[Rd] Wrong length of POSIXt vectors (PR#10507)

[Martin Maechler]

>>>>> "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