How to Use Timer in C++

1. Use the clock() Function to Implement a Timer in C++
2. Use the gettimeofday Function to Implement a Timer in C++

This article will demonstrate multiple methods of how to use a timer in C++.

Use the clock() Function to Implement a Timer in C++

The clock() function is a POSIX compliant method to retrieve the program’s processor time. The function returns the integer value that needs to be divided by a macro-defined constant called CLOCKS_PER_SEC to convert to several seconds.

The following sample code implements two functions for finding the maximum value in an int array. The max_index function does the searching based on the index of the elements, whereas the max_value is value-based. The goal is to calculate how much time they spend on finding the maximum int value in the 1,000,000 element array filled with random integers.

Notice that we call the clock() function twice - before the max_index call and then after the call. This time-measuring scheme can be employed generally without any regard to the specific function that retrieves the time. In this case, we may see (depending on the hardware system) that max_value gets the job done slightly faster than index-based search. Mind though, these maximum search algorithms have O(N) complexity and should not be employed in any professional codebase except for maybe experimentation.

#include <chrono>
#include <iostream>

using std::cout;
using std::endl;

int max_index(int arr[], int size) {
  size_t max = 0;

  for (int j = 0; j < size; ++j) {
    if (arr[j] > arr[max]) {
      max = j;
    }
  }
  return arr[max];
}

int max_value(int arr[], int size) {
  int max = arr[0];

  for (int j = 0; j < size; ++j) {
    if (arr[j] > max) {
      max = arr[j];
    }
  }
  return max;
}

constexpr int WIDTH = 1000000;

int main() {
  clock_t start, end;
  int max;

  int *arr = new int[WIDTH];

  std::srand(std::time(nullptr));
  for (size_t i = 0; i < WIDTH; i++) {
    arr[i] = std::rand();
  }

  start = clock();
  max = max_index(arr, WIDTH);
  end = clock();
  printf("max_index: %0.8f sec, max = %d\n",
         ((float)end - start) / CLOCKS_PER_SEC, max);

  start = clock();
  max = max_value(arr, WIDTH);
  end = clock();
  printf("max_value: %0.8f sec, max = %d\n",
         ((float)end - start) / CLOCKS_PER_SEC, max);

  exit(EXIT_SUCCESS);
}

Output:

max_value: 0.00131400 sec, max = 2147480499
max_value: 0.00089800 sec, max = 2147480499

Use the gettimeofday Function to Implement a Timer in C++

gettimeofday is a highly accurate time retrieval function in Linux based systems, which can be called from the C++ source code as well. The function was designed to get the time and timezone data, but the latter has been depreciated for some time, and the second argument should be nullptr instead of the valid timezone struct. gettimeofday stores the timing data in the special struct called timeval, that contains two data members tv_sec representing seconds and tv_usec for microseconds.

As a general rule, we declare and initialize the two timeval structures before calling the function gettimeofday. Once the function gets called, the data should be stored successfully in the corresponding struct if the return value of gettimeofday is 0. Otherwise, the failure is indicated by returning -1 value. Notice that, after the structs are filled with data, it needs conversion to time’s common unit value. This sample code implements the time_diff function that returns the time in seconds, which can be outputted to the console as needed.

#include <sys/time.h>

#include <ctime>
#include <iostream>

using std::cout;
using std::endl;

int max_index(int arr[], int size) {
  size_t max = 0;

  for (int j = 0; j < size; ++j) {
    if (arr[j] > arr[max]) {
      max = j;
    }
  }
  return arr[max];
}

int max_value(int arr[], int size) {
  int max = arr[0];

  for (int j = 0; j < size; ++j) {
    if (arr[j] > max) {
      max = arr[j];
    }
  }
  return max;
}

float time_diff(struct timeval *start, struct timeval *end) {
  return (end->tv_sec - start->tv_sec) + 1e-6 * (end->tv_usec - start->tv_usec);
}

constexpr int WIDTH = 1000000;

int main() {
  struct timeval start {};
  struct timeval end {};
  int max;

  int *arr = new int[WIDTH];

  std::srand(std::time(nullptr));
  for (size_t i = 0; i < WIDTH; i++) {
    arr[i] = std::rand();
  }

  gettimeofday(&start, nullptr);
  max = max_index(arr, WIDTH);
  gettimeofday(&end, nullptr);
  printf("max_index: %0.8f sec, max = %d\n", time_diff(&start, &end), max);

  gettimeofday(&start, nullptr);
  max = max_value(arr, WIDTH);
  gettimeofday(&end, nullptr);
  printf("max_value: %0.8f sec, max = %d\n", time_diff(&start, &end), max);

  exit(EXIT_SUCCESS);
}

Output:

max_value: 0.00126000 sec, max = 2147474877
max_value: 0.00093900 sec, max = 2147474877