From: Jim Frederick <James.Frederick_at_uncp.edu>

Date: Wed 19 Jul 2006 - 05:27:39 EST

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 Wed Jul 19 06:45:41 2006

Date: Wed 19 Jul 2006 - 05:27:39 EST

Hi,

When I used

x <- c(10,10,10,10,5,5,5,5)

sum(x)

x <- x - .Last.value/8

the result was zero, as I expected.

However, when I used

x <- rnorm(101,0,10)

sum(x)

x <- x - .Last.value/101

sum(x)

I did not get zero, but -2.664535e-15. OK, that's fairly close to
zero, but I tried to improve the figure. Each time I repeated the last
two commands, the sum got closer to zero. The series seemed to converge
on -1.30e-15, so changed the 101 to 50, then to 20, and then to 10. I
had sum(x) down to -5.551115e-17 before I quit.

If R can store values of x which sum to -5.551115e-17, then why can't I get that value on the first try? Is this a bug or just rounding error?

Jim Frederick

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 Wed Jul 19 06:45:41 2006

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