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

[Martyn Plummer]

On Tue, 2013-10-15 at 08:46 +0000, Martyn Plummer wrote:
> 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

I forgot to mention that this emulates rank() with argument
ties="first". If you want one of the other options it will be harder.

Martyn
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