The While-Loop

1. The syntax of the while-loop is:
   
   while (Boolean_Expression)
       Yes_Statement
   
   The execution sequence of the while-loop is as follows:
   
   
   • First the Boolean-Expression is test, if it is true, the loop statement is executed
     
   • The Boolean_expression is then retested
     
   • The loop statement is executed as long as (while) the Boolean_Expression is true.
     
   • When the Boolean_expression is tested and found to be false, the while-loop is exited and the next program statement after the while-loop is executed.
     
     
2. Note that if the Boolean_expression evaluates to false when the while-loop begin, then the loop body is never executed.
   
   
3. // Illustration of the use of the while-loop
   // The user first enters two integers
   // and then types in the answer for the sum of the integers.
   // If the answer is correct, then the program responds
   // with an encouraging statement.
   // The program should not end until the correct answer is entered.
   
   #include <iostream>
   using namespace std;
   
   int main ( )
   {
        int Num1, Num2, CorrectAnswer, Input;
        cout << "Type 2 integers separated by a space and press enter: ";
        cin >> Num1 >> Num2;
        CorrectAnswer = Num1 + Num2;
        cout << "Add up " << Num1 << " and " << Num2
                << " and enter your answer: ";
        cin >> Input;
   
        while (Input != CorrectAnswer)
   {
         cout << "Your answer is incorrect!\n";
         cout << "Please re-calculate the sum of " << Num1
                << " and " << Num2 << " and re-enter the result: ";
         cin >> Input;
   }
   
   cout << "Grrreat! Your answer is corrrect!" << endl;
   return 0;
   } //end main
   
   The following shows an example of the dialogue:
   
   Type 2 integers separated by a space and press enter: 11 6
   Add up 11 and 6 and enter your answer: 2
   Your answer is incorrect!
   Please re-calculate the sum of 11 and 6 and reenter the result: -23
   Your answer is incorrect!
   Please re-calculate the sum of 11 and 6 and reenter the result: 17
   Grrreat! Your answer is corrrect!
   
   
4. The following example is from the textbook:
   
   #include <iostream>
   using namespace std;
   
   int main( )
   {
       double balance = 50.00;
       int count = 0;
   
       cout << "This program tells you how long it takes\n"
          << "to accumulate a debt of $100, starting with\n"
          << "an initial balance of $50 owed.\n"
          << "The interest rate is 2% per month.\n";
   
       while (balance < 100.00)
       {
       balance = balance + 0.02 * balance;
       count++;
       }
   
       cout << "After " << count << " months,\n";
       cout.setf(ios::fixed);
       cout.setf(ios::showpoint);
       cout.precision(2);
       cout << "your balance due will be $" << balance << endl;
   
       return 0;
   }
   
   
5. // An example of a sentinel-controlled while-loop
   // The program asks the user to enter the final exam score
   // for each of the student in a class,
   // and then computes the average score for the class.
   // As usual, there are always some students who did not take the exam
   // (bad students), and the professor is lazy and does not want to count
   // the number of exam scores before entering them (bad professor).
   //
   // To compute the class average, we can declare a variable, say sum,
   // to represent the sum of the scores, and initialize it to 0.
   // After reading each score from the keyboard, it is added to sum.
   // After we are done reading all the scores, we need to divide the sum
   // by the total number of scores. We can obtain that number if we use a
   // counter which is initialized to 0 and is incremented by 1
   // after a score is read.
   // The remaining challenge is to come up with a way to know when
   // to stop the loop.
   // One way to get around the problem is to use a unique data value,
   // called the sentinel value, as the last data item.
   // Here we use a score of -1 as the sentinel value,
   // since we believe exam scores cannot be negative
   // no matter how crazy the professor may be.
   
   #include <iostream>
   using namespace std;
   
   int main ( )
   {
        const int SENTINEL = -1;
        int count = 0, sum = 0, score;
        float average;
   
        cout << "Enter scores one at a time as requested.\n";
        cout << "When done, enter " << SENTINEL << " to stop.\n";
        cout << "Enter the first score: ";
        cin >> score;
        while (score != SENTINEL)
        {
            count++;
            sum += score;
            cout << "Enter the next score: ";
            cout << "When done, enter " << SENTINEL << " to stop.\n";
            cin >> score;
        } // end while
   
        if (count==0)
           {
           cout << "You didn't enter a meaningful number.\n"
               << "No average is computed.\n";
           }
        else
           {
           // compute the class average
           average = float(sum)/float(count);
           // output the results
           cout << count << " exam scores were processed.\n";
           cout << "Average score is: " << average << endl;
           }
   
        return 0;
   } //end main