>>>>> Pavel Krivitsky
>>>>> on Wed, 28 May 2025 01:36:57 +0000 writes:
> Dear All,
> Thanks for looking into it, and apologies for bumping this.
> I think the strangest thing for me is that the C and the R
> implementations on Windows yield different results. I don't know if it
> warrants a deeper look.
This is not so strange, really:
In many cases (and some similar to this one), we *did* want R to
be platform independent and give the same results independent of
platform. ... consequently differing on some platforms from
their platform dependent C libraries ..
Martin
> Ultimately, I rewrote the code that relied on this behaviour. (I needed
> something that was a finite number when squared but was still as big as
> possible, so I divided it 1 + Machine epsilon, and it seems to work for
> my particular situation.)
> Best,
> Pavel
> On Tue, 2025-04-29 at 15:54 +0200, Tomas Kalibera wrote:
>>
>> On 4/29/25 12:00, Martin Maechler wrote:
>> > > > > > > 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$double.xmax)
>> > [1] 1.797693e+308
>> > > M+1 == M
>> > [1] TRUE
>> > > M*(1 + 2^-52)
>> > [1] Inf
>> > > print(1 + 2^-52, digits= 16)
>> > [1] 1
>> > > print(1 + 2^-52, digits= 17)
>> > [1] 1.0000000000000002
>> > What you see, I'd classify as quite related to R FAQ 7.31,
>> > := "the most frequently asked question about R
>> > among all the other frequently asked questions"
>> >
>> > A nice reading is the README you get here
>> > https://github.com/ThinkR-open/seven31
>> > which does link also to the R FAQ at
>> >
>> >
https://cran.r-project.org/doc/FAQ/R-FAQ.html#Why-doesn_0027t-R-think-these-numbers-are-equal_003f
>> >
>> > Of tangential interest only:
>> > You mention that it is R 4.5.0 you use on Windows.
>> > Would you (or anybody else) know if this is new behaviour or it
>> > also happened e.g. in R 4.4.x versions on Windows?
>>
>> This depends on C math function sqrt. With sqrt implemented in mingw-
>> w64
>> (mingwex), which is still the case of mingw-w64 v11, so still of
>> Rtools45, the result of sqrt(.Machine$double.xmax) is
>>
>> "0x1p+512"
>>
>> with mingw-w64 v12 (so with UCRT, and also on Linux, and also using
>> mpfr
>> library) it is
>>
>> "0x1.fffffffffffffp+511"
>>
>> It is not required by any standard for the math functions in the C
>> math
>> library to be precise (correctly rounded). But still, in this case,
>> the
>> correctly rounded value would be returned once Rtools can switch to
>> mingw-w64 v12 (which could be no sooner than Rtools46, as mingw-w64
>> is a
>> core component of the toolchain).
>>
>> A bit of advertising - I used Martin's Rmpfr package to compute this
>> using mpfr.
>> And there is a related blog post:
>>
https://blog.r-project.org/2025/04/24/sensitivity-to-c-math-library-and-mingw-w64-v12
>>
>> Best
>> Tomas
>>
>>
>> >
>> >
>> > Best regards,
>> > Martin
>> >
>> > --
>> > Martin Maechler
>> > ETH Zurich and R Core team
>> >
>> > ______________________________________________
>> > R-devel@r-project.org mailing list
>> > https://stat.ethz.ch/mailman/listinfo/r-devel
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