[R] Number changed weirdly when converting to numeric

J C Nash
2025-03-09T18:19:00Z

This may be way off the mark, but is it possible that the ARM machine is using
the "new" IEEE-754 arithmetic that does not have 80 bit extended? The
standard
was changed (in ways to allow non-compliant systems to be compliant) because
ARM does not have the hardware registers. There are reasons why this might be
sensible, but we need more awareness of the consequences to avoid some of the
resulting changes in results. I've had to "fix" things that
weren't broken because
M1 and later Macs gave different outputs, actually not in my code but in
vignettes
where I compared to other packages.

Cheers,

John Nash
(who actually was part of 1985 IEEE 754 committee, though a VERY minor player)

On 2025-03-09 14:06, Stephanie Evert wrote:
> For once, that doesn't seem to be the issue here. The bug only seems to
happen on arm64 and doesn't reproduce on x86_64 hardware.
>
>> x <- as.numeric("-177253333.333333343267441")
>> sprintf("%.15f", x)
> [1] "-177253333.333333373069763"
>
> This is the number adjacent to -177253333.333333343267441 in IEEE 754.
>
>> writeBin(x, raw(8))
> [1] ac aa aa aa 57 21 a5 c1
>
> If you look at the hexadecimal representation, the least significant bit
appears to be off by one: the first byte should be 0xAB rather than 0xAC
(according to online calculators such as
https://numeral-systems.com/ieee-754-converter/).
>
> Seems that decimal-to-float conversion has a bug on arm64. Note that I get
the same result with
>
>> x <- -177253333.333333343267441
>
> so it's not specific to as.numeric().
>
> Best,
> Stephanie
>
>
>
>
>> On 9 Mar 2025, at 18:46, Jeff Newmiller via R-help <r-help at
r-project.org> wrote:
>>
>>
https://cran.r-project.org/doc/FAQ/R-FAQ.html#Why-doesn_0027t-R-think-these-numbers-are-equal_003f
>>
>> https://0.30000000000000004.com/
>>
>> On March 9, 2025 10:12:47 AM PDT, Christofer Bogaso
<bogaso.christofer at gmail.com> wrote:
>>> Hi,
>>>
>>> I have below simple conversion
>>>
>>>> sprintf("%0.15f",
as.numeric("-177253333.333333343267441"))
>>>
>>> [1] "-177253333.333333373069763"
>>>
>>> I could not figure out why the input and output is different?
>>>
>>> Clearly this conversion is incorrect. Is there any way to convert
to
>>> numerical properly?
>>>
>>>> sessionInfo()
>>>
>>> R version 4.4.0 (2024-04-24)
>>>
>>> Platform: aarch64-apple-darwin20
>>>
>>> Running under: macOS 15.3.1
>>>
>>>
>>> Matrix products: default
>>>
>>> BLAS:
/Library/Frameworks/R.framework/Versions/4.4-arm64/Resources/lib/libRblas.0.dylib
>>>
>>> LAPACK:
/Library/Frameworks/R.framework/Versions/4.4-arm64/Resources/lib/libRlapack.dylib;
>>> LAPACK version 3.12.0
>>>
>>>
>>> locale:
>>>
>>> [1] C/UTF-8/C/C/C/C
>>>
>>>
>>> time zone: Asia
>>>
>>> tzcode source: internal
>>>
>>>
>>> attached base packages:
>>>
>>> [1] stats     graphics  grDevices utils     datasets  methods
base
>>>
>>>
>>> loaded via a namespace (and not attached):
>>>
>>> [1] compiler_4.4.0
>>>
>>> ______________________________________________
>>> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more,
see
>>> https://stat.ethz.ch/mailman/listinfo/r-help
>>> PLEASE do read the posting guide
https://www.R-project.org/posting-guide.html
>>> and provide commented, minimal, self-contained, reproducible code.
>>
>> --
>> Sent from my phone. Please excuse my brevity.
>>
>> ______________________________________________
>> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide
https://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>
>
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