Rcpp: Storing elements to subset of vector

Question

I'm still wrapping my head around Rcpp logic coming from an R context, so please be patient with me! From the following code:

Cjplus <- c(0,0)
Kseq <- c(1,2)
cand <- c(0,1)
cppFunction("NumericVector test(NumericVector Cjplus, NumericVector Kseq,
                   NumericVector cand, int i) {
  NumericVector A = as<NumericVector>(Cjplus[Kseq-1]);
  int B = cand[i-2];
  as<NumericVector>(Cjplus[Kseq-1]) = A + B;
  return Cjplus[Kseq-1];
}")
test(Cjplus, Kseq, cand, 3)

I expect to get [1] 1 1 as my output, but instead I get [1] 0 0. Am I indexing incorrectly here?

Answer 1

I don't understand why you try to use subset-assignment.

Cjplus <- c(0,0)
Kseq <- c(1,2)
cand <- c(0,1)

cppFunction("NumericVector test(NumericVector Cjplus, NumericVector Kseq,
                   NumericVector cand, int i) {
  NumericVector A = Cjplus[Kseq-1];
  double B = cand[i-2];
  A = A + B;
  return A;
}")
test(Cjplus, Kseq, cand, 3)
#[1] 1 1

Edit:

Here is a version that does subset-assignment.

library(Rcpp)
cppFunction("NumericVector test(const NumericVector Cjplus, NumericVector Kseq,
                   NumericVector cand, int i) {
  NumericVector C = clone(Cjplus);
  NumericVector A = C[Kseq-1];
  double B = cand[i-2];
  A = A + B;
  C[Kseq-1] = A;
  return C;
}")
test(Cjplus, Kseq, cand, 3)
#[1] 1 1