[Rd] Wrong length of POSIXt vectors (PR#10507)
Gabor Grothendieck
ggrothendieck at gmail.com
Sun Dec 16 01:20:07 CET 2007
If it were simply deprecated and then changed then
everyone using it would get a warning during the period
of deprecation so it would
not be so bad. Given that its current behavior is
not very useful I suspect its not widely used anyways.
| haven't followed the whole discussion so sorry if these
points have already been made.
On Dec 15, 2007 5:17 PM, Martin Maechler <maechler at stat.math.ethz.ch> wrote:
> >>>>> "TP" == Tony Plate <tplate at acm.org>
> >>>>> on Fri, 14 Dec 2007 13:58:30 -0700 writes:
>
>
> TP> Duncan Murdoch wrote:
> >> On 12/13/2007 1:59 PM, Tony Plate wrote:
> >>> Duncan Murdoch wrote:
> >>>> On 12/11/2007 6:20 AM, simecek at gmail.com wrote:
> >>>>> Full_Name: Petr Simecek
> >>>>> Version: 2.5.1, 2.6.1
> >>>>> OS: Windows XP
> >>>>> Submission from: (NULL) (195.113.231.2)
> >>>>>
> >>>>>
> >>>>> Several times I have experienced that a length of a POSIXt vector
> >>>>> has not been
> >>>>> computed right.
> >>>>>
> >>>>> Example:
> >>>>>
> >>>>> tv<-structure(list(sec = c(50, 0, 55, 12, 2, 0, 37, NA, 17, 3, 31
> >>>>> ), min = c(1L, 10L, 11L, 15L, 16L, 18L, 18L, NA, 20L, 22L, 22L
> >>>>> ), hour = c(12L, 12L, 12L, 12L, 12L, 12L, 12L, NA, 12L, 12L, 12L),
> >>>>> mday = c(13L, 13L, 13L, 13L, 13L, 13L, 13L, NA, 13L, 13L, 13L), mon
> >>>>> = c(5L, 5L, 5L, 5L, 5L, 5L, 5L, NA, 5L, 5L, 5L), year = c(105L,
> >>>>> 105L, 105L, 105L, 105L, 105L, 105L, NA, 105L, 105L, 105L), wday =
> >>>>> c(1L, 1L, 1L, 1L, 1L, 1L, 1L, NA, 1L, 1L, 1L), yday = c(163L, 163L,
> >>>>> 163L, 163L, 163L, 163L, 163L, NA, 163L, 163L, 163L), isdst = c(1L,
> >>>>> 1L, 1L, 1L, 1L, 1L, 1L, -1L, 1L, 1L, 1L)), .Names = c("sec", "min",
> >>>>> "hour", "mday", "mon", "year", "wday", "yday", "isdst"
> >>>>> ), class = c("POSIXt", "POSIXlt"))
> >>>>>
> >>>>> print(tv)
> >>>>> # print 11 time points (right)
> >>>>>
> >>>>> length(tv)
> >>>>> # returns 9 (wrong)
> >>>>
> >>>> tv is a list of length 9. The answer is right, your expectation is
> >>>> wrong.
> >>>>> I have tried that on several computers with/without switching to
> >>>>> English
> >>>>> locales, i.e. Sys.setlocale("LC_TIME", "en"). I have searched a
> >>>>> help pages but I
> >>>>> cannot imagine how that could be OK.
> >>>>
> >>>> See this in ?POSIXt:
> >>>>
> >>>> Class '"POSIXlt"' is a named list of vectors...
> >>>>
> >>>> You could define your own length measurement as
> >>>>
> >>>> length.POSIXlt <- function(x) length(x$sec)
> >>>>
> >>>> and you'll get the answer you expect, but be aware that length.XXX
> >>>> methods are quite rare, and you may surprise some of your users.
> >>>>
> >>>
> >>> On the other hand, isn't the fact that length() currently always
> >>> returns 9 for POSIXlt objects likely to be a surprise to many users
> >>> of POSIXlt?
> >>>
> >>> The back of "The New S Language" says "Easy-to-use facilities allow
> >>> you to organize, store and retrieve all sorts of data. ... S
> >>> functions and data organization make applications easy to write."
> >>>
> >>> Now, POSIXlt has methods for c() and vector subsetting "[" (and many
> >>> other vector-manipulation methods - see methods(class="POSIXlt")).
