[R] exactly representable numbers

[Sundar Dorai-Raj]

Robin Hankin said the following on 9/11/2006 3:52 AM:
> 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?
> 
> 
> 

I believe this is what .Machine$double.eps is. From ?.Machine

double.eps: the smallest positive floating-point number 'x' such that
           '1 + x != 1'.  It equals 'base^ulp.digits' if either 'base'
           is 2 or 'rounding' is 0;  otherwise, it is '(base^ulp.digits)
           / 2'.

See also .Machine$double.neg.eps. Is this what you need?

--sundar