one_array_two_threads.cpp

#include <iostream>
#include <thread>
#include <chrono>
#include <memory>
#include <vector>
#include <cmath>
#include <array>
#include <iterator>

constexpr int size = 100'000'000;

auto single_thread()
{
    auto t0 = std::chrono::steady_clock::now();
    std::vector<float> arr(size, 1.0f);

    for (int i=0; i<arr.size(); ++i) {
        arr[i] = arr[i] * sin(0.2 + i/5.0)*cos(0.2 + i/5.0) * cos(0.4 + i/2.0);
    }
    auto t1 = std::chrono::steady_clock::now();
    std::cout << "single: " << std::chrono::duration_cast<std::chrono::milliseconds>(t1-t0).count() << std::endl;
    return arr;
}

auto double_thread()
{
    auto t0 = std::chrono::steady_clock::now();
    std::vector<float> arr(size, 1.0f);
    auto h = size/2;
    std::vector<float> arr1(h);
    std::copy_n(arr.begin(), h, arr1.begin());

    std::vector<float> arr2(h);
    std::copy_n(arr.begin()+h, h, arr2.begin());

    std::thread th1([&arr1, h](){
        for (int i=0; i<arr1.size(); i++)
            arr1[i] = arr1[i] * sin(0.2 + i/5.0)*cos(0.2 + i/5.0) * cos(0.4 + i/2.0);
    });
    std::thread th2([&arr2, h](){
        for (int i=0; i<arr2.size(); i++) {
            int o = i+h;
            arr2[i] = arr2[i] * sin(0.2 + o/5.0)*cos(0.2 + o/5.0) * cos(0.4 + o/2.0);
        }
    });
    th1.join();
    th2.join();
    std::copy_n(arr1.begin(), h, arr.begin());
    std::copy_n(arr2.begin(), h, arr.begin()+h);

    auto t1 = std::chrono::steady_clock::now();
    std::cout << "double: " << std::chrono::duration_cast<std::chrono::milliseconds>(t1-t0).count() << std::endl;
    return arr;
}

int main()
{
    for (int i=0; i<100; ++i) {
        auto arr1 = single_thread();
        auto arr2 = double_thread();
        std::cout  << (arr1==arr2) << std::endl;
    }
    return 0;
}