[R] cumsum vs. sum

[Gabor Grothendieck]

Check out sum.exact and cumsum.exact in the caTools package.

> library(caTools)
Loading required package: bitops
> x <- 1/(12:14)
> sum(x) - cumsum(x)[3]
[1] 2.775558e-17
> sum.exact(x) - cumsum.exact(x)[3]
[1] 0


On Tue, Feb 17, 2009 at 5:12 PM, Stavros Macrakis <macrakis at alum.mit.edu> wrote:
> I recently traced a bug of mine to the fact that cumsum(s)[length(s)]
> is not always exactly equal to sum(s).
>
> For example,
>
>     x<-1/(12:14)
>     sum(x) - cumsum(x)[3]  => 2.8e-17
>
> Floating-point addition is of course not exact, and in particular is
> not associative, so there are various possible reasons for this.
> Perhaps sum uses clever summing tricks to get more accurate results?
> In some quick experiments, it does seem to get more accurate results
> than cumsum.
>
> It might be worth documenting.
>
>             -s
>
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