[R] stopifnot with logical(0)

[peter dalgaard]

> On 13 Dec 2015, at 20:31 , William Dunlap <wdunlap at tibco.com> wrote:
> 
>> as Bill or Jeff explained,  "the empty set is always true"
> 
> My wording was that any(logical(0)) is FALSE because "there are no
> TRUEs in logical(0)".

Yes. My mind still boggles over how the empty set slipped into Boolean algebra.

Another way of flogging the horse is to say that we want[*] 

any(c(x,y)) == any(x) | any(y)

even if x has length 0. In that case c(x,y) == y, so we must have

any(y) == any(logical(0)) | any(y)

which holds for all y if and only if any(logical(0)) is FALSE.

Also, as I tried to say before, any() is to "|" what sum() is to "+" and what all() is to "&" and prod() is to "*". All the operators have an identity element, namely FALSE, 0, TRUE, and 1 respectively, and the generic convention is that for an empty vector, we return the identity element, for the reason given above.  

Peter D.

[*] Assuming logical vectors here. Coercion might well do odd things...


> Bill Dunlap
> TIBCO Software
> wdunlap tibco.com
> 
> 
> On Sat, Dec 12, 2015 at 1:54 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}.
>> 
>> Someone writing the above would want stopifnot() to stop in the
>> case where x is a simple vector instead of a
>> matrix/data.frame/... with dimensions c(3,4) ... but it will not
>> because, as Bill or Jeff explained,  "the empty set is always
>> true", and so yes indeed,  you have to care about length-0
>> expressions in stopifnot().
>> 
>> Indeed, in the past, I had thought of  "improving" stopifnot()
>> by giving a warning or even stop()  for  logical(0)  expressions,
>> but I quickly dismissed that idea after some experiments.
>> 
>> My conclusion: Breaking such a fundamental lemma of logic as
>>  "the empty set is always true"
>> will lead to all kinds of illogical situations ... so don't do that!
>> 
>> Martin Maechler,
>> ETH Zurich
>> 
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> 
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
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> and provide commented, minimal, self-contained, reproducible code.

-- 
Peter Dalgaard, Professor,
Center for Statistics, Copenhagen Business School
Solbjerg Plads 3, 2000 Frederiksberg, Denmark
Phone: (+45)38153501
Office: A 4.23
Email: pd.mes at cbs.dk  Priv: PDalgd at gmail.com