[R] exactly representable numbers

Robin Hankin r.hankin at noc.soton.ac.uk
Mon Sep 11 10:52:03 CEST 2006


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  
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

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

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