[R] scan seems to modify the data

[Prof Brian Ripley]

On 31 Mar 2004, Peter Dalgaard wrote:

> Duncan Murdoch <dmurdoch at pair.com> writes:
> 
> > > print(x,17)
> > [1] 0.26600000000000001 0.25100000000000000
> ...
> > > print(x,17)
> > [1] 0.266
> > 
> > I don't know why the second print() prints 0.266 differently from the
> > first one.  (This is in the 1.9.0 beta in Windows).
> 
> To get the same number of decimals as the other guy. The interesting
> question is why 0.251 gets all those trailing zeros:
> 
> > print(0.251,17)
> [1] 0.25100000000000000
> > print(0.253,17)
> [1] 0.25300000000000000
> > print(0.255,17)
> [1] 0.255

Take a look at formatReal.  scientific thinks 0.251 has 17 digits and
0.255 has 3.  It really doesn't make any sense to ask for more precision 
than you have (.Machine$double.eps) and you do often get spurious 
errors if you attempt to do so.  So 15 digits is normally safe, but no 
more.

Note that there are decimal -> binary -> decimal conversions and you 
can't say which one introduced the small changes.

-- 
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