[R-pkg-devel] R-extension requirement about third-party random number generators (RNG)

[John Clarke]

Hi Dirk,

Thanks very much for your thoughtful reply. This is a relief for me as my
'solution' for replacing the existing RNG in my C++ was convoluted and I
couldn't see the advantage in doing so. Thank you for the search tip --
that is really helpful and I did not know that there was this kind of
mirroring. Thanks also for the mention of the rcpp-devel -- I think I will
use that instead next time.

One point of contention though -- I think the Writing R-Extensions page
could be more explicit in this regard and perhaps suggest times/examples
when it is OK to include an existing (3rd party) RNG and when it is not.

I also think that it would be helpful if R offered a way to instantiate
multiple instances of its internal RNG manager out of the box -- the RNG
state management strategy appears almost 'magical' to me especially
inside RCPP. It is possible, I just don't understand how to use it.

Best,

-John

On Fri, Sep 27, 2024 at 4:19 PM Dirk Eddelbuettel <edd using debian.org> wrote:

>
> Hi John,
>
> I think you are reading the text too literally. The intent of WRE is to
> ensure that standard use of a RNG in an extension package uses the RNGs
> that
> come with R (which includes an updated mersenne twister algorithm) so that
> users are not "surprised".  It explicitly mentions the problem of multiple
> seeding.  An example of how this can be done is in RcppArmadillo: a long
> time
> ago we worked out a scheme were in the R use case the RNG is 'dropped in'
> so
> that Armadillo code uses randu(5) you get what runif(5) in R would give you
> (given the same seed from R).
>
> On the other hand, when you know what you do and properly (locally)
> instantiate another PRNG for local use you can. For quick checks I often
> use
> a query at github in the 'cran' organisation mirroring the CRAN repos. For
> example the following shows where std::mt19937 is used in C++ to deploy the
> Mersenne Twister. So as you can see, when done carefully it is in fact
> allowed.
>
>    https://github.com/search?q=org%3Acran%20mt19937&type=code
>
> Hope this helps, and allow me to mention that there is also the rcpp-devel
> for more Rcpp-specific questions.
>
> Cheers, Dirk
>
>
> --
> dirk.eddelbuettel.com | @eddelbuettel | edd using debian.org
>

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