algo/foldopt.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{100, 2, 3, 4};
  std::println("coll: {}", coll);
    
  // compute ((E1 - E2) - E3) - ... :
  auto optDiff1 = std::ranges::fold_left_first(coll, std::minus{});
  if (optDiff1) {
    std::println("diffs from left:  {}", *optDiff1);
  }

  // compute ... - (En-2 - (En-1 - En)) :
  auto optDiff2 = std::ranges::fold_right_last(coll, std::minus{});
  if (optDiff2) {
    std::println("diffs from right: {}", *optDiff2);
  }

  // compute and use sum of empty collection:
  coll.clear();
  auto optDiff3 = std::ranges::fold_left_first(coll, std::plus{});
  std::println("diffs of empty collection: {}", optDiff3.value_or(0));
}