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

From: Peter Dalgaard <p.dalgaard_at_biostat.ku.dk>
Date: Fri, 20 Nov 2009 09:54:34 +0100

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?

[You need "wishlist" in the title for this sort of stuff.]

I'd be wary of this. Just the other day we found that identical() broke on some objects because a package had length() redefined as a class method. I.e. the danger is that something wants to use length() with its original low-level interpretation.

> 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
>
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-- 
    O__  ---- Peter Dalgaard             ุster Farimagsgade 5, Entr.B
   c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
  (*) \(*) -- University of Copenhagen   Denmark      Ph:  (+45) 35327918
~~~~~~~~~~ - (p.dalgaard_at_biostat.ku.dk)              FAX: (+45) 35327907

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Received on Fri 20 Nov 2009 - 08:56:40 GMT

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