From mark at celos.net  Thu Nov 19 23:30:13 2009
From: mark at celos.net (mark at celos.net)
Date: Thu, 19 Nov 2009 23:30:13 +0100 (CET)
Subject: [Rd] Surprising length() of POSIXlt vector (PR#14073)
Message-ID: <20091119223013.53B2C282EF57@mail.pubhealth.ku.dk>

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:
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[Rd] Surprising length() of POSIXlt vector (PR#14073)
mark at celos.net