Re: [R] stopifnot with logical(0)

[Martin Maechler]

> "DM" == Duncan Murdoch 
> on Sat, 12 Dec 2015 09:05:04 -0500 writes:

DM> On 12/12/2015 8:44 AM, peter dalgaard wrote:
>> 
>>> On 12 Dec 2015, at 10:54 , Martin Maechler
>>>  wrote:
>>> 
>>> My conclusion: Breaking such a fundamental lemma of
>>> logic as "the empty set is always true"
>> 
>> Umm, that doesn't make sense to me. Surely you mean that
>> "an AND-operation over an empty index set is TRUE"? A
>> similar OR operation is FALSE, i.e. they behave like
>> empty products and sums, respectively.
>> 

DM> How about "the empty set is all true, and all false."

or, what the I *meant* with the above:

  "All statements about elements of the empty set are true"

  ((and I still like the short form, even though it is not
correct strictly logically/mathematically))

Of course, Peter is correct,  and that
   any(logical(0))   is  FALSE
is really the only sensical way.

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Re: [R] stopifnot with logical(0)

[Hadley Wickham]

On Sat, Dec 12, 2015 at 1:51 PM, Martin Maechler
 wrote:
>> Hadley Wickham 
>> on Sat, 12 Dec 2015 08:08:54 -0600 writes:
>
> > On Sat, Dec 12, 2015 at 3:54 AM, Martin Maechler
> >  wrote:
> >>> Henrik Bengtsson  on
> >>> Fri, 11 Dec 2015 08:20:55 -0800 writes:
> >>
> >> > On Fri, Dec 11, 2015 at 8:10 AM, David Winsemius
> >>  wrote:
> >> >>
> >> >>> On Dec 11, 2015, at 5:38 AM, Dario Beraldi
> >>  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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Re: [R] stopifnot with logical(0)

[Duncan Murdoch]

On 14/12/2015 11:10 AM, Hadley Wickham wrote:

On Sat, Dec 12, 2015 at 1:51 PM, Martin Maechler
 wrote:
>> Hadley Wickham 
>> on Sat, 12 Dec 2015 08:08:54 -0600 writes:
>
> > On Sat, Dec 12, 2015 at 3:54 AM, Martin Maechler
> >  wrote:
> >>> Henrik Bengtsson  on
> >>> Fri, 11 Dec 2015 08:20:55 -0800 writes:
> >>
> >> > On Fri, Dec 11, 2015 at 8:10 AM, David Winsemius
> >>  wrote:
> >> >>
> >> >>> On Dec 11, 2015, at 5:38 AM, Dario Beraldi
> >>  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)
I don't think so.  We already have all(), so all(x == y) would do what 
you want.


Duncan Murdoch

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Re: [R] stopifnot with logical(0)

[William Dunlap]

Hadley wrote
> * all.equal is suboptimal because it returns TRUE or a text string

That feature works ok with stopifnot():
  > stopifnot(all.equal("one", 1))
  Error: all.equal("one", 1) are not all TRUE
and I suppose stopifnot could be enhanced to print the text strings that
all.equal() returns so the user has a better idea of what went wront.

I find that all.equal's comparing of names gets in the way of the intent
of stopifnot
  > stopifnot(all.equal(1:2, c(X=1,Y=2)))
  Error: all.equal(1:2, c(X = 1, Y = 2)) is not TRUE

When we use things like all.equal() or new operators that do
non-recycycling binary operations we may make things simpler
for the programmer, but harder for the user.  I think the user would
rather see a specific message about what is wrong rather than a
note that an obscure function did not return TRUE.
> f <- function(x) {
 stopifnot(length(x)==1, x>0)
 seq_len(x)
  }
> f(1:3)
Error: length(x) == 1 is not TRUE
> f(-4)
Error: x > 0 is not TRUE
> f(4)
[1] 1 2 3 4

Another step in this direction is to change the call to stop() in stopifnot
from what I assume is
stop(message, call.=FALSE)
to
stop(simpleError(message, sys.call(-1)))
so the error message included where error was:
> f(-1)
Error in f(-1) : x > 0 is not TRUE
(This is Bug 16188.)

