[R] exactly representable numbers

[Robin Hankin]

Hi

Given a real number x,  I want to know how accurately R can represent  
numbers near x.

In particular, I want to know the infinum of exactly representable
numbers greater than x, and the supremum of exactly representable  
numbers
less than x.  And then the interesting thing is the difference  
between these two.


I have a little function that does some of this:


f <- function(x,FAC=1.1){
   delta <- x
while(x+delta > x){
   delta <- delta/FAC
}
return(delta*FAC)
}

But this can't be optimal.

Is there a better way?



--
Robin Hankin
Uncertainty Analyst
National Oceanography Centre, Southampton
European Way, Southampton SO14 3ZH, UK
  tel  023-8059-7743