On 20/03/2013 12:56, Matthew Dowle wrote:
Hi,
Please consider the following :
x = as.integer(2^30-1)
[1] 1073741823
sum(c(rep(x, 10000000), rep(-x,9999999)))
[1] 1073741824
Tested on 2.15.2 and a recent R-devel (r62132).
I'm wondering if s in isum could be LDOUBLE instead of double, like
rsum, to fix this edge case?
No, because there is no guarantee that LDOUBLE differs from double (and
platform on which it does not).
Users really need to take responsibility for the numerical stability of
calcuations they attempt. Expecting to sum 20 million large numbers
exactly is unrealistic.
There are cases where 64-bit integer accumulators would be beneficial,
and this is one. Unfortunately C11 does not require them but some
optional moves in that direction are planned.
https://svn.r-project.org/R/trunk/src/main/summary.c
Thanks,
Matthew
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Brian D. Ripley, rip...@stats.ox.ac.uk
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