Duncan Murdoch wrote: > On 12/11/2007 6:20 AM, [EMAIL PROTECTED] 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 > > ______________________________________________ > R-devel@r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-devel > ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel