[R-pkg-devel] How to get arbitrary precise inputs from R for an Rcpp package?

[Duncan Murdoch]

On 2024-07-19 12:37 p.m., Martin Maechler wrote:
>>>>>> Khue Tran
>>>>>>      on Fri, 19 Jul 2024 09:32:14 +1000 writes:
> 
>      > Thank you for the suggestion, Denes, Vladimir, and Dirk. I have indeed
>      > looked into Rmpfr and while the package can interface GNU MPFR with R
>      > smoothly, as of right now, it doesn't have all the functions I need (ie.
>      > eigen for mpfr class) and when one input decimals, say 0.1 to mpfr(), the
>      > precision is still limited by R's default double precision.
> 
> Well, it depends *how* you input 0.1:
> the double precision representation of 0.1 is *NOT* the same as
> mathematically 1/10 and it cannot be.
> 
> What you really want is either
> 
> mpfr(0.1, 100) # 100 as an example ... to get a more precise
> 	       # version of the '0.1' (double prec).
> 
> ## *or*, e.g.,
> 
> 1 / mpfr(10, 100) # which is a more accurate approximation to
>                    # the mathematical fraction 1/10

It seems like mpfr("0.1", 100) works pretty well.  I'd assume it does 
the parsing, rather than parse 0.1 to the closest double, and convert that.

Duncan Murdoch


> 
> ## if you really want that, I'd also recommend truly exact fractions
> 
> require(gmp)
> as.bigq(1, 10) # or equivalently
> 
> 1 / as.bigz(10) # which automatically makes an exact fraction
> 
>      > Thank you for the note, Dirk. I will keep in mind to send any future
>      > questions regarding Rcpp to the Rcpp-devel mailing list. I understand that
>      > the type used in the Boost library for precision is not one of the types
>      > supported by SEXP, so it will be more complicated to map between the cpp
>      > codes and R. Given Rmpfr doesn't provide all necessary mpfr calculations
>      > (and embarking on interfacing Eigen with Rmpfr is not a small task), does
>      > taking input as strings seem like the best option for me to get precise
>      > inputs?
> 
> almost surely (that's also what you can do in Rmpfr)..
> 
> Martin
> 
> 
>      > Sincerely,
>      > Khue
> 
>      > On Fri, Jul 19, 2024 at 8:29 AM Dirk Eddelbuettel <edd using debian.org> wrote:
> 
>      >>
>      >> Hi Khue,
>      >>
>      >> On 19 July 2024 at 06:29, Khue Tran wrote:
>      >> | I am currently trying to get precise inputs by taking strings instead of
>      >> | numbers then writing a function to decompose the string into a rational
>      >> | with the denominator in the form of 10^(-n) where n is the number of
>      >> | decimal places. I am not sure if this is the only way or if there is a
>      >> | better method out there that I do not know of, so if you can think of a
>      >> | general way to get precise inputs from users, it will be greatly
>      >> | appreciated!
>      >>
>      >> That is one possible way. The constraint really is that the .Call()
>      >> interface
>      >> we use for all [1] extensions to R only knowns SEXP types which map to a
>      >> small set of known types: double, int, string, bool, ...  The type used by
>      >> the Boost library you are using is not among them, so you have to add code
>      >> to
>      >> map back and forth. Rcpp makes that easier; it is still far from automatic.
>      >>
>      >> R has packages such as Rmpfr interfacing GNU MPFR based on GMP. Maybe that
>      >> is
>      >> good enough?  Also note that Rcpp has a dedicated (low volume and friendly)
>      >> mailing list where questions such as this one may be better suited.
>      >>
>      >> Cheers, Dirk
>      >>
>      >> [1] A slight generalisation. There are others but they are less common /
>      >> not
>      >> recommended.
>      >>
>      >> --
>      >> dirk.eddelbuettel.com | @eddelbuettel | edd using debian.org
>      >>
> 
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> 
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