R calculates small numbers, where the result should be zero

See R Faq (http://cran.r-project.org/doc/FAQ/R-FAQ.html) with particular attention to FAQs 7.31 and 9.1 paragraph 3.

Essentially, computational precision is finite.

S Ellison
-----Original Message-----
From: r-help-bounces at r-project.org 
[mailto:r-help-bounces at r-project.org] On Behalf Of Andrea Gonnermann
Sent: 11 September 2012 15:24
To: r-help at r-project.org
Subject: [R] R calculates small numbers, where the result 
should be zero

Hi all,

my problem is that R does wrong calculations, when the result 
should be zero.
I have the following code:

 RD_Var1 <- ((0.1*(1-0.1))/100)+((0.2*(1-0.2))/100)
 RD_Var2 <- ((0.1*(1-0.1))/100)+((0.2*(1-0.2))/100)
 RD_1 <- 0.1-0.2
 RD_1
 RD_2 <- 0.1-0.2
 FEM_Eff_RD <- 
((1/RD_Var1)*RD_1+(1/RD_Var2)*RD_2)/(1/RD_Var1+1/RD_Var2)
 Q_RD    <- 
1/RD_Var1*((RD_1-FEM_Eff_RD)^2)+1/RD_Var2*((RD_2-FEM_Eff_RD)^2)
 Q_RD
My result is:
1.540744e-31
When I put the calculated numbers directly into the last 
formula, I get zero, which is the correct answer:
Q_RD    <- 1/0.0025*((-0.1--0.1)^2)+1/0.0025*((-0.1--0.1)^2)

Why does R calculate the other number instead of zero?

Thank you very much for your help!

______________________________________________
R-help at r-project.org 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.

*******************************************************************
This email and any attachments are confidential. Any use...{{dropped:8}}

R calculates small numbers, where the result should be zero

Thread (5 messages)