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

[Tony Plate]

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"?

-- Tony Plate

> Duncan Murdoch
> 
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