From: Robin Hankin <r.hankin_at_noc.soton.ac.uk>

Date: Mon 11 Sep 2006 - 10:15:04 GMT

Date: Mon 11 Sep 2006 - 10:15:04 GMT

Hi Sundar

thanks for this. But I didn't make it clear that I'm interested in
extreme numbers

such as 1e300 and 1e-300.

Then

> f(1e300)

[1] 7.911257e+283

is different from

1e300*.Machine$double.eps

[I'm interested in the gap between successive different exactly
representable

numbers right across the IEEE range]

rksh

On 11 Sep 2006, at 10:50, Sundar Dorai-Raj wrote:

*>
**>
*

> 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 infimum 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
*

-- Robin Hankin Uncertainty Analyst National Oceanography Centre, Southampton European Way, Southampton SO14 3ZH, UK tel 023-8059-7743 ______________________________________________ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.Received on Mon Sep 11 21:50:29 2006

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