[R] Precision in estimating log

From: Gichangi, Anthony <anthony_at_stat.sdu.dk>
Date: Fri 19 May 2006 - 22:25:51 EST

Hi R-users,

I have the following code:

f <-function(x,p)sqrt(-(x^2)-2*log(1-p))

r1 <-sqrt(-2*log(1-0.95))

r2 <-sqrt(-2*log(0.05))

on executing i get the following results

> f(r1,0.95)

[1] 0
>
> f(r2,0.95)

[1] NaN
Warning message:
NaNs produced in: sqrt(-(x^2) - 2 * log(1 - p))

I tried to track the problem and found that the answer to log(0.05) is different from the answer to log(1-0.95) which is ofcourse not true and hence it causes problems in the code

 print(log(0.05),digit=22)

[1] -2.9957322735539909
> print(log(1-0.95),digit=22)

[1] -2.99573227355399

Any possible explanation ?

Regards
Anthony



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