[Rcpp-devel] rgamma() behavior

[Kevin Ushey]

Hi Gregor, Dirk,

Added here: https://github.com/RcppCore/Rcpp/commit/f6e7e34b94ba52c1341142a5640d0ede674073c7

Please feel free to tweak for wording / style / clarity :)

Kevin

On Mon, Mar 24, 2014 at 5:05 PM, Dirk Eddelbuettel <edd at debian.org> wrote:
>
> On 24 March 2014 at 16:45, Kevin Ushey wrote:
> | Hi Gregor,
> |
> | This is probably worth a reminder as you say. Just for reference, the
> | motivation is that R's internal computations for rexp and rgamma
> | default to the 'scale' parameterization, rather than the 'rate'
> | parameterization seen at the R level. Rcpp uses that internal
> | operation and hence isn't consistent with what you see at the top
> | level (even though it's consistent with the internals).
> |
> | R-exts does give this a bit of treatment in
> | http://cran.r-project.org/doc/manuals/r-release/R-exts.html#Distribution-functions.
> |
> | It's a bit awkward that the parameterizations differ at the R and C
> | level, but it is what it is.
>
> Kevin beat me to it but that is what I was about to say.  Rcpp sugar does
> what the standalone R math library does (also at the C level).
>
> But you raise a good point, and we should add a note to the vignette.
>
> Dirk
>
>
> | Cheers,
> | Kevin
> |
> | On Mon, Mar 24, 2014 at 4:34 PM, Gregor Kastner <gregor.kastner at wu.ac.at> wrote:
> | > Dear all,
> | >
> | > I believe it's the second time I just realized that Rcpp::rgamma() behaves
> | > differently than to R's rgamma() when called without named arguments:
> | >
> | > ********** BEGIN code.R **********
> | >
> | > library(Rcpp)
> | >
> | > cppFunction('
> | >     SEXP rgamma2(const int n, const double a, const double b) {
> | >         RNGScope scope;
> | >         return Rcpp::rgamma(n, a, b);
> | >     }
> | > ')
> | >
> | > set.seed(123);
> | > print(rgamma(1, 2, 3));
> | >
> | > set.seed(123);
> | > print(rgamma2(1L, 2, 3));
> | >
> | > set.seed(123);
> | > print(rgamma2(1L, 2, 1/3));
> | >
> | > **********  END  code.R **********
> | >
> | > yields:
> | >
> | > [1] 0.2973645
> | > [1] 2.676281
> | > [1] 0.2973645
> | >
> | > Maybe this is worth a tiny note in e.g. the "Rcpp syntactic sugar" vignette?
> | > Might save me (and possibly others) from realizing a third time.
> | >
> | > Best and thanks,
> | > Gregor
> | > _______________________________________________
> | > Rcpp-devel mailing list
> | > Rcpp-devel at lists.r-forge.r-project.org
> | > https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/rcpp-devel
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> --
> Dirk Eddelbuettel | edd at debian.org | http://dirk.eddelbuettel.com