Chapter 28 Iterator

Iterator is an class used to access elements of Vector DataFrame List. If you want to use algorithms provided by standard C++, you need to understand iterator. Because many of the algorithms provided by standard C++ use iterators to specify location or range of data to apply the algorithms.

Specific iterator type is defined for each data structure of Rcpp.

NumericVector::iterator
IntegerVector::iterator
LogicalVector::iterator
CharacterVector::iterator
DataFrame::iterator
List::iterator

The figure below shows schematically how to access vector elements using an iterator.

• i = v.begin() : The iterator i points to the first element ofv.
• ++i : Updates i to the state pointing to the next element.
• --i : Updates i to the state pointing to the previous element.
• i + 1 : Represents an iterator pointing to the element 1 elements behind of i.
• i - 1 : Represents an iterator pointing to the element 1 elements ahead of i.
• *i : Represents the value of the element pointed by i.
• v.end() : Represents an iterator pointing to the end (one after the last element) of v.
• *(v.begin()+k) : Represents the value of the k-th element of v (v[k]).

The following code example shows an example of traversing all the elements of a NumericVector using iterator to calculate sum of the elements.

// [[Rcpp::export]]
double rcpp_sum(NumericVector x) {
  double total = 0;
  for(NumericVector::iterator i = x.begin(); i != x.end(); ++i) {
    total += *i;
  }
  return total;
}