[Rcpp-devel] Calling an internal do_* C level function

[Martyn Plummer]

On Mon, 2013-10-14 at 22:24 +0200, Romain Francois wrote:
> Le 14/10/13 21:47, Mark Fredrickson a écrit :
> > Thank you to Romain and Dirk. I've seen some C++ rank implementations,
> > and I'll probably copy one of those.
> >
> >     Lastly, not a milligram of Rcpp in this question. Plain R programming
> >     questions should go to r-devel, not rcpp-devel.
> >
> >
> > Noted. My apologies. I was hoping there was some magic/sugar for this
> > particular problem that I had missed in the docs. Perhaps something that
> > negotiated the look up table that R uses for these functions.
> >
> > -M
> 
> Here comes the magic.
> 
> The internal function table (R_FunTab) is not hidden. With a bit of 
> copying from R's unexported Defn.h, you can get the function pointer for 
> the internal rank. see this gist: 
> https://gist.github.com/romainfrancois/6981372
> 
> However, if you do this in a package, there is a good chance R CMD check 
> will moan about it because you use R_FunTab.
> 
> Romain

Calling non-API functions and subverting the mechanism for hiding them
is a good way to make yourself unpopular.

Anyway, Mark wants to use the rank() function from R. So here is some
code that I used to emulate it JAGS using the STL stable_sort algorithm.

//Create a vector of pointers to the elements of args and sort it
vector<double const*> argptrs(N);
for (int i = 0; i < N; ++i) {
   argptrs[i] = args + i;
}
stable_sort(argptrs.begin(), argptrs.end(), lt_doubleptr);

//Ranks can be inferred from the sorted vector of pointers
for (int i = 0; i < N; ++i) {
   value[argptrs[i] - args] = i + 1;
}

Here "args" is an array of doubles containing the input vector, "value"
is an array to hold the output vector, both of length "N", and
"lt_doubleptr" is a comparison function that works on the vector of
pointers:

static bool lt_doubleptr (double const *arg1, double const *arg2) {
  return *arg1 < *arg2;
}

Martyn
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