RE: [Rd] bug in modulus operator %% (PR#7852)

[Prof Brian Ripley]

I've now found a Windows system that does this.  This is also Windows XP, 
fully patched, and with the same rw2010.  So it may be chip-specific: the 
one that works is a P4 and the one that does not is a latest Pentium M.

I am not sure that the guarantee on the help page has been supported for a 
while.  I've altered the code so it is more likely to be.

BTW,
options(digits=20)
1 %% 0.001
[1] 0.0009999999999999792
shows why the original is not a bug in R.
On Thu, 12 May 2005 [EMAIL PROTECTED] wrote:
On Wed, 11 May 2005 [EMAIL PROTECTED] wrote:
Yes, you are correct. I had only checked one of my platforms. Linux
works as you suggest. But for me on Windows,
x <- 1
y <- 0.2
x %/% y
[1] 5  ## I get a 4 in Linux
I get 5 on Windows, but
(x %% y) + y * (x %/% y)
[1] 1
so is there a problem particular to your Windows runtime?

version
        _             =20
platform i386-pc-mingw32
arch     i386          =20
os       mingw32       =20
system   i386, mingw32 =20
status                 =20
major    2             =20
minor    1.0           =20
year     2005          =20
month    04            =20
day      18            =20
language R             =20
-----Original Message-----
From: Peter Dalgaard [mailto:[EMAIL PROTECTED]
Sent: Wednesday, May 11, 2005 4:14 PM
To: McGehee, Robert
Cc: [EMAIL PROTECTED]; Peter Dalgaard; [EMAIL PROTECTED];
[EMAIL PROTECTED]; r-devel@stat.math.ethz.ch
Subject: Re: [Rd] bug in modulus operator %% (PR#7852)
"McGehee, Robert" <[EMAIL PROTECTED]> writes:
Yes, but from ?"%%":
"It is guaranteed that 'x =3D=3D (x %% y) + y * (x %/% y)' (up to =
rounding
error) ..."
=20
(R 2.1.0)
x <- 1
y <- 0.2
x %% y
[1] 0.2
(x %% y) + y * (x %/% y)
[1] 1.2
=20
Certainly 1 does not equal 1.2 as the documentation would suggest, and
these seem like large enough numbers to not be effected by rounding
errors or lack of precision.
Now that looks a bit odd, but it isn't universal:
x <- 1
y <- 0.2
x %% y
[1] 0.2
x %/% y
[1] 4
(x %% y) + y * (x %/% y)
[1] 1
So what platform was that happening on?

--
Brian D. Ripley,                  [EMAIL PROTECTED]
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595
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