Re: [Rd] Modulus Bug (PR#7409)

From: <ripley_at_stats.ox.ac.uk>
Date: Thu 09 Dec 2004 - 19:42:12 EST


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

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Received on Thu Dec 09 19:56:40 2004

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