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 <e...@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 | e...@debian.org
>>
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