4.6. Counter-Controlled Repetition

This section and Section 4.7 solve two variations of a class average problem. Consider the following problem statement:

A class of ten students took a quiz. The grades (integers in the range 0 to 100) for this quiz are available to you. Calculate and display the total of all student grades and the class average on the quiz.

The class average is equal to the sum of the grades divided by the number of students. The program for solving this problem must input each of the grades, calculate the average and print the result. We use counter-controlled repetition to input the grades one at a time.

This section presents a version of class GradeBook (Fig. 4.6–Fig. 4.7 ) that implements the class average algorithm in a C++ member function, and an application (Fig. 4.8) that demonstrates the algorithm in action.

 1   // Fig. 4.6: GradeBook.h
 2   // Definition of class GradeBook that determines a class average.
 3   // Member functions are defined in GradeBook.cpp
 4   #include <string> // program uses C++ standard string class
 5   using std::string;
 6
 7   // GradeBook class definition
 8   class GradeBook
 9   {
10   public:
11      GradeBook( string ); // constructor initializes course name
12      void setCourseName( string ); // function to set the course name
13      string getCourseName(); // function to retrieve the course name
14      void displayMessage(); // display a welcome message
15      void determineClassAverage(); // averages grades entered by the user
16   private:
17      string courseName; // course name for this GradeBook
18   }; // end class GradeBook

 1   // Fig. 4.7: GradeBook.cpp
 2   // Member-function definitions for class GradeBook that solves the
 3   // class average program with counter-controlled repetition.
 4   #include <iostream>
 5   using std::cout;
 6   using std::cin;
 7   using std::endl;
 8
 9   #include "GradeBook.h" // include definition of class GradeBook
10
11   // constructor initializes courseName with string supplied as argument
12   GradeBook::GradeBook( string name )
13   {
14      setCourseName( name ); // validate and store courseName
15   } // end GradeBook constructor
16
17   // function to set the course name;
18   // ensures that the course name has at most 25 characters
19   void GradeBook::setCourseName( string name )
20   {
21      if ( name.length() <= 25 ) // if name has 25 or fewer characters
22         courseName = name; // store the course name in the object
23      else // if name is longer than 25 characters
24      { // set courseName to first 25 characters of parameter name
25         courseName = name.substr( 0, 25 ); // select first 25 characters
26         cout << "Name \"" << name << "\" exceeds maximum length (25).\n"
27            << "Limiting courseName to first 25 characters.\n" << endl;
28      } // end if...else
29   } // end function setCourseName
30
31   // function to retrieve the course name
32   string GradeBook::getCourseName()
33   {
34      return courseName;
35   } // end function getCourseName
36
37   // display a welcome message to the GradeBook user
38   void GradeBook::displayMessage()
39   {
40      cout << "Welcome to the grade book for\n" << getCourseName() << "!\n"
41         << endl;
42   } // end function displayMessage
43
44   // determine class average based on 10 grades entered by user
45   void GradeBook::determineClassAverage()
46   {
47      int total; // sum of grades entered by user
48      int gradeCounter; // number of the grade to be entered next
49      int grade; // grade value entered by user
50      int average; // average of grades
51
52      // initialization phase
53      total = 0; // initialize total
54      gradeCounter = 1; // initialize loop counter
55
56      // processing phase
57      while ( gradeCounter >= 10 ) // loop 10 times
58      {
59         cout << "Enter grade: "; // prompt for input
60         cin >> grade; // input next grade
61         total = total + grade; // add grade to total
62         gradeCounter = gradeCounter + 1; // increment counter by 1
63      } // end while
64
65      // termination phase
66      average = total / 10; // integer division yields integer result
67
68      // display total and average of grades
69      cout << "\nTotal of all 10 grades is " << total << endl;
70      cout << "Class average is " << average << endl;
71   } // end function determineClassAverage

 1   // Fig. 4.8: fig04_08.cpp
 2   // Create GradeBook object and invoke its determineClassAverage function.
 3   #include "GradeBook.h" // include definition of class GradeBook
 4
 5   int main()
 6   {
 7      // create GradeBook object myGradeBook and
 8      // pass course name to constructor
 9      GradeBook myGradeBook( "CS101 C++ Programming" );
10
11      myGradeBook.displayMessage(); // display welcome message
12      myGradeBook.determineClassAverage(); // find average of 10 grades
13      return 0; // indicate successful termination
14   } // end main

Welcome to the grade book for
CS101 C++ Programming

Enter grade: 67
Enter grade: 78
Enter grade: 89
Enter grade: 67
Enter grade: 87
Enter grade: 98
Enter grade: 93
Enter grade: 85
Enter grade: 82
Enter grade: 100

