[Rd] Wrong length of POSIXt vectors (PR#10507)
maechler at stat.math.ethz.ch
Sat Dec 15 23:17:49 CET 2007
>>>>> "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) (188.8.131.52)
>>>>> Several times I have experienced that a length of a POSIXt vector
>>>>> has not been
>>>>> computed right.
>>>>> 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 11 time points (right)
>>>>> # returns 9 (wrong)
>>>> tv is a list of length 9. The answer is right, your expectation is
>>>>> I have tried that on several computers with/without switching to
>>>>> 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
>>> > length(c(x0, x1))
>>>  2
>>> > length(c(x1,x10))
>>>  11
>>> > all(x3 == x3[seq(len=length(x3))])
>>>  TRUE
>>> > all(x3 == c(x3, x3, x3))
>>>  TRUE
>>> > length(c(x3, x10[5:7]))
>>>  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
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)
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