c++ vector for loop, iterator, range 속도 측정, For Loop을 꼭 써야 하는 경우

for loop < iterator < range 임

 

코드 >>

#include <iostream>
#include <vector>
#include <string>
#include <chrono>
 
 
int main()
{
    // Range based for loop이 가장 안전하고 확실한 방법임
    std::cout << "how many elements?" << std::endl;
    std::size_t n;
    std::cin >> n;
 
    std::vector<int> numsA(n, 1);
    std::vector<int> numsB(n, 1);
    std::vector<int> numsC(n, 1);
 
    
    auto start = std::chrono::high_resolution_clock::now();
    
    //index based
    for (int i = 0; i < 1000; i++) {
        for (std::size_t idx = 0; idx < numsA.size(); idx++)
        {
            numsA[idx] *= 2;
        }
    }
    auto end = std::chrono::high_resolution_clock::now();
    std::chrono::duration<double> diff = end - start;
    std::cout << "idx Loop time taken:" << diff.count() << "s\n";
    start = std::chrono::high_resolution_clock::now();
    
    //iterator based
    for (int i = 0; i < 1000; i++) {
        for (auto itr = numsB.begin(); itr != numsB.end(); itr++)
        {
            (*itr) *= 2;
        }
    }
    end = std::chrono::high_resolution_clock::now();
    diff = end - start;
    std::cout << "Iterator time taken:" << diff.count() << "s\n";
    start = std::chrono::high_resolution_clock::now();
    
    // range based loop (Best optimized!)
    for (int i = 0; i < 1000; i++) {
        for (auto& num : numsC)
        {
            num *= 2;
        }
    }
    
    end = std::chrono::high_resolution_clock::now();
    diff = end - start;
    std::cout << "Range Loop time taken:" << diff.count() << "s\n";
 
    return 0;
}

 

꼭 For loop을 써야 하는 경우

Loop 안에서 Vector의 Size가 변경된다면

For Loop을 쓰도록 하자

#include <iostream>
#include <vector>
#include <string>
#include <chrono>
 
// 벡터 내부에서 벡터의 사이즈가 변경되면 for loop 을 쓴다.
int main()
{
    std::vector<int> numsA{ 0,1,0,1 };
    // index version
    for (std::size_t idx = 0; idx < numsA.size(); idx++)
    {
        if (numsA[idx] == 1)
        {
            numsA.emplace_back(2);
        }
    }
    
    
    for (const int n : numsA)
    {
        std::cout << n;
    }
    std::cout << std::endl;
    // iterator version
    for (auto itr = numsA.begin(); itr != numsA.end(); itr++)
    {
        if ((*itr) == 0)
        {
            numsA.emplace_back(2);
        }
    }
 
 
    for (const int n : numsA)
    {
        std::cout << n;
    }
    std::cout << std::endl;
 
    // range version
    for (auto & num : numsA)
    {
        if (num == 0)
        {
            numsA.emplace_back(2);
        }
    }
 
 
    for (const int n : numsA)
    {
        std::cout << n;
    }
    std::cout << std::endl;
    return 0;
}