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

[Martin Maechler]

>>>>> 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

## 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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