Vector cloning

Unlike R, C++ uses a copy by reference system, meaning that if you copy a variable then make changes to the copy, the changes will also take place in the original.

// [[Rcpp::export]]
NumericVector always_returns_two(NumericVector x) {
  // Make a copy
  NumericVector y = x;
  // Modify the copy
  y[0] = 2;
  // The changes also happen in the original
  return x[0];
}

To prevent this behavior, you have to use the clone() function to copy the underlying data from the original variable into the new variable. The syntax is y = clone(x). In this exercise, we have defined two functions for you:

change_negatives_to_zero(): Takes a numeric vector, modifies by replacing negative numbers with zero, then returns both the original vector and the copy.
change_negatives_to_zero_with_cloning(): Does the same thing as above, but clones the original vector before modifying it.

This exercise is part of the course

Optimizing R Code with Rcpp

View Course

Exercise instructions

Complete the function definition of change_negatives_to_zero() by setting the_original to the_copy.
Complete the function definition of change_negatives_to_zero_with_cloning() by setting the_copy to the clone of the_original.
Read the contents of the console to compare the output of each function.

Hands-on interactive exercise

Have a go at this exercise by completing this sample code.

#include 
using namespace Rcpp;

// [[Rcpp::export]]
List change_negatives_to_zero(NumericVector the_original) {
  // Set the copy to the original
  NumericVector the_copy = ___;
  int n = the_original.size();
  for(int i = 0; i < n; i++) {
    if(the_copy[i] < 0) the_copy[i] = 0;
  }
  return List::create(_["the_original"] = the_original, _["the_copy"] = the_copy);
}

// [[Rcpp::export]]
List change_negatives_to_zero_with_cloning(NumericVector the_original) {
  // Clone the original to make the copy
  NumericVector the_copy = ___;
  int n = the_original.size();
  for(int i = 0; i < n; i++) {
    if(the_copy[i] < 0) the_copy[i] = 0;
  }
  return List::create(_["the_original"] = the_original, _["the_copy"] = the_copy);
}

/*** R
x <- c(0, -4, 1, -2, 2, 4, -3, -1, 3)
change_negatives_to_zero(x)
# Need to define x again because it's changed now
x <- c(0, -4, 1, -2, 2, 4, -3, -1, 3)
change_negatives_to_zero_with_cloning(x)
*/

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