Rcpp Function Compilation Error

Question

Original R function:

psiBiSquare<-function(r,c){
true<-abs(r)<=c
false<-abs(r)>c
psi<-true*(r*(1-(r/c)^2)^2)+false*0
return(psi)
}

I am replacing this with a faster Rcpp implementation below:

 #include <Rcpp.h>
#include <math.h>
using namespace Rcpp;

// [[Rcpp::export]]
NumericVector psiBS_rcpp(NumericVector r, double c) {
int n =r.size();
NumericVector y = clone(r);

for(int i=0;i<n;i++){
 if(abs(r[i])<=c){
  y[i]=(r*pow(1-pow(r/c,2),2));
 }
 else{
  y[i]=0;
 }
}
 return y;
}

The error message is "cannot convert rcpp::sugar::" What am I doing wrong?

Answer 1

Couple of things:

1. You have really poor code formatting. There is no extra fee on spaces. Use them.

2. You are undecided about r and use it at the same time as a vector, and its elements as a scalar. Your right hand side expression is vectorized, yet you try to assign to a scalar y[i].

Improved / corrected source file:

#include <Rcpp.h>
#include <math.h>
using namespace Rcpp;

// [[Rcpp::export]]    
NumericVector psiBS_rcpp(NumericVector r, double c) {
  int n = r.size();
  NumericVector y = clone(r);

  for (int i=0; i<n; i++) {
    if (abs(r[i]) <=c ) {
      y[i] = (r[i]*pow(1-pow(r[i]/c,2),2));
    }
    else{
      y[i]=0;
    }
  }
  return y;
}

/*** R
psiBiSquare <- function(r,c) {
  true <- abs(r) <= c
  false <- abs(r) > c 
  psi <- true * (r*(1-(r/c)^2)^2) + false*0
  return(psi)
}

r <- seq(-3,3)
c <- 1.5
psiBiSquare(r, c) 
psiBS_rcpp(r, c)
*/

Running it:

R> sourceCpp("/tmp/ij.cpp")

R> psiBiSquare <- function(r,c) {
+   true <- abs(r) <= c
+   false <- abs(r) > c
+   psi <- true * (r*(1-(r/c)^2)^2) + false*0
+   return(psi)
+ }

R> r <- seq(-3,3)

R> c <- 1.5

R> psiBiSquare(r, c)
[1]  0.000000  0.000000 -0.308642  0.000000  0.308642  0.000000  0.000000

R> psiBS_rcpp(r, c)
[1]  0.000000  0.000000 -0.308642  0.000000  0.308642  0.000000  0.000000
R>