C++ - Time execution 0s with Chrono Library - c++

Question

I'm sorting an array with merge sort. I use the Chrono library to measure the time, but, sometimes, the result is 0.
int main() {
srand(50000);
int A[N];
for (int i = 0; i < N; i++) {
A[i] = rand();
}
cout << "Array non ordinato\n";
Stampa(A, N);
auto start = std::chrono::system_clock::now();
QuickSort(A, 0, N - 1);
auto end = std::chrono::system_clock::now();
std::chrono::duration<double> elapsed = end - start;
cout << "\nArray ordinato\n";
Stampa(A, N);
cout << "Elapsed time in nanoseconds : "
<< chrono::duration_cast<chrono::nanoseconds>(end - start).count()
<< " ns" << endl;
cout << "Elapsed time in milliseconds : "
<< chrono::duration_cast<chrono::milliseconds>(end - start).count()
<< " ms" << endl;
cout << "Elapsed time: " << elapsed.count() << "s";
}
I defined N with #define N 300.
If N is 300, elapsed time (nanosecond, millisecond or second) is 0. If I increment N, I have an elapsed time greater than zero. I need time for small arrays too. How could I fix it?

Answer 1

If you want more precision for smaller N sorting, I recommend using the high resolution clock instead of the system clock.

Answer 2

When you are measuring time like in the question, you need to use std::chrono::steady_clock or any other clock where the is_steady property is true. std::chrono::system_clock is using the wall-clock which may change at any given time. This is the purpose of the std::chrono::system_clock: to represent the current time for the system.
You probably also want to use a higher number of elements to see any meaningful differences in time.
Although std::chrono::high_resolution clock is tempting going by it's name alone, it is worth noting that there is, technically, no high resolution clock in chrono. Rather, it is an alias to the clock that has the highest resolution available. Quoting from the standard:
Objects of class high_resolution_clock represent clocks with the
shortest tick period. high_resolution_clock may be a synonym for
system_clock or steady_clock.
when we check the implementations we see that MSVC has the following:
using high_resolution_clock = steady_clock;
while libstdc++-v3 has:
/**
* #brief Highest-resolution clock
*
* This is the clock "with the shortest tick period." Alias to
* std::system_clock until higher-than-nanosecond definitions
* become feasible.
*/
using high_resolution_clock = system_clock;