algo/folditer.cpp

The following code example is taken from the book
C++23 - The Complete Guide by Nicolai M. Josuttis, Leanpub, 2026
The code is licensed under a Creative Commons Attribution 4.0 International License.

---

// raw code

#include <print>
#include <vector>
#include <algorithm>

int main()
{
  std::vector coll{1, 2, 3, 4};
  std::println("coll: {}", coll);
    
  // compute (((100 + E1) + E2) + E3) + ... :
  auto [pos1, res1] = std::ranges::fold_left_with_iter(coll, 100, std::plus{});
  std::println("sum from left with init: {} (at end(): {})",
               res1, pos1 == coll.end());

  // compute ((E1 + E2) + E3) + ... :
  auto [pos2, optRes2] = std::ranges::fold_left_first_with_iter(coll,
                                                                std::plus{});
  if (optRes2) {
    std::println("sum from left (no init): {} (at end(): {})",
                 *optRes2, pos2 == coll.end());
  }
}