[R] Numerical precision problem

From: <C.Rosa_at_lse.ac.uk>
Date: Fri, 21 Dec 2007 19:52:29 -0000


Dear All,  

I have a numerical problem:

R, as other statistical software, can tell the difference between very small numbers and 0, while apparently cannot distinguish a number close to 1 and 1. In the example below, is it possible to instruct R to recognize that q is different from 1?

> p=.00000000000000000000000000000001

> p==0

[1] FALSE
> q=.99999999999999999999999999999

> q==1

[1] TRUE Interestingly,

> (1-p)==1

[1] TRUE The main problem I have is that I need to evaluate the inverse of the normal Cumulative Distribution Function near 1 (but not exactly 1) and the PC cannot recognize it.

Thank you very much for your help!

Carlo

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