[R-SIG-Mac] Inaccurate inaccuracies on Mac only?
Hans W Borchers
hwborchers at gmail.com
Sat Nov 30 14:54:47 CET 2013
Dear R Mac colleagues,
when I am running the following piece of code on Ubuntu Linux 12.04 LTS or on
Windows 7 -- with R 3.0.2 --, I get the results as indicated:
set.seed(8237)
x <- runif(32, -1, 1)
y <- runif(32, -1, 1)
u <- cbind(x[1], y[1])
u1 <- u %*% t(u)
u2 <- apply(u * u, 1, sum)
sqrt(u2 + u2 - 2*u1)
## [,1]
## [1,] NaN
## Warning message:
## In sqrt(u2 + u2 - 2 * u1) : NaNs produced
u2 + u2 - 2*u1
## [,1]
## [1,] -2.220446e-16
Theoretically, the last value should be zero. Of course, I am *not* surprised
to see this is not the case under finite precision arithmetics.
But what did surprise me was that under Mac OS X 10.6 and R 3.0.2 I get the
following, exact results:
set.seed(8237)
x <- runif(32, -1, 1)
y <- runif(32, -1, 1)
u <- cbind(x[1], y[1])
u1 <- u %*% t(u)
u2 <- apply(u * u, 1, sum)
sqrt(u2 + u2 - 2*u1)
## [,1]
## [1,] 0
u2 + u2 - 2*u1
## [,1]
## [1,] 0
I always thought that all these systems and versions comply to the IEEE
floating point standard and return the same (accurate or inaccurate) answers.
The problem here is: When I test a package on Mac, I cannot be sure it will
run without errors on other systems. Actully, I found this when running some
examples from help pages, written on the Mac.
Was I wrong, or is there something special about the Mac version of R ?
Many thanks
Hans Werner
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