[R] Number changed weirdly when converting to numeric

[Stephanie Evert]

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 using 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 using 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 using 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 using 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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