[R] cumsum vs. sum

Stavros Macrakis macrakis at alum.mit.edu
Wed Feb 18 18:41:27 CET 2009

Hmm.  Why not use the same method to guarantee the same result?  Or at
least document the possibility that cumsum(x)[length(x)] != sum(x)...
that seems like an easy trap to fall into.


On Wed, Feb 18, 2009 at 11:39 AM, Martin Maechler
<maechler at stat.math.ethz.ch> wrote:
>>>>>> "SM" == Stavros Macrakis <macrakis at alum.mit.edu>
>>>>>>     on Wed, 18 Feb 2009 10:00:40 -0500 writes:
>    SM> Nice!  Glad to hear it. It sounds as though it is still possible for
>    SM> cumsum(x)[length(x)] to not be exactly equal to sum, though?
> Well, possible, probably yes, platform-dependently;
> However I vaguely remember that I didn't see one such case in the few
> experiments I did.
> Martin
>    SM> On Wed, Feb 18, 2009 at 8:03 AM, Martin Maechler
>    SM> <maechler at stat.math.ethz.ch> wrote:
>    SM> ...
>    >> o   cumsum(x) and cumprod(x) for double precision x now use a long
>    >> double accumulator where available and so more closely match
>    >> sum() and prod() in potentially being more accurate.

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