How to get current time in milliseconds?

Question

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?

Answer 1

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.

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Do not forget to enable the optimization flags for your compiler, if you intend to do some benchmarking, e.g. for g++, 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?

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Ps: If your compiler doesn't support c++11, then you could look into other methods in my Time Measurements (C++).

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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! =)

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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

Answer 2

Simple working example

#include<iostream>
#include<chrono>
using namespace std;
using namespace std::chrono;

void longTask(){
    for(auto i = 0; i < INT_MAX; i++){
        //do something;
    }
}
int main(){
    auto startTime = high_resolution_clock().now();
    longTask();
    auto stopTime = high_resolution_clock().now();

    //Warning: can't print startTime or stopTime to cout withoutcasting
    //microseconds and milliseconds allowed types in duration_cast

    auto duration = duration_cast<milliseconds>(stopTime - startTime); 
    cout << "Time take to run longTask() in milliseconds: " << duration.count();
    return 0;
}

Output

Time take to run longTask() in milliseconds: 5274

Enjoy !!