[Rd] dput()

[Martin Maechler]

>>>>> 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.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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