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

[Prof Brian Ripley]

On Thu, 12 May 2005, Kjetil Brinchmann Halvorsen wrote:
Prof Brian Ripley wrote:
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.
Why is that not a bug in R? On my machine (windows XP, rw2010 from CRAN) I
Because 0 <= y %% x < y, so that adding any multiple of 0.001 takes it out 
of range.

get:
test <- function(x, y) (x %% y) + y * ( x %/% y ) # should be x
test(1, 0.001)
[1] 1.001
test(1, 0.1)
[1] 1.1
test(1, 1)
[1] 1
test(1, 0.01)
[1] 1.01
and this differences (well, not the third one) cannot be said to be rounding
error.
I think they can.  The problem is that x %/% y is affected by rounding 
error in the representation of 0.01, as in

1 %/% 0.01
[1] 99
on my machine.  That is due to rounding error.
--
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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