# [Rd] sum overflow (PR#1091)

**Uwe Ligges
**
ligges@statistik.uni-dortmund.de

*Thu, 13 Sep 2001 14:16:36 +0200*

Bill Simpson wrote:
>*
*>* > It's not a problem with sum:
*>* >
*>* > > sum(a*a)
*>* > [1] 333833500
*>* > > sum(b*b)
*>* > [1] 333833500
*>* >
*>* > are accurate.
*>* >
*>* > The overflow is in the integer arithmetic for *. That's a question for
*>* > your C run-time system. On a 64-bit machine you might get different
*>* > results (although most use 32-bit ints, including mine).
*>* >
*>* > If you use integers you need to be aware of the consequences. It's a
*>* > feature not a bug.
*>* I thought R used an internal rep that was double in all cases.
*>* Now I'm confused:
*>* > a<-(1:1000)
*>* > b<-(1:1000)
*>* > sum(a*a)*sum(b*b)
*>* [1] -652010736
*>* > a<-(1:1000)/1.0
*>* > b<-(1:1000)/1.0
*>* > sum(a*a)*sum(b*b)
*>* [1] 1.114448e+17
*>*
*>* So R somehow decides whether to use an integer or a double
*>* representation? Please tell me the rule used by R so I will know in the
*>* future.
*
It depends on the function you are using.
In your case you could use:
as.double(sum(a*a)) * as.double(sum(b*b))
or just check if the result is double:
is.double(sum(a*a))
Uwe Ligges
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