Bill Dunlap
TIBCO Software
wdunlap tibco.com


On Mon, Dec 14, 2015 at 8:10 AM, Hadley Wickham  wrote:
> On Sat, Dec 12, 2015 at 1:51 PM, Martin Maechler
>  wrote:
>>> Hadley Wickham 
>>> on Sat, 12 Dec 2015 08:08:54 -0600 writes:
>>
>> > On Sat, Dec 12, 2015 at 3:54 AM, Martin Maechler
>> >  wrote:
>> >>> Henrik Bengtsson  on
>> >>> Fri, 11 Dec 2015 08:20:55 -0800 writes:
>> >>
>> >> > On Fri, Dec 11, 2015 at 8:10 AM, David Winsemius
>> >>  wrote:
>> >> >>
>> >> >>> On Dec 11, 2015, at 5:38 AM, Dario Beraldi
>> >>  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/
>
> __
> 

Re: [R] stopifnot with logical(0)

[Duncan Murdoch]

On 14/12/2015 11:45 AM, Hadley Wickham wrote:

>> 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)
>
> I don't think so.  We already have all(), so all(x == y) would do what you
> want.

But that recycles, which is what we're trying to avoid here.


I think this is too special a case to need a function.  Usually all(x == 
y) is the test you want, because so many R functions will recycle.  It's 
not so when x is the dim of an array.  So I could see a weak argument 
for an equalDim() function, but I think it's better just to use


stopifnot(length(dim) == 2, all(dim == c(3,4)))

I know the all() in the second arg isn't needed, but I think it makes 
the intention clearer.  I think there would be less confusion if 
stopifnot() required its args to be single logical values, so I usually 
try to use it that way.


Duncan Murdoch

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Re: [R] stopifnot with logical(0)

[Hadley Wickham]

>> 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)
>
> I don't think so.  We already have all(), so all(x == y) would do what you
> want.

But that recycles, which is what we're trying to avoid here.

Hadley

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

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Re: [R] stopifnot with logical(0)

[William Dunlap]

>  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)".
Bill Dunlap
TIBCO Software
wdunlap tibco.com


On Sat, Dec 12, 2015 at 1:54 AM, Martin Maechler
 wrote:
>> Henrik Bengtsson 
>> on Fri, 11 Dec 2015 08:20:55 -0800 writes:
>
> > On Fri, Dec 11, 2015 at 8:10 AM, David Winsemius 
>  wrote:
> >>
> >>> On Dec 11, 2015, at 5:38 AM, Dario Beraldi  
> 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
>
> __
> R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

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Re: [R] stopifnot with logical(0)

[peter dalgaard]

> On 13 Dec 2015, at 20:31 , William Dunlap  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
>  wrote:
>>> Henrik Bengtsson 
>>>on Fri, 11 Dec 2015 08:20:55 -0800 writes:
>> 
>>> On Fri, Dec 11, 2015 at 8:10 AM, David Winsemius  
>>> wrote:
 
> On Dec 11, 2015, at 5:38 AM, Dario Beraldi  
> 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
>> 
>> __
>> R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
> 
> __
> R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> 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@cbs.dk  Priv: pda...@gmail.com

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Re: [R] stopifnot with logical(0)

[peter dalgaard]

> On 12 Dec 2015, at 10:54 , Martin Maechler  wrote:
> 
> My conclusion: Breaking such a fundamental lemma of logic as 
>  "the empty set is always true"

Umm, that doesn't make sense to me. Surely you mean that "an AND-operation over 
an empty index set is TRUE"? A similar OR operation is FALSE, i.e. they behave 
like empty products and sums, respectively.

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

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Re: [R] stopifnot with logical(0)

[Martin Maechler]

> Henrik Bengtsson 
> on Fri, 11 Dec 2015 08:20:55 -0800 writes:

> On Fri, Dec 11, 2015 at 8:10 AM, David Winsemius  
wrote:
>> 
>>> On Dec 11, 2015, at 5:38 AM, Dario Beraldi  
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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Re: [R] stopifnot with logical(0)

[Hadley Wickham]

On Sat, Dec 12, 2015 at 3:54 AM, Martin Maechler
 wrote:
>> Henrik Bengtsson 
>> on Fri, 11 Dec 2015 08:20:55 -0800 writes:
>
> > On Fri, Dec 11, 2015 at 8:10 AM, David Winsemius 
>  wrote:
> >>
> >>> On Dec 11, 2015, at 5:38 AM, Dario Beraldi  
> 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/

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Re: [R] stopifnot with logical(0)

[Duncan Murdoch]

On 12/12/2015 8:44 AM, peter dalgaard wrote:



On 12 Dec 2015, at 10:54 , Martin Maechler  wrote:

My conclusion: Breaking such a fundamental lemma of logic as
  "the empty set is always true"


Umm, that doesn't make sense to me. Surely you mean that "an AND-operation over an 
empty index set is TRUE"? A similar OR operation is FALSE, i.e. they behave like 
empty products and sums, respectively.



