[Rd] dput()

[Martin Maechler]

>>>>> 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.

Martin

    > On Sun, Mar 1, 2020 at 6:22 AM Duncan Murdoch
    > <murdoch.duncan using 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 using 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 using 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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