[Rd] RFC: is.whole() ? {"how to judge if a variable is an integer"}

[Duncan Murdoch]

On 28/10/2014, 5:32 AM, Martin Maechler wrote:
> Diverted to R-devel, as I'm requesting comments about a proposal
> to add is.whole() to R just so this issue does not trail on for
> centuries (;-), see below.
> 

I didn't read the thread in question, but I think Bill's comment is
crucial:  what properties of integers are you interested in testing?

One that comes up a lot is that some non-integral values print as
integers, and people get confused when using them as indices.  For
example, this prints results that surprise some people:

x <- 1:10
index <- 4 - 1.e-10
index
x[index]

I would think is.whole(index) should return FALSE here, but the sfsmisc
function defaults to a nonzero tolerance, and returns TRUE.

Duncan Murdoch

>>>>>> William Dunlap <wdunlap at tibco.com>
>>>>>>     on Sat, 18 Oct 2014 10:33:05 -0700 writes:
> 
>     > 3. all.equal(a, as.integer(a))
>     > Note that this one tests if 'a' can be stored accurately as a 32-bit signed
>     > integer.  If you want to know if 'a' can be used as an accurate count, then
>     > you want to test if a+1>a (use abs() in case a is negative).  E.g., try this
>     > for a<-2^49-1, about 5*10^14.
> 
>     > You have to decide what properties of integers you are interested in.
> 
>     > Bill Dunlap
>     > TIBCO Software
>     > wdunlap tibco.com
> 
> good point, thank you, Bill.
> 
> This whole issue comes up about once year (at least), it seems to me,
> and every time there are some good and some not so good propositions,
> some aiming for vectorized / whole object solutions some, only
> assuming scalar input.
> 
> Indeed, I'd claim that all three proposals cited below are not
> good enough in one way or the other.... though one
> could argue that a "tolerance = 0" version would be good enough,
> and hence the    
>     	  	  function(x) x %% 1  == 0
> would be sufficient. 
> 
> In the CRAN package 'sfsmisc' (which I maintain), Alain Hauser 
> recently added an
> 
> 	 is.whole()
> 
> function which works "vectorized" and uses a 'tolerance' and
> then all.equal(), but in better way than (most / all ?) what was proposed in
> this thread (line 486 ff of 
>  https://github.com/mmaechler/sfsmisc/blob/master/R/misc-goodies.R ).
> 
> Further, the CRAN packages
>  'gmp'   ("infinite" precision integer and rational arithmetic) and
>  'Rmpfr' ('infinite' precision floating point arithmetic) contain
> an S3 generic  is.whole() function and methods for their own
> number classes and a default method (( which "however" assumes
> a tolerance of zero, the same as the  x %% 1 == 0 proposal)).
> 
> Given all this, the r-help and stackoverflow threads,
> maybe we should decide that such an is.whole() function should
> be added to R and maintained (by me for the time), so we do have
> a better place to point people to, and well documented -- and
> eventually "optimized" -- behavior ?
> 
> Personally I do think I'd want a "signature" of
>     function (x, tolerance = sqrt(.Machine$double.eps))
> 
> 
> Martin
> 
> 
>     > On Sat, Oct 18, 2014 at 10:02 AM, PO SU <rhelpmaillist at 163.com> wrote:
>     >> 
>     >> Tks for your help, after investigate in your link, i find there seems three ways can be adoped:
>     >> 1.    is.wholenumber <- function(x, tol = .Machine$double.eps^0.5)  abs(x - round(x)) < tol)
>     >> e.g. is.wholenumber(1)
>     >> 2.   x%%1==0
>     >> 
>     >> 
>     >> 3. all.equal(a, as.integer(a))
>     >> 
>     >> 
>     >> and also included your last suggestion using floor. and also tks for other helpers！
>     >> --
>     >> 
>     >> PO SU
>     >> mail: desolator88 at 163.com
>     >> Majored in Statistics from SJTU
>     >> 
>     >> At 2014-10-18 22:48:15, "Sergio Fonda" <sergio.fonda99 at gmail.com> wrote:
>     >> 
>     >> Sorry for my previous hurry misunderstanding.
>     >> Try this link:
>     >> http://stackoverflow.com/questions/3476782/how-to-check-if-the-number-is-integer
>     >> 
>     >> 2014-10-18 16:25 GMT+02:00 PO SU <rhelpmaillist at 163.com>:
>     >> 
>     >> It's due to that, 1 is a numeric, 1.2 is a numeric, though it's true. but deeply, when i want to know 1 is an integer,  there seems no easy way to get the answer.
>     >> 
>     >> So, is there anyone happen to know it?
>     >> 
> 
>     >> 
>     >> At 2014-10-18 20:10:09, "S Ellison" <S.Ellison at LGCGroup.com> wrote:
>     >> 
>     >>>> But i use a<-10/b ,  b is some value ,may be  5, maybe 5.5
>     >> 
>     >>> If you do floating point arithmetic on integers you'll usually get floating point answers, including the 5.0.
>     >> 
>     >>> 
>     >> 
>     >>> See FAQ 7.31 for the usual floating point problem, and ?all.equal for the usual answer to it. You could see if a result is close to an integer by,for example, using all.equal to compare it to itself after rounding.
>     >> 
>     >>> 
>     >> 
>     >>> S
> 
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