From ripley at stats.ox.ac.uk  Thu Dec  9 09:42:12 2004
From: ripley at stats.ox.ac.uk (ripley@stats.ox.ac.uk)
Date: Thu Dec  9 09:42:17 2004
Subject: [Rd] Modulus Bug (PR#7409)
Message-ID: <20041209084212.4D19D10F20@slim.kubism.ku.dk>

This is nothing to do with integers: 1e18 and 11 are doubles here.

It is a result of rounding error: 1e18/11 is not representable accurately, 
and this should have been a warning to you that your calculations were
unreasonable.

The C code is

double myfmod(double x1, double x2)
{
     double q = x1 / x2;
     return x1 - floor(q) * x2;
}

We can improve the answer, but what you are doing is fundamentally flawed 
and it is hard to detect whether rounding error has affected this. A 
warning rather than an error seems appropriate.

If you really want to do things like this, try the gmp package (which 
seems to give the wrong answer here) or a more appropriate calculator.


On Thu, 9 Dec 2004 Robert.McGehee@geodecapital.com wrote:

> R Developers,
>
> 1000000000000000000 %% 11
> [1] -32
>
> I now understand that integers cannot be larger than
> .Machine$integer.max, but because the above produces a result than is
> patently wrong instead of an error, I'm reporting this as a bug.

-- 
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


[Rd] Modulus Bug (PR#7409)
ripley@stats.ox.ac.uk