Question

I have the following two loops:

#include <iostream>
#include <stdlib.h>
#include <time.h>

using namespace std;
int main(){

    int start=clock();
    for (int i=0;i<100;i++)
        cout<<i<<" "<<endl;
    cout<<clock()-start<<"\n"<<endl;
    cout<<"\n";

    int start1=clock();
    for (int j=0;j<100;++j)
        cout<<j<<" "<<endl;
    cout<<"\n";
    cout<<clock()-start1<<" \n "<<endl;

    return 0;
}

I ran that three times. On the first two runs, the second loop was fastest but, on the third run, the first loop was fastest. What does this mean? Which is better? Does it depend on the situation?

Answer 1

In your case it is probably standard measurement error and it does not matter do you use post-increment or pre-increment. For standard types (int, byte …) it does not matter.

BUT you should get used to using pre-increment because there are performance implications if you are using them on a Class, depending how those operators are implemented. Post-increment operator i++ has to make a copy of the object

For example:

class integer
{
public:
  integer(int t_value)
    : m_value(t_value)
  {
  }

  int get_value() { return m_value; }

  integer &operator++() // pre increment operator
  {
    ++m_value;
    return *this;
  }

  integer operator++(int) // post increment operator
  {
    integer old = *this; // a copy is made, it's cheap, but it's still a copy
    ++m_value;
    return old; // Return old copy
  }

private:
  int m_value;

Take a look at the following answer

StackOverflow: i++ less efficient than ++i, how to show this?