How about "the empty set is all true, and all false."

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Re: [R] stopifnot with logical(0)

[Henrik Bengtsson]

On Sat, Dec 12, 2015 at 6:08 AM, Hadley Wickham  wrote:
> On Sat, Dec 12, 2015 at 3:54 AM, Martin Maechler
>  wrote:
>>> Henrik Bengtsson 
>>> on Fri, 11 Dec 2015 08:20:55 -0800 writes:
>>
>> > On Fri, Dec 11, 2015 at 8:10 AM, David Winsemius 
>>  wrote:
>> >>
>> >>> On Dec 11, 2015, at 5:38 AM, Dario Beraldi  
>> 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)))

Kids, this one of the rare cases where you should not listen to Hadley
;)  Because,

> x <- matrix(1:12, nrow=3, ncol=4)
> dim(x)
[1] 3 4
> identical(dim(x), c(3,4))
[1] FALSE

Why, because:

> storage.mode(dim(x))
[1] "integer"
> storage.mode(c(3,4))
[1] "double"

My rule of thumb is that identical() is awesome, but you really have
to know the inner bits and pieces (*).  When in doubt, use
all.equal(), e.g.

> all.equal(dim(x), c(3,4))
[1] TRUE

Related to Hadley's point, is that using all(x == y) is risky because
R loops of one of the two vectors if one is longer than the other,
e.g.

> all(dim(x) == c(3,4))
[1] TRUE
> all(dim(x) == c(3,4,3,4))
[1] TRUE
> all(dim(x) == c(3,4,3,4,3,4))
[1] TRUE

so one really need to check the lengths as well, e.g.

> all(length(dim(x)) == length(c(3,4)), dim(x) == c(3,4))
[1] TRUE


(*) ADVANCED: I would say its risky to use:

> identical(dim(x), c(3L,4L))
[1] TRUE

because, who knows, in a future version of R we might see
matrices/arrays that support dimensions longer than
.Machine$integer.max which in case dimensions may be stored as
doubles.  This is what we already have for very long vectors today,
cf. help("length").

Henrik

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

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Re: [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
 wrote:

Henrik Bengtsson 
 on Fri, 11 Dec 2015 08:20:55 -0800 writes:


 > On Fri, Dec 11, 2015 at 8:10 AM, David Winsemius 
 wrote:
 >>
 >>> On Dec 11, 2015, at 5:38 AM, Dario Beraldi  
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

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Re: [R] stopifnot with logical(0)

[Martin Maechler]

> Hadley Wickham 
> on Sat, 12 Dec 2015 08:08:54 -0600 writes:

> On Sat, Dec 12, 2015 at 3:54 AM, Martin Maechler
>  wrote:
>>> Henrik Bengtsson  on
>>> Fri, 11 Dec 2015 08:20:55 -0800 writes:
>> 
>> > On Fri, Dec 11, 2015 at 8:10 AM, David Winsemius
>>  wrote:
>> >>
>> >>> On Dec 11, 2015, at 5:38 AM, Dario Beraldi
>>  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...

Martin

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Re: [R] stopifnot with logical(0)

[Henrik Bengtsson]

On Fri, Dec 11, 2015 at 8:10 AM, David Winsemius  wrote:
>
>> On Dec 11, 2015, at 5:38 AM, Dario Beraldi  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

>
>
>>
>> stopifnot(logical(0) == 1)
>> stopifnot(logical(0) == TRUE)
>> stopifnot(logical(0) == FALSE)
>>
>> My understanding is that logical(0) is an empty set, so I would expect the
>> above tests to fail.
>>
>> (I got bitten by this in a piece of code where "x" happened to be
>> logical(0) and stopifnot didn't catch it)
>>
>> Thanks!
>> Dario
> --
>
> David Winsemius
> Alameda, CA, USA
>
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> R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see
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Re: [R] stopifnot with logical(0)

[Jeff Newmiller]

The goal of the comparison operators is to obtain a logical value.  Why compare 
logical values... you clearly already have that?

stopifnot( logicalvariable ) and 
stopifnot( !logicalvariable )

are sensible,  but not

stopifnot( logicalvariable == TRUE ) or 
stopifnot( logicalvariable == FALSE )

That said, R lets you construct the department of redundancy department  
anyway,  but you have to have a value to compare.  logical(0) is the absence of 
a logical value, so there is nothing to compare, so the result has to also be 
the absence of a logical value (logical(0)).

