[R] stopifnot with logical(0)
William Dunlap
wdunlap at tibco.com
Mon Dec 14 17:31:29 CET 2015
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 <h.wickham at gmail.com> wrote:
> 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/
>
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