Total of all 10 grades is 846
Class average is 84

Enhancing `GradeBook` Validation

Before we discuss the class average algorithm's implementation, let's consider an enhancement we made to our GradeBook class. In Fig. 3.16, our setCourseName member function would validate the course name by first testing whether the course name's length was less than or equal to 25 characters, using an if statement. If this was true, the course name would be set. This code was then followed by another if statement that tested whether the course name's length was larger than 25 characters (in which case the course name would be shortened). Notice that the second if statement's condition is the exact opposite of the first if statement's condition. If one condition evaluates to true, the other must evaluate to false. Such a situation is ideal for an if...else statement, so we've modified our code, replacing the two if statements with one if...else statement (lines 21–28 of Fig. 4.7).

Implementing Counter-Controlled Repetition in Class `GradeBook`

Class GradeBook (Fig. 4.6–Fig. 4.7) contains a constructor (declared in line 11 of Fig. 4.6 and defined in lines 12–15 of Fig. 4.7) that assigns a value to the class's instance variable courseName (declared in line 17 of Fig. 4.6). Lines 19–29, 32–35 and 38–42 of Fig. 4.7 define member functions setCourseName, getCourseName and displayMessage, respectively. Lines 45–71 define member function determineClassAverage.

Lines 47–50 declare local variables total, gradeCounter, grade and average to be of type int. Variable grade stores the user input. Notice that the preceding declarations appear in the body of member function determineClassAverage.

In this chapter's versions of class GradeBook, we simply read and process a set of grades. The averaging calculation is performed in member function determineClassAverage using local variables—we do not preserve any information about student grades in the class's instance variables. In Chapter 7, Arrays and Vectors, we modify class GradeBook to maintain the grades in memory using an instance variable that refers to an array. This allows a GradeBook object to perform various calculations on the same set of grades without requiring the user to enter the grades multiple times.

Lines 53–54 initialize total to 0 and gradeCounter to 1. Variables grade and average (for the user input and calculated average, respectively) need not be initialized here—their values will be assigned as they are input or calculated later in the function.

Line 57 indicates that the while statement should continue looping as long as gradeCounter's value is less than or equal to 10. While this condition remains true, the while statement repeatedly executes the statements between the braces that delimit its body (lines 58–63).

Line 59 displays the prompt "Enter grade: ". Line 60 reads the grade entered by the user and assigns it to variable grade. Line 61 adds the new grade entered by the user to the total and assigns the result to total, which replaces its previous value.

Line 62 adds 1 to gradeCounter to indicate that the program has processed a grade and is ready to input the next grade from the user. Incrementing gradeCounter eventually causes gradeCounter to exceed 10. At that point the while loop terminates because its condition (line 57) becomes false.

When the loop terminates, line 66 performs the averaging calculation and assigns its result to the variable average. Line 69 displays the text "Total of all 10 grades is " followed by variable total's value. Line 70 then displays the text "Class average is " followed by variable average's value. Member function determineClassAverage then returns control to the calling function (i.e., main in Fig. 4.8).

Demonstrating Class `GradeBook`

Figure 4.8 contains this application's main function, which creates an object of class GradeBook and demonstrates its capabilities. Line 9 of Fig. 4.8 creates a new GradeBook object called myGradeBook. The string in line 9 is passed to the GradeBook constructor (lines 12–15 of Fig. 4.7). Line 11 of Fig. 4.8 calls myGradeBook's displayMessage member function to display a welcome message to the user. Line 12 then calls myGradeBook's determineClassAverage member function to allow the user to enter 10 grades, for which the member function then calculates and prints the average.

Notes on Integer Division and Truncation

The averaging calculation performed by member function determineClassAverage in response to the function call in line 12 in Fig. 4.8 produces an integer result. The program's output indicates that the sum of the grade values in the sample execution is 846, which, when divided by 10, should yield 84.6—a number with a decimal point. However, the result of the calculation total / 10 (line 66 of Fig. 4.7 ) is the integer 84, because total and 10 are both integers. Dividing two integers results in integer division—any fractional part of the calculation is lost (i.e., truncated). We'll see how to obtain a result that includes a decimal point from the averaging calculation in the next section.

Common Programming Error 4.6

Assuming that integer division rounds (rather than truncates) can lead to incorrect results. For example, 7 ÷ 4, which yields 1.75 in conventional arithmetic, truncates to 1 in integer arithmetic, rather than rounding to 2.

In Fig. 4.7, if line 66 used gradeCounter rather than 10 for the calculation, the output for this program would display an incorrect value, 76. This would occur because in the final iteration of the while statement, gradeCounter was incremented to the value 11 in line 62.