[R] stopifnot with logical(0)

[Duncan Murdoch]

On 12/12/2015 9:08 AM, Hadley Wickham wrote:
> 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)))

identical() is a little pickier than people might think:

 > identical(dim(matrix(0, 3,4)), c(3,4))
[1] FALSE
 > identical(dim(matrix(0, 3,4)), c(3L,4L))
[1] TRUE

Duncan Murdoch