bug in sum() on integer vector

Wed, Dec 14, 2011 1:16 PM

I agree that where the overflow occurs is not critical (one can go back to cumsum and find
out). I am assuming that Uwe still wants to know there has been an overflow at some point
i.e., a warning. This could become more "interesting" as parallel computation causes
different summation orderings on sums of large numbers of items.

JN

On 12/14/2011 03:58 PM, Uwe Ligges wrote:


On 14.12.2011 17:19, peter dalgaard wrote:

On Dec 14, 2011, at 16:19 , John C Nash wrote:

Following this thread, I wondered why nobody tried cumsum to see where the integer
overflow occurs. On the shorter xx vector in the little script below I get a message:

Warning message:
Integer overflow in 'cumsum'; use 'cumsum(as.numeric(.))'

But sum() does not give such a warning, which I believe is the point of contention. Since
cumsum() does manage to give such a warning, and show where the overflow occurs, should
sum() not be able to do so? For the record, I don't class the non-zero answer as an error
in itself. I regard the failure to warn as the issue.

It (sum) does warn if you take the two "halves" separately. The issue is that the
overflow is detected at the end of the summation, when the result is to be saved to an
integer (which of course happens for all intermediate sums in cumsum)

x<- c(rep(1800000003L, 10000000), -rep(1200000002L, 15000000))
sum(x[1:10000000])

[1] NA
Warning message:
In sum(x[1:1e+07]) : Integer overflow - use sum(as.numeric(.))

sum(x[10000001:25000000])

[1] NA
Warning message:
In sum(x[10000001:1.5e+07]) : Integer overflow - use sum(as.numeric(.))

sum(x)

[1] 4996000

There's a pretty easy fix, essentially to move

     if(s>  INT_MAX || s<  R_INT_MIN){
         warningcall(call, _("Integer overflow - use sum(as.numeric(.))"));
         *value = NA_INTEGER;
     }

inside the summation loop. Obviously, there's a speed penalty from two FP comparisons
per element, but I wouldn't know whether it matters in practice for anyone.


I don't think I am interested in where the overflow happens if I call sum()...

Uwe

bug in sum() on integer vector

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