On Sat, Dec 12, 2015 at 1:51 PM, Martin Maechler
<maech...@stat.math.ethz.ch> wrote:
>>>>>> Hadley Wickham <h.wick...@gmail.com>
>>>>>> on Sat, 12 Dec 2015 08:08:54 -0600 writes:
>
> > On Sat, Dec 12, 2015 at 3:54 AM, Martin Maechler
> > <maech...@stat.math.ethz.ch> wrote:
> >>>>>>> Henrik Bengtsson <henrik.bengts...@gmail.com> on
> >>>>>>> Fri, 11 Dec 2015 08:20:55 -0800 writes:
> >>
> >> > On Fri, Dec 11, 2015 at 8:10 AM, David Winsemius
> >> <dwinsem...@comcast.net> wrote:
> >> >>
> >> >>> On Dec 11, 2015, at 5:38 AM, Dario Beraldi
> >> <dario.bera...@gmail.com> wrote:
> >> >>>
> >> >>> Hi All,
> >> >>>
> >> >>> I'd like to understand the reason why
> >> stopifnot(logical(0) == x) doesn't >>> (never?) throw an
> >> exception, at least in these cases:
> >> >>
> >> >> The usual way to test for a length-0 logical object is
> >> to use length():
> >> >>
> >> >> x <- logical(0)
> >> >>
> >> >> stopifnot( !length(x) & mode(x)=="logical" )
> >>
> >> > I found
> >>
> >> > stopifnot(!length(x), mode(x) == "logical")
> >>
> >> > more helpful when troubleshooting, because it will tell
> >> you whether > it's !length(x) or mode(x) == "logical"
> >> that is FALSE. It's as if you > wrote:
> >>
> >> > stopifnot(!length(x)) > stopifnot(mode(x) == "logical")
> >>
> >> > /Henrik
> >>
> >> Yes, indeed, thank you Henrik --- and Jeff Newmiller
> >> who's nice humorous reply added other relevant points.
> >>
> >> As author stopifnot(), I do agree with Dario's "gut
> >> feeling" that stopifnot() "somehow ought to do the right
> >> thing" in cases such as
> >>
> >> stopifnot(dim(x) == c(3,4))
> >>
> >> which is really subtle version of his cases {But the gut
> >> feeling is wrong, as I argue from now on}.
>
> > Personally, I think the problem there is that people
> > forget that == is vectorised, and for a non-vectorised
> > equality check you really should use identical:
>
> > stopifnot(identical(dim(x), c(3,4)))
>
> You are right "in theory" but practice is less easy:
> identical() tends to be too subtle for many users ... even
> yourself (;-), not really of course!), Hadley, in the above case:
>
> Your stopifnot() would *always* stop, i.e., signal an error
> because typically all dim() methods return integer, and c(3,4)
> is double.
> So, if even Hadley gets it wrong so easily, I wonder if its good
> to advertize to always use identical() in such cases.
> I indeed would quite often use identical() in such tests, and
> you'd too and would quickly find and fix the "trap" of course..
> So you are mostly right also in my opinion...
Ooops, yes - but you would discover this pretty quickly if you weren't
coding in a email client ;)
I wonder if R is missing an equality operator for this case. Currently:
* == is suboptimal because it's vectorised
* all.equal is suboptimal because it returns TRUE or a text string
* identical is suboptimal because it doesn't do common coercions
Do we need another function (equals()?) that uses the same coercion
rules as == but isn't vectorised? (Like == it would only work with
vectors, so you'd still need identical() for (e.g.) comparing
environments)
Hadley
--
http://had.co.nz/
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