R devel - May 2017 - precision of do_arith() in arithmetic.c

To the R development team:

First of all, thank you so much for maintaining wonderful R software.

Perhaps, Dr. Ahn has just reported an error on the wilcox.test() function,
and suggesting that an error may arise from abs() and rank().


I just had a quick check that the problem may come from the precision of
the results of arithmetic functions.


87.7-89.1+1.4
# > 87.7-89.1+1.4
# [1] 8.437695e-15

I checked that do_arith() in arithmetic.c is using double type (8 byte) for
PLUSOP, MINUSOP, TIMESOP, DIVOP etc.

https://github.com/wch/r-source/blob/f68b30e3b5479d84adbff516d48d4722a574dc82/src/main/arithmetic.c


I have two thoughts:

(1) in the rank() function, we may round at less than 14 decimal places by
default?

(2) using long double type (10 byte) instead of double type (8 byte) by
default in the arithmetic function could be helpful for preventing
embarassment?

Perhaps, R may provide the arithmetic function with various variable types,
if some application needs.


Thanks.

Min-hyung

	[[alternative HTML version deleted]]

> On 24 May 2017, at 04:24, M Kim <mkim0710 at gmail.com> wrote:
> 
> To the R development team:
> 
> First of all, thank you so much for maintaining wonderful R software.
> 
> Perhaps, Dr. Ahn has just reported an error on the wilcox.test() function,
> and suggesting that an error may arise from abs() and rank().
> 
> 
> I just had a quick check that the problem may come from the precision of
> the results of arithmetic functions.
> 
> 
> 87.7-89.1+1.4
> # > 87.7-89.1+1.4
> # [1] 8.437695e-15
> 
R FAQ 7.31.
See
https://cran.r-project.org/doc/FAQ/R-FAQ.html#Why-doesn_0027t-R-think-these-numbers-are-equal_003f

Berend
> I checked that do_arith() in arithmetic.c is using double type (8 byte) for
> PLUSOP, MINUSOP, TIMESOP, DIVOP etc.
> 
>
https://github.com/wch/r-source/blob/f68b30e3b5479d84adbff516d48d4722a574dc82/src/main/arithmetic.c
> 
> 
> I have two thoughts:
> 
> (1) in the rank() function, we may round at less than 14 decimal places by
> default?
> 
> (2) using long double type (10 byte) instead of double type (8 byte) by
> default in the arithmetic function could be helpful for preventing
> embarassment?
> 
> Perhaps, R may provide the arithmetic function with various variable types,
> if some application needs.
> 
> 
> Thanks.
> 
> Min-hyung
> 
> 	[[alternative HTML version deleted]]
> 
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