[Rd] sum overflow (PR#1091)

[Bill Simpson]

> 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.

Thanks!

Bill

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