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

[Steven McKinney]

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
> 
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