The do-while-Loop

1. The syntax of the do-while-loop is:
   
   do
       Yes_Statement
   while (Boolean_Expression);
   
   As usual the single Yes_Statement can always be replaced by a compound statement (a group of statements enclosed by curely braces).
   
   The execution sequence of the do-while-loop is as follows:
   
   
   • First the statement in the loop body is executed
     
   • The Boolean_expression is then evaluated. If it is true then the loop is executed again; the Boolean expression is checked again, and so forth.
     
   • When the Boolean_expression is tested and found to be false, the do-while-loop is exited and the next program statement after the while-loop is executed.
     
     
2. Note that the loop body is executed at least once even if the Boolean_expression evaluates to false when the do-while-loop begin.
   
   
3. // Illustration of the use of the do-while-loop.
   // The user is asked to enter two numbers of type double.
   // The program then computes and prints out the answer for the
   // product of the numbers.
   // The user is then asked if (s)he wants the program to do another
   // calculation.
   // The user answers "yes" by typing the letter y in upper or lower case.
   // and the process of asking to input two numbers and computing and
   // printing out the product is repeated until the user responds by
   // typing any key other than the letter y.
   
   #include <iostream>
   using namespace std;
   
   int main ( )
   {
        double Num1, Num2, Product;
        char Key;
        cout.setf(ios::fixed);
        cout.setf(ios::showpoint);
        cout.precision(7);
   
        do
        {
              cout << "Enter two numbers of type double: ";
              cin>> Num1 >> Num2;
              Product = Num1 * Num2;
              cout << "The product of " << Num1 << " and " << Num2
                     << " is: " << Product << endl << endl;
              cout << "If you want me to calculate another product for you\n"
                      << "then press \'Y\' or \'y\',\n"
                     << "otherwise press any other key to end the calculation.\n";
              cin >> Key;
        } while (Key == 'Y' || Key == 'y');
   
        cout << "It is always so nice working with you. Bye! \n\n";
        return 0;
   } //end main
   
   A possible dialogue is:
   
   Enter two numbers of type double: -2.13 3.1415962
   The product of -2.13 and 3.1415962 is: -6.6915999
   
   If you want me to calculate another product for you
   then press 'Y' or 'y'
   otherwise press any other key to end the calculation.
   y
   Enter two numbers of type double: 1.2345678 8.7654321e-1
   The product of 1.2345678 and 0.8765432 is: 1.0821520
   
   If you want me to calculate another product for you
   then press 'Y' or 'y'
   otherwise press any other key to end the calculation.
   n
   It is always so nice working with you. Bye!
   
   
4. The do-while-loop is often used to verify that the input data from the keyboard are of the correct form. The following code illustrates how that can be done:
   
   char ch;
   do
   {
        cout << "Enter a digit: ";
        cin >> ch;
   } while ((ch < '0') || ('9' < ch));
   
   The loop never ends until the user supplies the correct data.
   
   
5. If the loop condition turns out to be always satisfied, the loop will never end. Be careful not to have these so called infinite loops.
   
   
6. Note that the do-while loop ends with a semicolon.