[SOLVED] How to get current time in milliseconds?

Issue

I am new to C++ and I don’t know much about its library. I need to do time analysis of different sorting algorithms, for which I need to get the current time in milliseconds. Is there any way to do that?

Solution

Simply use std::chrono. The general example below times the task "of printing 1000 stars":

#include <iostream>
#include <ctime>
#include <ratio>
#include <chrono>
 
int main ()
{
  using namespace std::chrono;
 
  high_resolution_clock::time_point t1 = high_resolution_clock::now();
 
  std::cout << "printing out 1000 stars...\n";
  for (int i=0; i<1000; ++i) std::cout << "*";
  std::cout << std::endl;
 
  high_resolution_clock::time_point t2 = high_resolution_clock::now();
 
  duration<double, std::milli> time_span = t2 - t1;
 
  std::cout << "It took me " << time_span.count() << " milliseconds.";
  std::cout << std::endl;
 
  return 0;
}

Instead of printing the stars, you will place your sorting algorithm there and time measure it.


Do not forget to enable the optimization flags for your compiler, if you intend to do some benchmarking, e.g. for , you need -O3. This is serious, check what happened to me when I didn’t do so: Why emplace_back is faster than push_back?


Ps: If your compiler doesn’t support , then you could look into other methods in my Time Measurements (C++).


A specific (toy) example, by using my Quicksort (C++), would be:

#include <iostream>
#include <ctime>
#include <ratio>
#include <chrono>

void quickSort(int a[], int first, int last);
int pivot(int a[], int first, int last);
void swap(int& a, int& b);
void swapNoTemp(int& a, int& b);

using namespace std;
using namespace std::chrono;

int main()
{
    int test[] = { 7, -13, 1, 3, 10, 5, 2, 4 };
    int N = sizeof(test)/sizeof(int);

    cout << "Size of test array :"  << N << endl;

    high_resolution_clock::time_point t1 = high_resolution_clock::now();

    // I want to measure quicksort
    quickSort(test, 0, N-1);

    high_resolution_clock::time_point t2 = high_resolution_clock::now();

    duration<double> time_span = t2 - t1;

    std::cout << "It took me " << time_span.count() << " seconds.";
    std::cout << std::endl;

    return 0;
}

and the output now is:

Georgioss-MacBook-Pro:~ gsamaras$ g++ -Wall -std=c++11 -O3 main.cpp 
Georgioss-MacBook-Pro:~ gsamaras$ ./a.out 
Size of test array :8
It took me 3.58e-07 seconds.

It’s as simple as that. Happy benchmarking! =)


EDIT:

high_resolution_clock::now() function returns time relative to which time?

From std::chrono:

Time points

A reference to a specific point in time, like one’s
birthday, today’s dawn, or when the next train passes. In this
library, objects of the time_point class template express this by
using a duration relative to an epoch (which is a fixed point in time
common to all time_point objects using the same clock).

where one could check this epoch and time_point example, which outputs:

time_point tp is: Thu Jan 01 01:00:01 1970

Answered By – gsamaras

Answer Checked By – Pedro (BugsFixing Volunteer)

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