>>>>> Andy Teucher
>>>>> on Fri, 11 Aug 2023 16:07:36 -0700 writes:
> I understand that `as.character.POSIXt()` had an overhaul in R 4.3
(https://github.com/wch/r-source/commit/f6fd993f8a2f799a56dbecbd8238f155191fc31b),
and I have come across a new behaviour and I wonder if it is unintended?
Well, as the NEWS entry says
(partly visible in the url above -- which only shows one part of
the several changes for R 4.3) :
• as.character(<POSIXt>) now behaves more in line with the methods
for atomic vectors such as numbers, and is no longer influenced
by options(). Ditto for as.character(<Date>). The
as.character() method gets arguments digits and OutDec with
defaults _not_ depending on options(). Use of as.character(*,
format = .) now warns.
It was "inconsistent" to have as.character(.) basically use format(.) for
these datatime objects.
as.character(x) for basic R types such as numbers, strings, logicals,...
fulfills the important property
as.character(x)[j] === as.character(x[j])
whereas that is very much different for format() where indeed,
the formatting of x[1] may quite a bit depend on the other
x[j]'s values:
> as.character(c(1, pi, pi/2^20))
[1] "1" "3.14159265358979" "2.99605622633914e-06"
> format(c(1, pi, pi/2^20))
[1] "1.000000e+00" "3.141593e+00" "2.996056e-06"
> format(c(1, pi))
[1] "1.000000" "3.141593"
> format(c(1, 10))
[1] " 1" "10"
>
> When calling `as.character.POSIXt()` on a vector that contains elements
where the time component is midnight (00:00:00), it drops the time component of
that element in the resulting character vector. Previously the time component
was retained:
> In R 4.2.3:
> ```
> R.version$version.string
> #> [1] "R version 4.2.3 (2023-03-15)"
> (t <- as.POSIXct(c("1975-01-01 00:00:00", "1975-01-01 15:27:00")))
> #> [1] "1975-01-01 00:00:00 PST" "1975-01-01 15:27:00 PST”
> (tc <- as.character(t))
> #> [1] "1975-01-01 00:00:00" "1975-01-01 15:27:00”
> ```
> In R 4.3.1:
> ```
> R.version$version.string
> #> [1] "R version 4.3.1 (2023-06-16)"
> (t <- as.POSIXct(c("1975-01-01 00:00:00", "1975-01-01 15:27:00")))
> #> [1] "1975-01-01 00:00:00 PST" "1975-01-01 15:27:00 PST”
> (tc <- as.character(t))
> #> [1] "1975-01-01" "1975-01-01 15:27:00”
> ```
You should have used format() here or at least should do so now.
> This has consequences when round-tripping from POSIXt ->
> character -> POSIXt,
Well, I'd argue that such a "round trip" is not a "good idea"
anyway, as there are quite a few platform (local timezone for
one) issues, and precision is lost, notably for POSIXlt which
may be more precise than you typically get, etc.
> since `as.POSIXct.character()` drops the time component from the entire
vector if any element does not have a time component:
Well, there *is* no as.POSIXct.character() {but we understand what you mean}:
If you look at the help page you'd see that there's as.POSIXlt.character()
{which is called from as.POSIXct.default()}
with a 3rd argument 'format' and a 4th argument 'tryFormats'
{and a lot more information -- the whole topic is far from trivial}.
Now, indirectly you would want R to be "smart", i.e. the
as.POSIXlt.character() method "guess better" about what the
user wants. ...
... and I agree that is not an unreasonable expectation, e.g.,
for your example of wanting
c("1975-01-01", "1975-01-01 15:27:00")
to "work".
as.POSIXlt.character() is well documented to be trying all of
the `tryFormats` in order, until it finds one that works for all
vector components (or fail / use NA if none works);
and here it's only a format which drops the time that works for
all (i.e. both, in the example).
{ Even though its behavior is well documented,
one could even argue that by default you'd want a warning in
such a case where "so much" is lost.
I think however that introducing such a warning may trip too
much current code relying .. also, the extra *checking* maybe
somewhat costly .. (?) .... anyway that's an interesting side topic
}
Instead what you want here is for each string (element of the
character vector) to try the `tryFormats and using the best
available *individually* {smart R users ==> "think lapply(.)"} :
Currently, this would be "something like" unlist(lapply(x, as.POSIXlt))
well, and then you need to jump a hoop additionally.
If you want POSIXct, like this :
.POSIXct(unlist(lapply( * , as.POSIXct))))
For your example
ch <- c("1975-01-01", "1975-01-01 15:27:00")
> str(.POSIXct(unlist(lapply(ch, as.POSIXct))))
POSIXct[1:2], format: "1975-01-01 00:00:00" "1975-01-01 15:27:00"
---
After all that, yes, I agree that we should consider making
this much easier. E.g., by adding an optional argument to
as.POSIXlt.character() say, `each` with default FALSE such
that as.POSIXlt(*, each=TRUE)
{and also as.POSIXct(*, each=TRUE) } would follow the above
strategy.
?
Martin
--
Martin Maechler
ETH Zurich and R Core tam
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