为什么在根据测量位置测量经过时间时会有很大差异？

Question

My question is about the difference of the elapsed time according to the point.

For finding the largest portion of the total elapsed time when executing in my code, I used clock function.

source : calculating time elapsed in C++

First, I put the clock function at the start and end of the main function. (Actually, there are some declaration of variables but I deleted them for readability of my questions). Then I think I will be able to measure the total elapsed time.

int main(){

    using clock = std::chrono::system_clock;
    using sec = std::chrono::duration<double>;
    const auto before = clock::now();

...

    std::cin >> a >> b;
    lgstCommSubStr findingLCSS(a,b,numberofHT,cardi,SubsA);

    const sec duration = clock::now() - before;

    std::cout << "It took " << duration.count() << "s in main function" << std::endl;

    return 0;
}

Second, I put the clock function at the class findingLCSS . This class is for finding longest common sub-string between two string. It is the class that actually do my algorithm. I write the code for finding it in its constructor. Therefore, when making this class, it returns longest common sub-string information. I think this elapsed time will be the actual algorithm running time.

public:
    lgstCommSubStr(string a, string b, int numHT, int m, vector <int> ** SA):
        strA(a), strB(b), hashTsize(numHT), SubstringsA(SA),
        primeNs(numHT), xs(numHT),
        A_hashValues(numHT), B_hashValues(numHT),
        av(numHT), bv(numHT), cardi(m)
{

        using clock = std::chrono::system_clock;
        using sec = std::chrono::duration<double>;
        const auto before = clock::now();
...

        answer ans=binarySearch(a,b, numHT);

        std::cout << ans.i << " " << ans.j << " " << ans.length << "\n";


        const sec duration = clock::now() - before;

        std::cout << "It took " << duration.count() << "s in the class" << std::endl;

}

The output is as below.

tool coolbox
1 1 3
It took 0.002992s in inner class
It took 4.13945s in main function

It means 'tool' and 'coolbox' have a substring 'ool'

But I am confused that there is a big difference between two times.

Because the first time is total time and the second time is algorithm running time, I have to think its difference time is the elapsed time for declaration variables.

But it looks weird because I think declaration variables time is short.

Is there a mistake in measuring the elapsed time?

Please give me a hint for troubleshoot. Thank you for reading!

Answer 1

Taking a snapshot of the time before std::cin >> a >> b; leads to an inaccurate measurement as you're likely starting the clock before you type in the values for a and b . Generally you want to put your timing as close as possible to the thing you're actually measuring.