wrong answer for simple expressions

I am experiencing strange (to me) output when trying to do simple
calculations.  Expressions that should equal zero yield non-zero
values.

No. There is no reason why these expressions should yield zero values. 
Remember that computers work in base 2, and that 0.1 has an infinitely 
recurring binary expansion in base 2.  You should expect that 0.1 computed 
two different ways should differ in the last few bits. You have managed to 
get zero to 52 bits accuracy, which is not bad when you consider that the 
machine only works to 54 bits.

 	-thomas

Examples:

a <- 4.1-3.1
b <- 5.1-4.1
a-b

[1] -4.440892e-16

(4.1-3.1)-(5.1-4.1)

[1] -4.440892e-16


When this last expression is expanded, I get the right answer:

4.1-3.1-5.1+4.1

[1] 0


I am using the binary packaged version R-2.0.0-0.fdr.1.fc2.i386.rpm for
Linux Fedora Core 2.  I had the same problem with version 1.9.0-0

Can anyone tell me what is going on?

Thanks,


Drew Hoysak

______________________________________________
R-help at 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

Thomas Lumley			Assoc. Professor, Biostatistics
tlumley at u.washington.edu	University of Washington, Seattle