[Rd] Modulus Bug (PR#7409)
tlumley at u.washington.edu
Thu Dec 9 16:02:04 CET 2004
On Thu, 9 Dec 2004 ripley at stats.ox.ac.uk wrote:
> 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
It was -- the poster originally asked about large integer representations
and said he was planning to use other software for large integers.
At the time I said that the fact %% didn't give some sort of error or
warning was a bug, and was planning to return either a warning or NA or
> 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 at geodecapital.com wrote:
>> R Developers,
>> 1000000000000000000 %% 11
>>  -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 at 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
> R-devel at stat.math.ethz.ch mailing list
More information about the R-devel