[Rd] Fwd: Rcpp sugar dpois

[Dirk Eddelbuettel]

On 3 November 2015 at 12:16, Enrico Bibbona wrote:
| Hi. Here is a piece of cpp code.

Somewhat wrong list -- try rcpp-devel for Rcpp questions.

| It works, but I do not understand the rational for the use of
| "R::dpois" to call the function dpois since in the examples I have always

Choice is good.  In Rcpp, the R namespace also offers the scalar variants of
the d/p/q/r functions described in Rmath.h -- besides the vectorised variants
in the Rcpp namespace.

| found directly "dpois" or "Rcpp::dpois" that both do not work in my code.

As you do not show the non-working code, I cannot help with your problem.
Here is simple feasibility test:

R> cppFunction("NumericVector dp(NumericVector x, double lambda) { return dpois(x, lambda, false); }")
R> dp(0:4, 0.5)
[1] 0.60653066 0.30326533 0.07581633 0.01263606 0.00157951
R> dpois(0:4, 0.5)
[1] 0.60653066 0.30326533 0.07581633 0.01263606 0.00157951
R> 

This uses Rcpp::dpois works with a vectorised first argument at the C++
level, and then behaves identical to the R function we call for
comparison. R::dpois, which you use, takes a scalar 'double' which is why you
need to loop by hand.

Please consider posting on Rcpp-devel next time. Remeber to subscribe before
you attempt to post.

Dirk

| Could anyone be so patient to explain me why should it be like that?
| Thaks a lot, Enrico
| 
| #include <Rcpp.h>
| using namespace Rcpp;
| NumericVector pcUtildedatoU(NumericVector z, double j, double par)
| 
| {
|     int l_z=z.size();
|     NumericVector out(l_z);
|     for (int w=0; w<= (l_z-1); w++)
|     {
|         double outcand=0;
|         double d=z[w];
|         if (d >= j)
|             outcand=1;
|         else if (d>= -j)
|         {
|             for(int i=-j; i<=d; i+=2)
|                 outcand+= R::dpois((i+j)/2, par,false)* R::dpois((j-i)/2,
| par,false);
|             outcand=outcand/R::dpois(j, 2*par,false);
|         }
|         out[w]=outcand;
|     }
|     return out;
| }
| 
| -- 
| Enrico Bibbona
| Dipartimento di Matematica
| Università di Torino
| https://sites.google.com/site/enricobibbona/
| 
| 
| 
| -- 
| Enrico Bibbona
| Dipartimento di Matematica
| Università di Torino
| https://sites.google.com/site/enricobibbona/
| 
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| 
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http://dirk.eddelbuettel.com | @eddelbuettel | edd at debian.org