Polygon circumference/radius ratio.

#include <iostream>
#include <iomanip>
#include <algorithm>
#include <cmath>
#if defined(__cpp_lib_math_constants)
# include <numbers>
#endif

//  π(n) = n . sin(180 / n)

#define FUNCTIONAL_

#if defined(__cpp_lib_math_constants)
constexpr
double pi = std::numbers::pi_v<double>;
#elif defined(M_PI)
constexpr
double pi = M_PI;
#else
constexpr
double pi = 2.0 * std::acos(0.0);
#endif

auto to_radian = [](double angle) { return angle * (pi / 180.0); };
auto poly_pi_l = [](long sides) {
  return (1.0 * sides) * std::sin(to_radian(180.0) / (1.0 * sides));
};
auto poly_pi = [](long sides) { return (1.0 * sides) * std::sin(pi / (1.0 * sides)); };

int main() {
  std::cout << "C++ Version: " << __cplusplus << '\n';
#if defined(__cpp_lib_math_constants)
  std::cout << "C++ Math Constants Version: " << __cpp_lib_math_constants << '\n';
#endif

  std::cout << std::fixed << std::setprecision(12);
  std::cout << "π: " << pi << '\n';

  auto polygons = { 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 50, 100, 200, 500,
                         1'000,      5'000,
                        10'000,     20'000,
                       100'000,    200'000,
                       200'000,    400'000,
                     1'000'000,  2'000'000,  3'000'000,
                     4'000'000,  4'100'000,  4'200'000,
                    10'000'000, 20'000'000, 25'000'000, };

#ifndef FUNCTIONAL_
  std::cout << std::string(60, '=') << std::endl;

  for (auto side : polygons) {
    std::cout << std::setw(12) << side << " : "
              << std::setw(15) << poly_pi(side)
              << " - "
              << std::setw(15) << poly_pi_l(side)
              << '\n';
  }
#else
  std::cout << std::string(60, '.') << std::endl;

  std::for_each(polygons.begin(), polygons.end(), [](auto const side) {
    std::cout << std::setw(12) << side << " : "
              << std::setw(15) << poly_pi(side)
              << " - "
              << std::setw(15) << poly_pi_l(side)
              << '\n';
  });
#endif

  return 0;
}