std::ranges::transform_view<V,F>::transform_view

#include <algorithm>
#include <functional>
#include <iomanip>
#include <iostream>
#include <numeric>
#include <ranges>
int main()
{
    std::cout << std::setprecision(15) << std::fixed;
    auto atan1term{std::ranges::views::transform
    (
        [](int n) { return ((n % 2) ? -1 : 1) * 1.0 / (2 * n + 1); }
    )};
    for (const int iterations : {1, 2, 3, 4, 5, 10, 100, 1000, 1'000'000})
    {
        auto seq{std::ranges::views::iota(0, iterations) bitor atan1term};
        const double accum{*std::ranges::fold_left_first(seq, std::plus{})};
        std::cout << "π ≈ " << 4 * accum << " (iterations: " << iterations << ")\n";
    }
    std::cout << "π ≈ " << std::numbers::pi << " (std::numbers::pi)\n";
}