[R] Floating point imprecision in sum() under R-2.3.1?

[Brahm, David]

After upgrading to R-2.3.1 on Linux Redhat, I was suprised by this:

R> x <- c(721.077, 592.291, 372.208, 381.182)
R> sum(x) - 2066.758
   [1] 4.547474e-13

Now I understand that floating point arithmetic is not precise, but
1) the result is exactly 0 in R-2.2.1 (patched) on the same machine,
2) .Machine$double.eps = 2.2e-16, so the error seems quite large.

Also note I get the same result on R-2.3.1 under Windows, and that
R> (721.077 + 592.291 + 372.208 + 381.182) - 2066.758
   [1] 0

Is this related to the (2.3.0) NEWS item:
  sum(), prod(), mean(), rowSums() and friends use a long double
  accumulator where available and so may be more accurate. 
and should I be concerned?  Thanks.

-- David Brahm (brahm at alum.mit.edu)


Version:
 platform = i686-pc-linux-gnu
 arch = i686
 os = linux-gnu
 system = i686, linux-gnu
 status = 
 major = 2
 minor = 3.1
 year = 2006
 month = 06
 day = 01
 svn rev = 38247
 language = R
 version.string = Version 2.3.1 (2006-06-01)

Locale:
C

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