[Rd] Calling C implementations of rnorm and friends

[Gabriel Becker]

Well,

For this particular use case why not just transform the parameters at the R
level and then call the existing function? Is there not a closed form
mapping?

~G
On Jul 1, 2016 2:50 PM, "Joshua Ulrich" <josh.m.ulrich at gmail.com> wrote:

> On Fri, Jul 1, 2016 at 6:13 AM, Luis Usier
> <luis.henrique.usier at gmail.com> wrote:
> > Gabriel,
> >
> > Thanks for that! I guess I really should have figured that one out
> sooner,
> > huh?
> >
> > I understand why that wouldn't be CRAN-compliant. But then, what *is* the
> > proper way to do it? Is there any way I can call unexported functions
> from
> > another package and have it accepted by CRAN?
> >
> There may be ways to call unexported functions that R CMD check will
> not detect, but you should not look for ways to violate the spirit of
> the policy.  Work-arounds include asking the package maintainer to
> export the functionality you want, or you can copy the desired
> functionality to your package (with attribution, of course).
>
> In this particular case, you can probably get what you want via the
> Rmath library.  It provides C-level access to all the distribution
> functions (and more).  You would need something like this in your C
> code:
>
> #include <Rmath.h>
> GetRNGstate();
> double rn = rnorm(0.0, 1.0);
> PutRNGstate()
>
> Once Dirk recovers from useR!, he'll tell you how this is all
> super-easy if you just use Rcpp. ;)
>
> > Also, if I instead re-write the random variable generating functions, do
> > you have any idea of where the source code is in the stats package? As I
> > said above, I can't seem to find the source code for the functional
> forms.
> >
> See the sections "Functions that call compiled code" and "Compiled
> code in a base package" from this answer:
> http://stackoverflow.com/a/19226817/271616
>
> That should give you a few pointers on where/how to look.
>
> > Thanks,
> >
> > Luis
> >
> > On Thu, Jun 30, 2016 at 10:38 PM, Gabriel Becker <gmbecker at ucdavis.edu>
> > wrote:
> >
> >> Luis,
> >>
> >> C_rnorm is a symbol but it's not exported.  This means that you *can* do
> >> this by using stats:::C_rnorm.
> >>
> >> That said, it's not exported, which means that it's not supported to do
> >> this. So your package likely would not be allowed on CRAN, for example.
> >>
> >> Best,
> >> ~G
> >> On Jun 30, 2016 2:08 PM, "Luis Usier" <luis.henrique.usier at gmail.com>
> >> wrote:
> >>
> >>> Hi all,
> >>>
> >>> Looking at the body for the function rnorm, I see that the body of the
> >>> function is:
> >>>
> >>>     .Call(C_rnorm, n, mean, sd)
> >>>
> >>> I want to implement functions that generate normal (and other) random
> >>> variables. Now, I understand that I can perfectly well just call the R
> >>> wrapper for these functions and that will be almost indistinguishable
> for
> >>> most purposes, but for whatever reason I wanted to try and call the C
> >>> function directly. My first instinct was to call them as:
> >>>
> >>>     .Call(C_rnorm, 1, 1, 1)
> >>>
> >>> This doesn't work because R assumes C_rnorm is an object. Looking at
> the
> >>> documentation for .Call, I try passing it in as a string:
> >>>
> >>>     .Call("C_rnorm", 1, 1, 1, PACKAGE = "stats")
> >>>
> >>> This doesn't work either. The help page links to getNativeSymbolInfo(),
> >>> which I can't make work either. It also refers me to the dyn.load()
> >>> function and the "Writing R  Extensions" manual.
> >>>
> >>> After reading and trying to digest those, I try
> >>>
> >>>     getDLLRegisteredRoutines("stats")
> >>>
> >>> which shows me all the C and Fortran functions as registered routines,
> >>> along with their number of parameters. I retrieve rnorm from the list
> and
> >>> pass it on to .Call, which then works fine.
> >>>
> >>> However, is there an easier way to do this? Specifically, I would like
> to
> >>> call the DLL registered routines from within functions in a package I
> am
> >>> writing. The manual tells me I should use useDynLib(). So if I added
> >>> useDynLib("stats") to my namespace, would that work? Could I then have
> a
> >>> function such as:
> >>>
> >>>     function(x, y, z) .Call(C_rnorm, x, y, z)
> >>>
> >>> in my package? If not, what is the proper way of calling these
> functions
> >>> from other packages? Should I use "C_rnorm" or "norm"?
> >>>
> >>> Also, I was looking for the C source code of rnorm, because I wanted to
> >>> understand how the function works. Looking at Winston Chang's github R
> >>> mirror, I found rnorm in the random.c file in the stats package.
> However,
> >>> the code I find for it:
> >>>
> >>>
> >>>
> >>> #define DEFRAND2_REAL(name) \
> >>> SEXP do_##name(SEXP sn, SEXP sa, SEXP sb) { \
> >>>     return random2(sn, sa, sb, name, REALSXP); \
> >>> }
> >>> DEFRAND2_REAL(rnorm)
> >>>
> >>>
> >>> Doesn't help me at all in understanding how it works. It should create
> a
> >>> function random2(sn, sa, sb, norm, REALSXP); I understand that is a
> >>> version
> >>> of the random2 function that returns a real S expression taking sn, sa
> and
> >>> sb as parameters. But how does find the actual functional form for the
> >>> normal distribution?
> >>>
> >>> I am asking because I would like to rewrite some of the other
> functions,
> >>> such as parameterizing rbeta by the mean and sample size rather than by
> >>> the
> >>> number of successes and failures and rgamma by the mean and total time
> >>> elapsed instead of the number of events. Once I understand how the C
> >>> source
> >>> code works, it would be hopefully not very difficult to reparameterize
> >>> them.
> >>>
> >>> Thanks,
> >>>
> >>> Luis Usier
> >>>
> >>>         [[alternative HTML version deleted]]
> >>>
> >>> ______________________________________________
> >>> R-devel at r-project.org mailing list
> >>> https://stat.ethz.ch/mailman/listinfo/r-devel
> >>>
> >>
> >
> >         [[alternative HTML version deleted]]
> >
> > ______________________________________________
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> > https://stat.ethz.ch/mailman/listinfo/r-devel
>
>
>
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