[R] question about precision, floor, and powers of two.

[Peter Dalgaard]

Uwe Ligges <ligges at statistik.uni-dortmund.de> writes:

> Dr Carbon wrote:
> 
> > At the risk of being beaten about the face and body, can somebody explain
> > why the middle example: log2(2^3); floor(log2(2^3)) is different than
> > examples 1 and 3?
> 
> 
> Because
> 
>  > log2(2^3) - 3
> [1] -4.440892e-16
> 
> see the R FAQ "Why doesn't R think these numbers are equal?".
> 
> Uwe Ligges

In this particular case, it is slightly odd that we can't get an exact
answer for operations that could in principle be carried out using
integer arithmetic, but we're actually calculating log(8)/log(2).

(Curiously, the same effect is not seen on Linux or Solaris until 

 > log2(2^29)-29
[1] 3.552714e-15

)
 
> > 
> >>log2(2^2); floor(log2(2^2))
> > 
> > [1] 2
> > [1] 2
> > 
> >>log2(2^3); floor(log2(2^3))
> > 
> > [1] 3
> > [1] 2
> > 
> >>log2(2^4); floor(log2(2^4))
> > 
> > [1] 4
> > [1] 4
> > 
> > 
> > DrC
> > 
> > 	[[alternative HTML version deleted]]
> > 
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