>>>>> Duncan Murdoch
>>>>> on Mon, 2 Mar 2020 04:43:53 -0500 writes:
> On 02/03/2020 3:24 a.m., Martin Maechler wrote:
>>>>>>> robin hankin
>>>>>>> on Sun, 1 Mar 2020 09:26:24 +1300 writes:
>>
>> > Thanks guys, I guess I should have referred to FAQ 7.31
>> > (which I am indeed very familiar with) to avoid
>> > misunderstanding. I have always used dput() to clarify
>> > 7.31-type issues.
>>
>> > The description in ?dput implies [to me at any rate] that
>> > there will be no floating-point roundoff in its output. I
>> > hadn't realised that 'deparsing' as discussed in dput.Rd
>> > includes precision roundoff issues.
>>
>> > I guess the question I should have asked is close to
>> > Ben's: "How to force dput() to return an exact
>> > representation of a floating point number?". Duncan's
>> > reply is the insight I was missing: exact decimal
>> > representation of a double might not be possible (this had
>> > not occurred to me). Also, Duncan's suggestion of control
>> > = c("all", "hexNumeric") looks good and I will experiment
>> > with this.
>>
>> This was not Duncan's suggestion but rather Duncan's *citation* :
>> Note that he used " .... " !
>>
>> The citation is from ?deparseOpts (to which one is pointed when
reading ?dput),
>> <rant>
>> but unfortunately many people nowadays have stopped reading texts
>> that are longer than a tweet... ;-)
>> <rant/>
>> ... and indeed, ?dput and ?deparse use 'control = "all"'
>> instead of c("all", "hexNumeric") when talking about getting
>> close to an inverse of parse()
>>
>> As a matter of fact, within R Core we had discussed this, many
>> moons ago and actually had more or less decided to make "all"
>> to *include* "digits17".
>>
>> "digits17" is "almost always" (I'm sorry I cannot quantify the
>> 'almost' here) sufficient ... and is obviously conflicting with
>> using hexadecimals instead of digits.
>>
>> For R 4.0.0, I think we should finally consider doing something
>> here :
>>
>> 1) define "all" to include "digits17"
>> so new "all" is current c("all", "digits17")
>> {in a way such that c("all", "hexNumeric") implicitly removes
>> "digits17" (as it's in contradiction with "hexNumeric").
>>
>> 2) add a new option "AllHex" := c("all", "hexNumeric"),
>> (Note the capital "A": such that match.arg()-like abbreviation
>> of .deparseOpts() arguments remain possible and notably "all"
>> does not suddenly become ambiguous)
>>
>> Of course, '1)' is well possible without '2)',
>> but '2)' would allow to use dput(*, control = "All")
>> which is somewhat easier to readers & writers.
> I think 1) is a good idea, and adding something with the meaning of
> AllHex seems useful: but that's not a name I'd choose, since it's not
> consistent with the other names (which are almost all camelCase). I'd
> choose something like "exact" (even though it isn't :-).
Thank you -- you are right;
all "AllHex" is too non-orthodox and hence a pain for people to
get right, remember, etc.
In light of Steven Dirkse's reply (and other much older e-mails
by others I remember only vaguely), it seems we still need to
find an example (with numbers) where it is not exact ...
which makes "exact" even more appropriate.
Martin
>> > On Sun, Mar 1, 2020 at 6:22 AM Duncan Murdoch
>> > <murdoch.dun...@gmail.com> wrote:
>> >>
>> >> On 29/02/2020 4:19 a.m., Ben Bolker wrote:
>> >> >
>> >> > I think Robin knows about FAQ 7.31/floating point
>> >> (author of > 'Brobdingnag', among other numerical
>> >> packages). I agree that this is > surprising (to me).
>> >> >
>> >> > To reframe this question: is there way to get an
>> >> *exact* ASCII > representation of a numeric value (i.e.,
>> >> guaranteeing the restored value > is identical() to the
>> >> original) ?
>> >> >
>> >> > .deparseOpts has
>> >> >
>> >> > ‘"digits17"’: Real and finite complex numbers are
>> >> output using > format ‘"%.17g"’ which may give more
>> >> precision than the > default (but the output will depend
>> >> on the platform and there > may be loss of precision when
>> >> read back).
>> >> >
>> >> > ... but this still doesn't guarantee that all precision
>> >> is kept.
>> >>
>> >> "Using control = c("all", "hexNumeric") comes closest to
>> >> making deparse() an inverse of parse(), as representing
>> >> double and complex numbers as decimals may well not be
>> >> exact. However, not all objects are deparse-able even
>> >> with this option. A warning will be issued if the
>> >> function recognizes that it is being asked to do the
>> >> impossible."
>> >>
>> >> >
>> >> > Maybe
>> >> >
>> >> > saveRDS(x,textConnection("out","w"),ascii=TRUE) >
>> >> identical(x,as.numeric(out[length(out)])) ## TRUE
>> >> >
>> >> > ?
>> >> >
>> >> >
>> >> >
>> >> >
>> >> > On 2020-02-29 2:42 a.m., Rui Barradas wrote: >> Hello,
>> >> >>
>> >> >> FAQ 7.31
>> >> >>
>> >> >> See also this StackOverflow post:
>> >> >>
>> >> >>
>> >>
https://stackoverflow.com/questions/9508518/why-are-these-numbers-not-equal
>> >> >>
>> >> >> Hope this helps,
>> >> >>
>> >> >> Rui Barradas
>> >> >>
>> >> >> Às 00:08 de 29/02/20, robin hankin escreveu: >>> My
>> >> interpretation of dput.Rd is that dput() gives an exact
>> >> ASCII form >>> of the internal representation of an R
>> >> object. But:
>> >> >>>
>> >> >>> rhankin@cuttlefish:~ $ R --version >>> R version
>> >> 3.6.2 (2019-12-12) -- "Dark and Stormy Night" >>>
>> >> Copyright (C) 2019 The R Foundation for Statistical
>> >> Computing >>> Platform: x86_64-pc-linux-gnu (64-bit)
>> >> >>>
>> >> >>> [snip]
>> >> >>>
>> >> >>> rhankin@cuttlefish:~ $ R --vanilla --quiet >>>> x <-
>> >> sum(dbinom(0:20,20,0.35)) >>>> dput(x) >>> 1 >>>> x-1 >>>
>> >> [1] -4.440892e-16
>> >> >>>>
>> >> >>>> x==1 >>> [1] FALSE
>> >> >>>>
>> >> >>>
>> >> >>> So, dput(x) gives 1, but x is not equal to 1. Can
>> >> anyone advise?
>> >> >>>
>> > ______________________________________________
>> > R-devel@r-project.org mailing list
>> > https://stat.ethz.ch/mailman/listinfo/r-devel
>>
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