[Rd] order of operations

[Gabor Grothendieck]

It could be that the two sides of * are run in parallel in the future and maybe
not having a guarantee would simplify implementation?


On Fri, Aug 27, 2021 at 12:35 PM Avi Gross via R-devel
<r-devel using r-project.org> wrote:
>
> Does anyone have a case where this construct has a valid use?
>
> Didn't Python  add a := operator recently that might be intended more for
> such uses as compared to using the standard assignment operators? I wonder
> if that has explicit guarantees of what happens in such cases, but that is
> outside what this forum cares about. Just for the heck of it, I tried the
> example there:
>
>         >>> (x := 1) * (x := 2)
>         2
>         >>> x
>         2
>
> Back to R, ...
>
> The constructs can get arbitrarily complex as in:
>
> (x <- (x <- 0) + 1) * (x <- (x <-2) + 1)
>
> My impression is that when evaluation is left to right and also innermost
> parentheses before outer ones, then something like the above goes in stages.
> The first of two parenthetical expressions is evaluated first.
>
> (x <- (x <- 0) + 1)
>
> The inner parenthesis set x to zero then the outer one increments x to 1.
> The full sub-expression evaluates to 1 and that value is set aside for a
> later multiplication.
>
> But then the second parenthesis evaluates similarly, from inside out:
>
> (x <- (x <-2) + 1)
>
> It clearly resets x to 2 then increments it by 1 to 3 and returns a value of
> 3. That is multiplied by the first sub-expression to result in 3.
>
> So for simple addition, even though it is commutative, is there any reason
> any compiler or interpreter should not follow rules like the above?
> Obviously with something like matrices, some operations are not abelian and
> require more strict interpretation in the right order.
>
> And note the expressions like the above can run into more complex quandaries
> such as when you have a conditional with OR or AND parts that may be
> short-circuited and in some cases, a variable you expected to be set, may
> remain unset or ...
>
> This reminds me a bit of languages that allow pre/post increment/decrement
> operators like ++ and -- and questions about what order things happen.
> Ideally, anything in which a deterministic order is not guaranteed should be
> flagged by the language at compile time (or when interpreted) and refuse to
> go on.
>
> All I can say with computer languages and adding ever more features,
>         with greater power comes greater responsibility and often greater
> confusion.
>
>
> -----Original Message-----
> From: R-devel <r-devel-bounces using r-project.org> On Behalf Of Gabor
> Grothendieck
> Sent: Friday, August 27, 2021 11:32 AM
> To: Thierry Onkelinx <thierry.onkelinx using inbo.be>
> Cc: r-devel using r-project.org
> Subject: Re: [Rd] order of operations
>
> I agree and personally never do this but I would still like to know if it is
> guaranteed behavior or not.
>
> On Fri, Aug 27, 2021 at 11:28 AM Thierry Onkelinx <thierry.onkelinx using inbo.be>
> wrote:
>
> > IMHO this is just bad practice. Whether the result is guaranteed or
> > not, doesn't matter.
> >
> > ir. Thierry Onkelinx
> > Statisticus / Statistician
> >
> > Vlaamse Overheid / Government of Flanders INSTITUUT VOOR NATUUR- EN
> > BOSONDERZOEK / RESEARCH INSTITUTE FOR NATURE AND FOREST Team Biometrie
> > & Kwaliteitszorg / Team Biometrics & Quality Assurance
> > thierry.onkelinx using inbo.be Havenlaan 88 bus 73, 1000 Brussel www.inbo.be
> >
> >
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> > a given body of data.
> > ~ John Tukey
> >
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> >
> > <https://www.inbo.be>
> >
> >
> > Op vr 27 aug. 2021 om 17:18 schreef Gabor Grothendieck <
> > ggrothendieck using gmail.com>:
> >
> >> Are there any guarantees of whether x will equal 1 or 2 after this is
> run?
> >>
> >> (x <- 1) * (x <- 2)
> >> ## [1] 2
> >> x
> >> ## [1] 2
> >>
> >> --
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> >>
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> >>
> >
>
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