Somewhere, the test that lead to the existence of this empty logical vector had 
no data. That absence of data is what you need to test for. Or, at the very 
least, you need to verify that the length of your logical variable is greater 
than zero before checking its value. 
-- 
Sent from my phone. Please excuse my brevity.

On December 11, 2015 5:38:47 AM PST, Dario Beraldi  
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:
>
>stopifnot(logical(0) == 1)
>stopifnot(logical(0) == TRUE)
>stopifnot(logical(0) == FALSE)
>
>My understanding is that logical(0) is an empty set, so I would expect
>the
>above tests to fail.
>
>(I got bitten by this in a piece of code where "x" happened to be
>logical(0) and stopifnot didn't catch it)
>
>Thanks!
>Dario
>
>   [[alternative HTML version deleted]]
>
>__
>R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see
>https://stat.ethz.ch/mailman/listinfo/r-help
>PLEASE do read the posting guide
>http://www.R-project.org/posting-guide.html
>and provide commented, minimal, self-contained, reproducible code.

[[alternative HTML version deleted]]

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Re: [R] stopifnot with logical(0)

[Giorgio Garziano]

I think the inspection of the "stopifnot()" source code may help.

> stopifnot
function (...)
{
n <- length(ll <- list(...))
if (n == 0L)
return(invisible())
mc <- match.call()
for (i in 1L:n) if (!(is.logical(r <- ll[[i]]) && !anyNA(r) &&
all(r))) {
ch <- deparse(mc[[i + 1]], width.cutoff = 60L)
if (length(ch) > 1L)
ch <- paste(ch[1L], "")
stop(sprintf(ngettext(length(r), "%s is not TRUE", "%s are not all 
TRUE"),
ch), call. = FALSE, domain = NA)
}
invisible()
}


The following code may help in understanding.

(arg <- (logical(0) == 1))
(arg <- (logical(0) == TRUE))
(arg <- (logical(0) == FALSE))

n <- length(ll <- list(arg))
n

if (n == 0L)
  return(invisible())

for (i in 1L:n) {
  print(is.logical(r <- ll[[i]]))
  print(!anyNA(r))
  print(all(r))
  if (! (is.logical(r <- ll[[i]]) && !anyNA(r) &&  all(r)) ) {
print("stop")
  }
}

Executing such code:

> (arg <- (logical(0) == 1))
logical(0)
> (arg <- (logical(0) == TRUE))
logical(0)
> (arg <- (logical(0) == FALSE))
logical(0)
>
> n <- length(ll <- list(arg))
> n
[1] 1
>
> if (n == 0L)
+   return(invisible())
>
> for (i in 1L:n) {
+   print(is.logical(r <- ll[[i]]))
+   print(!anyNA(r))
+   print(all(r))
+   if (! (is.logical(r <- ll[[i]]) && !anyNA(r) &&  all(r)) ) {
+ print("stop")
+   }
+ }
[1] TRUE
[1] TRUE
[1] TRUE
>

See also help(all) and what its "Note" states about all(logical(0))
and consider that:

> is.logical(logical(0))
[1] TRUE


--
GG

[[alternative HTML version deleted]]

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Re: [R] stopifnot with logical(0)

[David Winsemius]

> On Dec 11, 2015, at 5:38 AM, Dario Beraldi  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" )


> 
> stopifnot(logical(0) == 1)
> stopifnot(logical(0) == TRUE)
> stopifnot(logical(0) == FALSE)
> 
> My understanding is that logical(0) is an empty set, so I would expect the
> above tests to fail.
> 
> (I got bitten by this in a piece of code where "x" happened to be
> logical(0) and stopifnot didn't catch it)
> 
> Thanks!
> Dario
-- 

David Winsemius
Alameda, CA, USA

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Re: [R] stopifnot with logical(0)

[William Dunlap]

The reason is probably that
   any(logical())
and
   any(!logical())
return FALSE (there are no TRUEs in logical(0)).
Bill Dunlap
TIBCO Software
wdunlap tibco.com


On Fri, Dec 11, 2015 at 5:38 AM, Dario Beraldi  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:
>
> stopifnot(logical(0) == 1)
> stopifnot(logical(0) == TRUE)
> stopifnot(logical(0) == FALSE)
>
> My understanding is that logical(0) is an empty set, so I would expect the
> above tests to fail.
>
> (I got bitten by this in a piece of code where "x" happened to be
> logical(0) and stopifnot didn't catch it)
>
> Thanks!
> Dario
>
> [[alternative HTML version deleted]]
>
> __
> R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

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