Duncan Murdoch]

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
> 
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> 
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