bug in modulus operator %% (PR#7852)

Thu, May 12, 2005 2:38 AM

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,

[1] 0.0009999999999999792

shows why the original is not a bug in R.

On Thu, 12 May 2005 ripley@stats.ox.ac.uk wrote:

On Wed, 11 May 2005 Robert.McGehee@geodecapital.com 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:p.dalgaard@biostat.ku.dk]=20
Sent: Wednesday, May 11, 2005 4:14 PM
To: McGehee, Robert
Cc: ted.harding@nessie.mcc.ac.uk; Peter Dalgaard; R-bugs@biostat.ku.dk;
kjetil@acelerate.com; r-devel@stat.math.ethz.ch
Subject: Re: [Rd] bug in modulus operator %% (PR#7852)


"McGehee, Robert" <Robert.McGehee@geodecapital.com> 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,                  ripley@stats.ox.ac.uk
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

bug in modulus operator %% (PR#7852)

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