[Rd] identical(0, -0)

[Duncan Murdoch]

Martin Maechler wrote:
>>>>>> William Dunlap <wdunlap at tibco.com>
>>>>>>     on Thu, 6 Aug 2009 15:06:08 -0700 writes:
>>>>>>             
>
>     >> -----Original Message----- From:
>     >> r-help-bounces at r-project.org
>     >> [mailto:r-help-bounces at r-project.org] On Behalf Of
>     >> Giovanni Petris Sent: Thursday, August 06, 2009 3:00 PM
>     >> To: milton.ruser at gmail.com Cc: r-help at r-project.org;
>     >> Daniel.Gerlanc at geodecapital.com Subject: Re: [R] Why is 0
>     >> not an integer?
>     >> 
>     >> 
>     >> I ran an instant experiment...
>     >> 
>     >> > typeof(0) [1] "double" > typeof(-0) [1] "double" >
>     >> identical(0, -0) [1] TRUE
>     >> 
>     >> Best, Giovanni
>
>     > But 0.0 and -0.0 have different reciprocals
>
>     >> 1.0/0.0
>     >    [1] Inf
>     >> 1.0/-0.0
>     >    [1] -Inf
>
>     > Bill Dunlap TIBCO Software Inc - Spotfire Division wdunlap
>     > tibco.com
>
> yes.  {finally something interesting in this boring thread !}
> ---> diverting to R-devel
>
> In April, I've had a private e-mail communication with John
> Chambers [father of S, notably S4, which also brought identical()]
> and Bill about the topic,
> where I had started suggesting that  R  should be changed such
> that
>     identical(-0. , +0.)
> would return FALSE.
> Bill did mention that it does so for (newish versions of) S+
> and that he'd prefer that, too,
> and John said
>
>  >> I agree on having a preference for a bitwise comparison for
>  >> identical()---that's what the name means after all.  But since
>  >> someone implemented the numerical case as the C == it's probably
>  >> going to be more hassle than it's worth to change it.  But we
>  >> should make the implementation clear in the documentation.
>
> so in principle, we all agreed that R's identical() should be
> changed here, namely by using something like  memcmp() instead
> of simple '==' ,  however we haven't bothered to actually 
> *implement* this change.
>
> I am currently testing a patch  which would lead to
> identical(0, -0)  return FALSE.
>   
I don't think that would be a good idea.  Other expressions besides "-0" 
calculate the zero with the negative sign bit, e.g. the following sequence:

pos <- 1
neg <- -1
zero <- 0
y <- zero*pos
z <- zero*neg
identical(y, z)

I think most R users would expect the last expression there to be TRUE 
based on the previous two lines, given that pos and neg both have finite 
values.  In a simple case like this y == z would be a better test to 
use, but if those were components of a larger structure, identical() is 
all we've got, and people would waste a lot of time tracking down why 
structures differing only in the sign of zero were not identical, even 
though every element tested equal.

Duncan Murdoch
> Martin Maechler, ETH Zurich
> 	    
>     >> > By the way:
>     >> > 
>     >> > Are there difference between -0 and 0?
>
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