Tip (5 minute solution): The Stopwatch

[Valmont]

Students often ask how they can test their algorithms for speed. I'll present you a simple, platform independent class to measure speeds. A function main() is included to demonstrate its usage.

Question: After testing the three differenct loop-functions, why would anyone prefer a certain version?

stopwatch.h

#ifndef STOPWATCH_H
#define STOPWATCH_H

#include <ctime>

inline static double seconds()
{
  const double secs_per_tick = 1.0 / std::CLOCKS_PER_SEC;
  return ( (double) std::clock() ) * secs_per_tick;
}

class Stopwatch 
{
private:
  bool running_;
  double start_time_;
  double total_;
public:
  inline Stopwatch();
  inline void start();
  inline double stop();
  inline double read();
  inline void resume();
  inline int running();
};

inline Stopwatch::Stopwatch() : 
  running_(0), start_time_(0.0), total_(0.0) 
{}

void Stopwatch::start() 
{
  running_ = true;
  total_ = 0.0;
  start_time_ = seconds();
}

double Stopwatch::stop()  
{
  if (running_)
  {
    total_ += (seconds() - start_time_);
    running_ = 0;
  }
  return total_; 
}

inline void Stopwatch::resume()
{
  if (!running_)
  {
    start_time_ = seconds();
    running_ = 1;
  }
}
    

inline double Stopwatch::read()   
{
  if (running_)
  {
    stop();
    resume();
  }
  return total_;
}

#endif //STOPWATCH_H

main.cpp

#include "stopwatch.h"
#include <iostream>

using namespace std;

unsigned dTestArray[200000000];

void do_loop()
{
  unsigned i = 0;
  do
  {
    dTestArray[i] = i;
    ++i;   
  } while(i < 200000000);
}  

void while_loop()
{
  unsigned i = 0;
  while(i < 200000000)
  {
    dTestArray[i] = i;
    ++i;
  }  
}

void for_loop()
{
  for(unsigned i = 0; i < 200000000; ++i)
  {
    dTestArray[i] = i;
  }  
}  

int main()
{
  Stopwatch W;
  
  W.start();
  for(unsigned i = 0; i < 20; ++i)
  {
    //Change the loop versions here to compare the loop speeds.
    for_loop();
  }  
  W.stop();
  
  std::cout<<W.read()<<std::endl;
  std::cin.get();
  
  return 0;
}