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 :-).
Duncan Murdoch
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?
>> >>>
>> >>> ______________________________________________ >>>
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>> >>>
>> >>
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