Question

我写了这样的测试-

std::vector<int> test_vector;                                                        

for (int i = 0; i < 100000000; ++i) {                                                
    test_vector.push_back(i);                                                        
}                                                                                    

QElapsedTimer timer;                                                                 
timer.start();                                                                       

std::sort(test_vector.begin(),test_vector.end(), [](int a, int b) { return a < b; });

qDebug() << "The slow operation took" << timer.elapsed() << "milliseconds";          
qDebug() << "The slow operation took" << timer.nsecsElapsed() << "nanoseconds";

在这里我开始重新排序和结果

The slow operation took 4091 milliseconds
The slow operation took 4091842000 nanoseconds

但是当我改变

std::sort(test_vector.begin(),test_vector.end(), [](int a, int b) { return a > b; });

结果

The slow operation took 2867 milliseconds
The slow operation took 2867591800 nanoseconds

我在Qt_5_12_3_MinGW_64_bit-Release上进行了测试，无法理解为什么反向排序比重新排序更快？

已解决！

我在Qt_5_12_3_MSVC2017_64bit上测试了相同的示例，问题已解决，问题出在MinGW_64中

但是，我仍然有一个问题，为什么我要将向量归类为供稿中所有10个元素，

#include <chrono>
#include<iostream>
#include <vector>
#include <algorithm>



    int main() {

        std::vector<int> test_vector;

        for (int i = 0; i < 100000000; ++i) {
            test_vector.push_back(10);
        }

        auto begin = chrono::high_resolution_clock::now();

        std::sort(test_vector.begin(), test_vector.end(), [](int a, int b) { return a < b; });


        auto end = std::chrono::high_resolution_clock::now();
        auto dur = end - begin;
        auto ms = std::chrono::duration_cast<std::chrono::milliseconds>(dur).count();
        std::cout << ms << endl;


        return 0;
    }

结果167毫秒，

并重新排序2553毫秒

for (int i = 0; i < 100000000; ++i) {
        test_vector.push_back(i);
    }

为什么反向排序比重新排序更快

已解决！

0 个答案: