On Tue, Aug 23, 2011 at 6:56 PM, Daniel Lai <dan...@bccrc.ca> wrote:
> Hi Barry,
>
> Shock and horror indeed, addition is not _associative_, at least for
> floating point arithmetic [1].
>
> Flipping the order of the operands seems to be the explanation of the
> discrepancies between R and MATLAB as you suggest:
>
> In R:
> a = 0.812672
> b = 0.916541
> c = 0.797810
> sprintf('%.30f', a+b+c)
> [1] "2.527023000000000241271891354700"
> sprintf('%.30f', c+a+b)
> [1] "2.527022999999999797182681504637"
> sprintf('%.30f', sum(c(a,b,c)))
> [1] "2.527022999999999797182681504637"
>
> In MATLAB:
> a = 0.812672;
> b = 0.916541;
> c = 0.797810;
> sprintf('%.30f', a+b+c)
> ans = 2.527023000000000241271891354700
> sprintf('%.30f', c+a+b)
> ans = 2.527022999999999797182681504637
> sprintf('%.30f', sum([a,b,c]))
> ans = 2.527023000000000241271891354700
>
> From this, I'll have to conclude that R's sum() is correct, but probably
> internally flipping operands.
>Also check out sumexact in the caTools package. -- Statistics & Software Consulting GKX Group, GKX Associates Inc. tel: 1-877-GKX-GROUP email: ggrothendieck at gmail.com ______________________________________________ R-help@r-project.org 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.
