Re: [Rd] RFC: Redefining length(<POSIXlt>) ?

From: Gabor Grothendieck <ggrothendieck_at_gmail.com>
Date: Mon 24 Jul 2006 - 12:37:23 GMT

I think Spencer's point of it likely being compatible with most current code is important (since it currently always returns 9 so its unlikely to be used meaningfully in existing code).

On 7/24/06, Martin Maechler <maechler@stat.math.ethz.ch> wrote:
> So I did open a new subject and move the discussion to R-devel
> now.
>
> >>>>> "MM" == Martin Maechler <maechler@stat.math.ethz.ch>
> >>>>> on Mon, 24 Jul 2006 11:46:05 +0200 writes:
>
> >>>>> "Gabor" == Gabor Grothendieck <ggrothendieck@gmail.com>
> >>>>> on Sun, 23 Jul 2006 09:02:35 -0400 writes:
>
> Gabor> Looking at the diff.POSIXt code we see the problem is
> Gabor> that it takes the length of the input using length
> Gabor> which is wrong since in the case of POSIXlt the
> Gabor> length is always 9 (or maybe length should be defined
> Gabor> differently for POSIXlt?).
>
> MM> Though I agree with Spencer that a user may expect
> MM> length() to behave differently, but I don't think this
> MM> would be a good idea. Yes, length() is generic, but its
> MM> help() emphasizes that for lists, length() should be the
> MM> number of list elements. Of course anyone one *can*
> MM> define length() methods that behave differently for
> MM> his/her classes, but then one would also want to make
> MM> sure that e.g. x[length(x)] or 'x[length(x)] <- value'
> MM> works and -- in a case of simple S3 class built on a
> MM> list, would work differently than if x was a the simple list.
>
> Hmm, after thinking a bit more, and particularly after seeing
> all the other methods we already have working for "POSIXlt",
> I'm much less sure if (re)definining length(<POSIXlt>)
> would be such a bad idea.
> Indeed, "[", and "[<-" and "[[" already work with an implicit
> length of 'length_of_series' and not length 9, and so does str().
>
> So it seems to me, there is more reasonable momentum *for* a
> length.POSIXlt() method than I first considered.
>
> Still not sure what the wisest decision would be, and I am
> asking for more opinions or arguments pro / con.
> Since Brian Ripley has done the most work on the POSIX[cl]?t
> classes, we should also wait on his opinion.
>
> Martin
>
> MM> In my view, I would only consider redefing length() for
> MM> "non-basic" S4 classes, i.e. those with slots, where no
> MM> confusion is possible, since these objects are
> MM> definitely not simple vectors nor lists (aka "generic"
> MM> vectors).
>
>



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