Understanding do/while loop C++

Question

I do not understand how this loop repeats when the user inputs 'Y'. All of the code is in the "do" part; and the do part repeats once? After that while statement is just a return statement.

int main()
{   
   int score1, score2, score3; //three scores
   double average;             // Average score
   char again;                 // To hold Y or N input

   do
   {
       /Get three Scores.
       cout << "Enter 3 scores and i will average them: ";
       cin >> score1 >> score2 >> score 3;

       //calculate and display the average
       average = (score1 + score2 + score 3)/3.0;
       cout << "The average is " << average << :.\n;

       // Does the user want to average another set?
       cout << "Do you want to average another set? (Y/N) ":
       cin >> again;
   } while(again == 'Y' || again == 'y');

    return 0;   

}

The text book explanation is too brief and not clicking in my head. Thank you for your time.

Answer 1

What you are looking at is a do-while loop. It is a different construct than a while loop.

From cppreference.com, emphasis mine:

---

'while' loop

while ( <condition> )
{
    // code
}

Executes a statement repeatedly, until the value of condition becomes false. The test takes place before each iteration.

---

'do-while' loop

do
{
     // code
} while ( <condition> );

Executes a statement repeatedly, until the value of expression becomes false. The test takes place after each iteration.

Answer 2

You don't have a while loop after the do. You don't have a while loop anywhere! The while is just a way of terminating the do loop.

Answer 3

I'll reduce it to the relevant parts, and in pseudo code:

do
{
   // do stuff that's irrelevant here
   ask user to type Y or N and store into "again" // that's the cin >> again line
}
while (again is Y or y)

The key is that the loop runs until the variable "again" is either Y or y.

And the variable again is set inside the loop, based on the user input.

Take these two things together, and you find that the loop runs until the user inputs Y or y.

Answer 4

I know i am missing something but there is nothing in the while loop, how does entering 'Y' make it go back? do is supposed to happen once and then the while loop executed. I am not understanding what is making the loop go back around

The while keyword expects a condition to determine whether the code within its body should be executed or not. In your case you have given a condition that the loop should be executed if the variable again is set to either 'y' or 'Y'.

} while(again == 'Y' || again == 'y');

Refer to this link for a detailed explanation on while vs do-while loop constructs.

Answer 5

The body of the loop starts at the { after the do and ends at the } before the while.

It seems like you are familiar with the while loop and now you just need to understand that a do-while loop has slightly different structure (condition comes after the body as opposed to before).

Example:

while ( condition ) {    // while loop
     body
}

do {                     // do-while loop
    body
} while (condition);     

Note that they are not equivalent, you typically choose one over the other depending on whether it is more natural to check the condition before or after doing the iterations.