[Rd] mean(x) != mean(rev(x)) different with x <- c(NA, NaN) for some builds

[Hervé Pagès]

On 03/31/2017 10:14 PM, Prof Brian Ripley wrote:
> From ?NA
>
>      Numerical computations using ‘NA’ will normally result in ‘NA’: a
>      possible exception is where ‘NaN’ is also involved, in which case
>      either might result.
>
> and ?NaN
>
>      Computations involving ‘NaN’ will return ‘NaN’ or perhaps ‘NA’:
>      which of those two is not guaranteed and may depend on the R
>      platform (since compilers may re-order computations).
>
> fortunes::fortune(14) applies (yet again).

The problem is that TFM often contradicts itself e.g. in ?prod:

      If ‘na.rm’ is ‘FALSE’ an ‘NA’ value in any of the arguments will
      cause a value of ‘NA’ to be returned, otherwise ‘NA’ values are
      ignored.

which is clearly not the case (at least for me):

   > x <- c(NaN, NA)
   > prod(x)
   [1] NaN

H.

>
> On 01/04/2017 04:50, Henrik Bengtsson wrote:
>> In R 3.3.3, I observe the following on Ubuntu 16.04 (when building
>> from source as well as for the sudo apt r-base build):
>>
>>> x <- c(NA, NaN)
>>> mean(x)
>> [1] NA
>>> mean(rev(x))
>> [1] NaN
>>
>>> rowMeans(matrix(x, nrow = 1, ncol = 2))
>> [1] NA
>>> rowMeans(matrix(rev(x), nrow = 1, ncol = 2))
>> [1] NaN
>>
>>> .rowMeans(x, m = 1, n = 2)
>> [1] NA
>>> .rowMeans(rev(x), m = 1, n = 2)
>> [1] NaN
>>
>>> .rowSums(x, m = 1, n = 2)
>> [1] NA
>>> .rowSums(rev(x), m = 1, n = 2)
>> [1] NaN
>>
>>> rowSums(matrix(x, nrow = 1, ncol = 2))
>> [1] NA
>>> rowSums(matrix(rev(x), nrow = 1, ncol = 2))
>> [1] NaN
>>
>> I'd expect NA to trump NaN in all cases (with na.rm = FALSE).  sum()
>> does not have this problem and returns NA in both cases (*).
>>
>> For the same R version build from source on RHEL 6.6 system
>> (completely different architecture), I get the expected result (= NA)
>> for all of the above cases, e.g.
>>
>>> x <- c(NA, NaN)
>>> mean(x)
>> [1] NA
>>> mean(rev(x))
>> [1] NA
>> [...]
>>
>> Before going insane trying to troubleshoot this, I have a vague memory
>> that this, or something related to this, has been discussed
>> previously, but I cannot locate it.
>>
>> Is the above a bug in R, a FAQ, a build error, overzealous compiler
>> optimization, and / or ...?
>>
>> Thanks,
>>
>> Henrik
>
>

-- 
Hervé Pagès

Program in Computational Biology
Division of Public Health Sciences
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