[R] as.character(seq(-.35,.95,.1))

[Petr PIKAL]

Hi

r-help-bounces at r-project.org napsal dne 20.11.2007 17:07:42:

> On 11/20/2007 10:50 AM, Ken Fullish wrote:
> >  > as.character(seq(-.25,.95,.1))
> >   [1] "-0.25" "-0.15" "-0.05" "0.05"  "0.15"  "0.25"  "0.35"  "0.45" 
> > "0.55"  "0.65"  "0.75"  "0.85"  "0.95"
> > 
> >  > as.character(seq(-.35,.95,.1))
> >   [1] "-0.35"               "-0.25" 
> > "-0.15"               "-0.0499999999999999" "0.05"
> >   [6] "0.15"                "0.25" 
> > "0.35"                "0.45"                "0.55"
> > [11] "0.65"                "0.75" 
> > "0.85"                "0.95"
> > 
> > Not a big deal, just curiosity:
> > Why do I obtain this "ugly" "-0.0499999999999999" instead of the 
> > expected "-0.05" ?
> 
> Because as.character() tries to do an accurate conversion, and the 
> number in your vector is closer to -0.0499999999999999 than to -0.05.
> You could get the "-0.05" by something like round( seq(...), 2).
> 
> The reason seq() doesn't give you exactly -0.05 is that the starting 
> values and step size you've chosen are not exactly representable in R's 
> floating point format.   It can only store fractions exactly when the 
> denominator is a power of 2.

And it is not related exclusively to R but to floating point arithmetic in 
computer binary language.

Regards
Petr

> 
> Duncan Murdoch
> 
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