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

From: Gabor Grothendieck <ggrothendieck_at_gmail.com>
Date: Mon, 23 Nov 2009 21:57:24 -0500

I agree it should be changed. Perhaps there could be a global option that gives the previous behavior. The global option would be deprecated after a while but in the interim it would give package developers a change to move over and to try it under both definitions.

On Fri, Nov 20, 2009 at 1:05 PM, <maechler_at_stat.math.ethz.ch> wrote:
>>>>>> "PD" == Peter Dalgaard <p.dalgaard@biostat.ku.dk>
>>>>>>     on Fri, 20 Nov 2009 09:54:34 +0100 writes:
>
>    PD> 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?
>
>
>    PD> [You need "wishlist" in the title for this sort of stuff.]
>
>    PD> I'd be wary of this. Just the other day we found that identical() broke
>    PD> on some objects because a package had length() redefined as a class
>    PD> method. I.e. the danger is that something wants to use length() with its
>    PD> original low-level interpretation.
>
> Yes, of course.
> and Romain mentioned  str().  Note that we have needed to define
> a "POSIXt" method for str(), partly just *because* of the
> current anomaly:
> As Tony Plate, e.g., has argued, entirely correctly in my view,
> the anomaly is that    length() and "["   are not compatible;
> and while I think no R language definition says that they should
> be, I still believe that you need very good reasons for them to
> be incompatible, as they are for POSIXlt.
>
> In the current case, for me the only good reason is backwards
> compatibility.
> My personal taste would be to change it and see what happens.
> I would be willing to clean up after that change within R 'base'
> and all packages I am coauthoring (quite a few), but of course
> there are still a thousand more R packages..
> My strong bet would be that less than 1% would be affected,
> and my point guess for the percentage affected would be
> rather in the order of  1/1000.
>
> The question is if we (you too!), the R community, are willing to
> bear the load of cleanup, after such a change which would really
> *improve* consistency of that small corner of R.
> For me, as I indicated above, I am willing to bear my share
> (and actually have got it ready for R-devel)

>
> Martin Maechler, ETH Zurich (and R Core Team)
>
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