[Rd] for in r-devel

[Michael Lawrence]

Thanks for pointing this out. The commit you cite was reverted a couple
days later, because we are still thinking about how it should work. While
it appears Date would benefit, weird things happen e.g. with POSIXlt and
other data structures in the wild. This is due to inconsistency in the
behaviors of length() and [[() methods. One thought is introducing a
generic that coerces to something with consistent behavior, e.g.
iterable(). We were even thinking that an iterable might be allowed to
return NA for length, in which case it would need to support methods like
next() and hasNext(). Ideas welcome.

Michael





On Fri, Mar 18, 2016 at 7:25 AM, Gabor Grothendieck <ggrothendieck at gmail.com
> wrote:

> Regarding, this news item for r-devel:
>
> ‘for()’ loops are generalized to iterate over any object with ‘[[’ and
> ‘length()’ methods. Thanks to Hervé Pagès for the idea and the patch.
>
> Below dd is an object for which [[ and length work but the result is
> still numeric rather than Date class in  "R Under development
> (unstable) (2016-03-15 r70334)" as observed in the comments to:
>
> http://stackoverflow.com/questions/36074344/why-does-for-convert-date-to-numeric#comment59794873_36074344
> Expanding on that:
>
>     dd <- Sys.Date() + 0:1
>
>     dd[[1]]  # [[ works
>     ## [1] "2016-03-18"
>
>     length(dd)  # length works
>     ## [1]  2
>
>     for(d in dd) str(d)  # gives numeric rather than Date class
>     ## num 16878
>     ## num 16879
>
>
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