>>>>> Pavel Krivitsky via R-devel
>>>>> on Mon, 28 Apr 2025 05:13:41 +0000 writes:
> Hello, Under R 4.5.0 on Windows (x86-64), I get:
>> sqrt(.Machine$double.xmax)^2
> [1] Inf
>> sqrt(.Machine$double.xmax)*sqrt(.Machine$double.xmax)
> [1] Inf
> On other hand on other platforms, including Debian Linux
> (x86-64), I get:
d> sqrt(.Machine$double.xmax)^2
> [1] 1.797693134862315508561e+308
d> sqrt(.Machine$double.xmax)*sqrt(.Machine$double.xmax)
> [1] 1.797693134862315508561e+308
> Windows is running inside a VirtualBox instance on the
> same host as Linux. I don't have direct results from the
> MacOS platforms, but based on the symptoms that had led me
> to investigate, the behaviour is as the Linux.
> Adding to the mystery, if I implement the same operation in C, e.g.,
> library(inline)
> sqrsqrt <- cfunction(sig = c(x = "numeric"), language = "C", "
> double sqrtx = sqrt(Rf_asReal(x));
> return Rf_ScalarReal(sqrtx*sqrtx);
> ")
> R on Linux yields:
d> sqrsqrt(.Machine$double.xmax)
> [1] 1.797693134862315508561e+308
> i.e., the same number, whereas R on Windows yields:
d> sqrsqrt(.Machine$double.xmax)
> [1] 1.797693134862315508804e+308
> which is not Inf but not the same as Linux either.
> Lastly, on both platforms,
d> sqrsqrt(.Machine$double.xmax) < .Machine$double.xmax
> [1] TRUE
> I am not sure if this is a bug, intended behaviour, or something else.
"something else": It is not a bug, nor intended, but should
also *not* be surprising nor a mistery: The largest possible
double precision number is by definition "very close to
infinity" (in the space of double precision numbers)
[R 4.5.0 patched on Linux (Fedora 40; x86_64)] :
> (M <- .Machine$double.xmax)
[1] 1.797693e+308
> M+1 == M
[1] TRUE
> M*(1 + 2^-52)
[1] Inf
> print(1 + 2^-52, digits= 16)
[1] 1
> print(1 + 2^-52, digits= 17)
[1] 1.0000000000000002
>
What you see, I'd classify as quite related to R FAQ 7.31,
:= "the most frequently asked question about R
among all the other frequently asked questions"
A nice reading is the README you get here
https://github.com/ThinkR-open/seven31
which does link also to the R FAQ at
https://cran.r-project.org/doc/FAQ/R-FAQ.html#Why-doesn_0027t-R-think-these-numbers-are-equal_003f
Of tangential interest only:
You mention that it is R 4.5.0 you use on Windows.
Would you (or anybody else) know if this is new behaviour or it
also happened e.g. in R 4.4.x versions on Windows?
Best regards,
Martin
--
Martin Maechler
ETH Zurich and R Core team
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