[Rd] double in summary.c : isum

[Matthew Dowle]

On 23.03.2013 12:01, Prof Brian Ripley wrote:
> On 20/03/2013 12:56, Matthew Dowle wrote:
>>
>> Hi,
>>
>> Please consider the following :
>>
>>> x = as.integer(2^30-1)
>> [1] 1073741823
>>> sum(c(rep(x, 10000000), rep(-x,9999999)))
>> [1] 1073741824
>>
>> Tested on 2.15.2 and a recent R-devel (r62132).
>>
>> I'm wondering if s in isum could be LDOUBLE instead of double, like
>> rsum, to fix this edge case?
>
> No, because there is no guarantee that LDOUBLE differs from double
> (and platform on which it does not).

That's a reason for not using LDOUBLE at all isn't it? Yet src/main/*.c 
has 19 lines using LDOUBLE e.g. arithmetic.c and cum.c as well as 
summary.c.

I'd assumed LDOUBLE was being used by R to benefit from long double (or 
equivalent) on platforms that support it (which is all modern Unix, Mac 
and Windows as far as I know). I do realise that the edge case wouldn't 
be fixed on platforms where LDOUBLE is defined as double.

What have I misunderstood?

>
> Users really need to take responsibility for the numerical stability
> of calcuations they attempt.  Expecting to sum 20 million large
> numbers exactly is unrealistic.

Trying to take responsibility, but you said no. Changing from double to 
LDOUBLE would mean that something that wasn't realistic, was then 
realistic (on platforms that support long double).

And it would bring open source R into line with TERR, which gets the 
answer right, on 64bit Windows at least. But I'm not sure I should be as 
confident in TERR as I am in open source R because I can't see its 
source code.


> There are cases where 64-bit integer accumulators would be
> beneficial, and this is one.  Unfortunately C11 does not require them
> but some optional moves in that direction are planned.
>
>>
>> https://svn.r-project.org/R/trunk/src/main/summary.c
>>
>> Thanks,
>> Matthew
>>
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