> >>> Hence, from the point of view of intending to supply "easy-to-use
> >>> facilities ... [for] all sorts of data", isn't it a little
> >>> incongruous that length() is not also provided -- as 3 functions (any
> >>> others?) comprise a core set of vector-manipulation functions?
> >>>
> >>> Would it make sense to have an informal prescription (e.g., in
> >>> R-exts) that a class that implements a vector-like object and
> >>> provides at least of one of functions 'c', '[' and 'length' should
> >>> provide all three? It would also be easy to describe a test-suite
> >>> that should be included in the 'test' directory of a package
> >>> implementing such a class, that had some tests of the basic
> >>> vector-manipulation functionality, such as:
> >>>
> >>> > # at this point, x0, x1, x3, & x10 should exist, as vectors of the
> >>> > # class being tested, of length 0, 1, 3, and 10, and they should
> >>> > # contain no duplicate elements
> >>> > length(x0)
> >>> [1] 1
> >>> > length(c(x0, x1))
> >>> [1] 2
> >>> > length(c(x1,x10))
> >>> [1] 11
> >>> > all(x3 == x3[seq(len=length(x3))])
> >>> [1] TRUE
> >>> > all(x3 == c(x3[1], x3[2], x3[3]))
> >>> [1] TRUE
> >>> > length(c(x3[2], x10[5:7]))
> >>> [1] 4
> >>> >
> >>>
> >>> It would also be possible to describe a larger set of vector
> >>> manipulation functions that should be implemented together, including
> >>> e.g., 'rep', 'unique', 'duplicated', '==', 'sort', '[<-', 'is.na',
> >>> head, tail ... (many of which are provided for POSIXlt).
> >>>
> >>> Or is there some good reason that length() cannot be provided (while
> >>> 'c' and '[' can) for some vector-like classes such as "POSIXlt"?
> >>
> >> What you say sounds good in general, but the devil is in the details.
> >> Changing the meaning of length(x) for some objects has fairly
> >> widespread effects. Are they all positive? I don't know.
> >>
> >> Adding a prescription like the one you suggest would be good if it's
> >> easy to implement, but bad if it's already widely violated. How many
> >> base or CRAN or Bioconductor packages violate it currently? Do the
> >> ones that provide all 3 methods do so in a consistent way, i.e. does
> >> "length(x)" mean the same thing in all of them?
> TP> I'm not sure doing something like this would be so bad even if it is
> TP> already widely violated. R has evolved significantly over time, and
> TP> many rough edges have been cleaned up, sometimes in ways that were not
> TP> backward compatible. This is a great thing & my thanks go to the people
> TP> working on R.
>
> TP> If some base or CRAN or Bioconductor packages currently don't implement
> TP> vector operations consistently, wouldn't it be good to know that?
> TP> Wouldn't it be useful to have an automatic way of determining whether a
> TP> particular vector-like class is consistent with generally agreed set of
> TP> principles for how basic vector operations should work -- things like
> TP> length(x)+length(y)==length(c(x,y))? This could help developers check,
> TP> document & improve their code, and it could help users understand how to
> TP> use a class, and to evaluate the software quality of a class
> TP> implementation and whether or not it provides the functionality they need.
> >> I agree that the current state is less than perfect, but making it
> >> better would really be a lot of work. I suspect there are better ways
> >> to spend my time, so I'm not going to volunteer to do it. I'm not
> >> even going to invite someone else to do it, or offer to review your
> >> work if you volunteer. I think this falls into the class of "next
> >> time we write a language, let's handle this better" problems.
>
> TP> Thanks very much for the thoughtful (and honest) feedback! I suspect
> TP> that the current state could be improved with just a little work, and
> TP> without forcing anyone to do any work they don't want to do. I'll think
> TP> about this more and try to come back with a better & more concrete
> TP> suggestion.
>
> Good. From "the outside" (i.e. superficial gut feeling :-)
> I've sympathized with your suggestion, Tony, quite a bit.
> Further, my own taste would probably also have lead me to define
> length.POSIXlt differently ..
> OTOH, I agree with Duncan that it may be too late to change it
> and even more to enforce the consistency rules you propose.
> If with a small bit of code (and some patience) we could check
> all of CRAN and hopefully bioconductor packages and find only a
> very few where it was violated, the whole endeavor may be worth it
> ... for the sake of making R more consistent, easier to teach, etc..
>
> Unfortunately I don't remember now what happened many months ago
> when I indeed did experiment with having something like
>
> length.POSIXlt <- function(x) length(x$sec)
>
> Martin Maechler
>
>
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