[R] stopifnot with logical(0)

[Hadley Wickham]

On Sat, Dec 12, 2015 at 1:51 PM, Martin Maechler
<maechler at stat.math.ethz.ch> wrote:
>>>>>> Hadley Wickham <h.wickham at gmail.com>
>>>>>>     on Sat, 12 Dec 2015 08:08:54 -0600 writes:
>
>     > On Sat, Dec 12, 2015 at 3:54 AM, Martin Maechler
>     > <maechler at stat.math.ethz.ch> wrote:
>     >>>>>>> Henrik Bengtsson <henrik.bengtsson at gmail.com> on
>     >>>>>>> Fri, 11 Dec 2015 08:20:55 -0800 writes:
>     >>
>     >> > On Fri, Dec 11, 2015 at 8:10 AM, David Winsemius
>     >> <dwinsemius at comcast.net> wrote:
>     >> >>
>     >> >>> On Dec 11, 2015, at 5:38 AM, Dario Beraldi
>     >> <dario.beraldi at 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/