[Rd] Friday question: negative zero

[Steven McKinney]

Seems the same on this Apple Mac OSX platform:

> y <- 0
> 1/y
[1] Inf
> y <- -y
> 1/y
[1] -Inf
> x <- complex(real = -1)
> x
[1] -1+0i
> 1/x
[1] -1+0i
> x^(1/3)
[1] 0.5+0.8660254i
> (1/x)^(1/3)
[1] 0.5-0.8660254i

> sessionInfo()
R version 2.5.1 (2007-06-27) 
powerpc-apple-darwin8.9.1 

locale:
en_CA.UTF-8/en_CA.UTF-8/en_CA.UTF-8/C/en_CA.UTF-8/en_CA.UTF-8

attached base packages:
[1] "stats"     "graphics"  "grDevices" "utils"     "datasets"  "methods"   "base"     
> system("uname -a")
Darwin Dapple.local 8.10.0 Darwin Kernel Version 8.10.0: Wed May 23 16:50:59 PDT 2007; root:xnu-792.21.3~1/RELEASE_PPC Power Macintosh powerpc
> version
               _                           
platform       powerpc-apple-darwin8.9.1   
arch           powerpc                     
os             darwin8.9.1                 
system         powerpc, darwin8.9.1        
status                                     
major          2                           
minor          5.1                         
year           2007                        
month          06                          
day            27                          
svn rev        42083                       
language       R                           
version.string R version 2.5.1 (2007-06-27)


Steven McKinney





-----Original Message-----
From: r-devel-bounces at r-project.org on behalf of Duncan Murdoch
Sent: Fri 8/31/2007 5:39 PM
To: R Devel
Subject: [Rd] Friday question: negative zero
 
The IEEE floating point standard allows for negative zero, but it's hard 
to know that you have one in R.  One reliable test is to take the 
reciprocal.  For example,

 > y <- 0
 > 1/y
[1] Inf
 > y <- -y
 > 1/y
[1] -Inf

The other day I came across one in complex numbers, and it took me a 
while to figure out that negative zero was what was happening:

  > x <- complex(real = -1)
  > x
[1] -1+0i
  > 1/x
[1] -1+0i
  > x^(1/3)
[1] 0.5+0.8660254i
  > (1/x)^(1/3)
[1] 0.5-0.8660254i

(The imaginary part of 1/x is negative zero.)

As a Friday question:  are there other ways to create and detect 
negative zero in R?

And another somewhat more serious question:  is the behaviour of 
negative zero consistent across platforms?  (The calculations above were 
done in Windows in R-devel.)

Duncan Murdoch

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