Comparing prime generation the old way and the C++20 way

primes.cpp

#include <chrono>
#include <iostream>
#include <numeric>
#include <ranges>
#include <vector>

const int N = 10000000;

int main() {
  auto t1 = std::chrono::steady_clock::now();

  auto isprime_old = [](int n) {
    if (n < 2) return false;
    for (int i = 2; i * i <= n; ++i) {
      if (n % i == 0) return false;
    }
    return true;
  };

  long long psum = 0;
  for (int i = 1; i < N; ++i) {
    if (isprime_old(i)) psum += i;
  }
  std::cout << psum << std::endl;

  auto t2 = std::chrono::steady_clock::now();
  std::chrono::duration<double> dt1 = t2 - t1;
  std::cout << dt1.count() << " seconds" << std::endl;

  auto isprime_new = [](int n) {
    using namespace std::views;
    return n > 1 and (
      iota(2) | 
      take_while([n](int k) { return k * k <= n; }) |
      filter([n](int k) { return n % k == 0; })
    ).empty();
  };
  
  auto primes = std::views::iota(1, N) | std::views::filter(isprime_new);
  std::cout << std::accumulate(primes.begin(), primes.end(), 0ll) << std::endl;

  auto t3 = std::chrono::steady_clock::now();
  std::chrono::duration<double> dt2 = t3 - t2;
  std::cout << dt2.count() << " seconds" << std::endl;

  return 0;
}