[R] Question about summing to zero
Jim Frederick
James.Frederick at uncp.edu
Tue Jul 18 21:27:39 CEST 2006
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
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