rcpp updating data in base environment - r

Question

I am working on a simple matching algorithm in Rcpp which is taking a number of individuals (I), a number of schools (J), a number of submitted choices (nc), a priority ranking of individuals (pos), the number of vacancy (emp), and the true choices..
#include <Rcpp.h>
using namespace Rcpp;
//[[Rcpp::export]]
NumericVector gs2(int I, int J, int nc, NumericVector pos, NumericVector emp, NumericMatrix choices) {
NumericVector admits(J);
NumericVector out(I);
std::fill(out.begin(),out.end(),J+1);
for (int i=0;i<I;i++){
NumericVector apply = choices(pos(i),_)-1;
for (int j=0;j<nc;j++){
if (emp(apply(j))>0)
{
out(pos(i)) = apply(j)+1;
admits(apply(j)) = admits(apply(j)) + 1;
emp(apply(j)) = emp(apply(j)) - 1;
break;
}
}
}
return out;
}
The code works fine.. Except that it looks like it is messing with my data.. after running the code my size variable has been changed...Am I missing something? Thanks
set.seed(123)
rank = (1:20)-1
stuchoice = matrix(sample(1:3,6*20,replace=T),byrow=T,ncol=6,nrow=20)
size = c(7,11,4)
gs2(20,3,6,rank,size,stuchoice)
size

Answer 1

Your size variable is changing because you are changing it in your C++ code. In particular this line:
emp(apply(j)) = emp(apply(j)) - 1;
Rcpp passes variables by reference so anything you do to them inside will be reflected in your top R variables. If you want to avoid this, then you want to clone your variable. Changing your code to the following corrects the problem.
#include <Rcpp.h>
using namespace Rcpp;
// Note the change in the name of 'emp' to 'emp_'!!!
//[[Rcpp::export]]
NumericVector gs2(int I, int J, int nc, NumericVector pos, NumericVector emp_, NumericMatrix choices) {
NumericVector admits(J);
NumericVector out(I);
// clone your emp
NumericVector emp = clone(emp_);
std::fill(out.begin(),out.end(),J+1);
for (int i=0;i<I;i++){
NumericVector apply = choices(pos(i),_)-1;
for (int j=0;j<nc;j++){
if (emp(apply(j))>0)
{
out(pos(i)) = apply(j)+1;
admits(apply(j)) = admits(apply(j)) + 1;
emp(apply(j)) = emp(apply(j)) - 1;
break;
}
}
}
return out;
}
Test
library(Rcpp)
sourceCpp("test.cpp")
set.seed(123)
rank = (1:20)-1
stuchoice = matrix(sample(1:3,6*20,replace=T),byrow=T,ncol=6,nrow=20)
size = c(7,11,4)
gs2(20,3,6,rank,size,stuchoice)
size
[1] 7 11 4