Re: [Rd] Surprising length() of POSIXlt vector (PR#14073)

From: Steven McKinney <smckinney_at_bccrc.ca>
Date: Thu, 19 Nov 2009 19:47:54 -0800

> -----Original Message-----
> From: Benilton Carvalho [mailto:bcarvalh_at_jhsph.edu]
> Sent: Thursday, November 19, 2009 6:59 PM
> To: Steven McKinney
> Cc: 'mark_at_celos.net'; 'r-devel_at_stat.math.ethz.ch'
> Subject: Re: [Rd] Surprising length() of POSIXlt vector (PR#14073)
>
> Steve,
>
> I'm no expert on this, but my understanding is that the choice was to
> stick to the definition.
>
> The help file for length() [1] says:
>
> "For vectors (including lists) and factors the length is the number of
> elements."
>
> The help file for POSIXlt [2] (for example) says:
>
> "Class '"POSIXlt"' is a named list of vectors representing (...)"
>
> and then lists the 9 elements (sec / min / hour / mday / mon / year /
> wday / yday / isdst).
>
> So, by [1] length of POSIXlt objects is 9, because it "is a named list
> of vectors representing (...)".

Thanks, a most sensible description.
After how many bug reports does it qualify for addition to the FAQ?!

Steve McKinney

>
> b
>
> On Nov 20, 2009, at 12:19 AM, Steven McKinney wrote:
>
> >
> > I've checked the archives, and this problem crops up every
> > few months going back for years.
> >
> > What I was not able to find was an explanation of why a
> > function such as
> > length.POSIXlt <- function(x) { length(x$sec) }
> > is a Bad Idea, or what it would break. listserv threads
> > seem to end without presenting an answer. R News 2001
> > Vol 1/2 discusses that "lots of methods are needed..."
> > (page 11) but I haven't found discussion of why a length
> > method isn't feasible.
> >
> > Can anyone clarify this, or point me at the right
> > archive or documentation source that discusses why
> > objects of class POSIXlt always need to return a
> > length of 9?
> >
> > Thanks
> > Steve McKinney
> >
> >
> >> -----Original Message-----
> >> From: r-devel-bounces_at_r-project.org [mailto:r-devel-bounces_at_r-
> >> project.org] On Behalf Of Benilton Carvalho
> >> Sent: Thursday, November 19, 2009 4:29 PM
> >> To: mark_at_celos.net
> >> Cc: r-devel_at_stat.math.ethz.ch
> >> Subject: Re: [Rd] Surprising length() of POSIXlt vector (PR#14073)
> >>
> >> Check the documentation and the archives. Not a bug. b
> >>
> >> On Nov 19, 2009, at 8:30 PM, mark_at_celos.net wrote:
> >>
> >>> Arrays of POSIXlt dates always return a length of 9. This
> >>> is correct (they're really lists of vectors of seconds,
> >>> hours, and so forth), but other methods disguise them as
> >>> flat vectors, giving superficially surprising behaviour:
> >>>
> >>> strings <- paste('2009-1-', 1:31, sep='')
> >>> dates <- strptime(strings, format="%Y-%m-%d")
> >>>
> >>> print(dates)
> >>> # [1] "2009-01-01" "2009-01-02" "2009-01-03" "2009-01-04"
> >>> "2009-01-05"
> >>> # [6] "2009-01-06" "2009-01-07" "2009-01-08" "2009-01-09"
> >>> "2009-01-10"
> >>> # [11] "2009-01-11" "2009-01-12" "2009-01-13" "2009-01-14"
> >>> "2009-01-15"
> >>> # [16] "2009-01-16" "2009-01-17" "2009-01-18" "2009-01-19"
> >>> "2009-01-20"
> >>> # [21] "2009-01-21" "2009-01-22" "2009-01-23" "2009-01-24"
> >>> "2009-01-25"
> >>> # [26] "2009-01-26" "2009-01-27" "2009-01-28" "2009-01-29"
> >>> "2009-01-30"
> >>> # [31] "2009-01-31"
> >>>
> >>> print(length(dates))
> >>> # [1] 9
> >>>
> >>> str(dates)
> >>> # POSIXlt[1:9], format: "2009-01-01" "2009-01-02" "2009-01-03"
> >>> "2009-01-04" ...
> >>>
> >>> print(dates[20])
> >>> # [1] "2009-01-20"
> >>>
> >>> print(length(dates[20]))
> >>> # [1] 9
> >>>
> >>> I've since realised that POSIXct makes date vectors easier,
> >>> but could we also have something like:
> >>>
> >>> length.POSIXlt <- function(x) { length(x$sec) }
> >>>
> >>> in datetime.R, to avoid breaking functions (like the
> >>> str.POSIXt method) which use length() in this way?
> >>>
> >>> Thanks,
> >>> Mark <><
> >>>
> >>> ------
> >>>
> >>> Version:
> >>> platform = i686-pc-linux-gnu
> >>> arch = i686
> >>> os = linux-gnu
> >>> system = i686, linux-gnu
> >>> status =
> >>> major = 2
> >>> minor = 10.0
> >>> year = 2009
> >>> month = 10
> >>> day = 26
> >>> svn rev = 50208
> >>> language = R
> >>> version.string = R version 2.10.0 (2009-10-26)
> >>>
> >>> Locale:
> >>> C
> >>>
> >>> Search Path:
> >>> .GlobalEnv, package:stats, package:graphics, package:grDevices,
> >>> package:utils, package:datasets, package:methods, Autoloads,
> >>> package:base
> >>>
> >>> ______________________________________________
> >>> R-devel_at_r-project.org mailing list
> >>> https://stat.ethz.ch/mailman/listinfo/r-devel
> >>
> >> ______________________________________________
> >> R-devel_at_r-project.org mailing list
> >> https://stat.ethz.ch/mailman/listinfo/r-devel



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