Need to understand for loop better - post increment operator

Question

I was not really clear with the post increment operator that I always used with for loops. My latest and newly acquired understanding of post increment operator is the following:

int a = 5
int b = a++ //a will increment and return back its old value 5
so b = 5

Armed with this new knowledge i decided to understand/apply it to the places where i commonly used the post increment operator as in a for loop . Now it seems like I am lost since I am ending up with the wrong output theoretically

Consider the following code

for(int i=0 ; i< 3 ; i++)
{
  std::cout << i; 
}

First loop

i starts with 0 which is less than 3 so ( increment by 1 however since its i++ it returns old value 0) 
so cout should display 1 // But it displays 0

Second Loop

 i is now 1 which is less than 3 so i++ is applied - Now i is 2 and returns back 1
 so cout should display 2 //But it display 1

Third Loop

 i is now 2 which is less than 3 so i++ is applied - Now i is 3 and returns back 2
 so cout should display 3 //But it display 2

Fourth Loop

 i is now 3 which is not less than 3 so loop exits

Could anyone please clear my understanding and point me in the right direction. The output should be 0,1,2 where am i going wrong ?

Answer 1

What you're missing is when each of those sections of the for statement happen:

for (int i = 0 ; i < 3 ; i++)
//   111111111   22222   333

• The first bit happens once before any iterations are done.
• The second expression is evaluated before each potential iteration and, if false, no further iterations are done.
• The third bit is done at the end of each iteration, before returning to evaluate the second bit.

Now re-read that last bullet point carefully. The i++ is done at the end of an iteration, after the cout << i. And, immediately after that, the continuation condition is checked (the second part).

So the loop is effectively the same as:

{   // Outer braces just to limit scope of i, same as for loop.
    int i = 0;
    while (i < 3) {
        cout << i;
        i++;
    } 
}

That's why you get 0 1 2.