[R] stopifnot with logical(0)

[Hadley Wickham]

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)))

Hadley

-- 
http://had.co.nz/