Conditional update of NumericVectors in Rcpp

Question

In R, I would use this to update values in a vector matching a conditional criteria:

a <- rep(seq(1:20),5)
a[a==5] <- 100 

How would I go about doing this using Rcpp if i had a NumericVector of a?

I am new to Rcpp and the only way I can think of at the moment is to loop over each value in a. I am using this:

cppFunction('NumericVector test(NumericVector a){
            int b = a.size();
            for (int i = 0; i < b; i++) {
            if (a[i] == 5) {
            a[i] = 100;
            }
      }
      return(a);

}')

Is there a way of doing this without the loop or with fewer lines of code?

Any help much appreciated.

Answer 1

For standard algorithms like this one, you may find it already implemented in the standard library:

#include <Rcpp.h>
using namespace Rcpp;

// [[Rcpp::export]]
NumericVector replace(NumericVector x) {
  std::replace(x.begin(), x.end(), 5, 100);
  return x;
}


/*** R
a <- rep(seq(1:20),5)
replace(a)
a
*/

Beware that if input a is already of type double, it will get modified.