Figure 7.12 shows the output that summarizes the 10 grades we store in an object of class GradeBook (Fig. 7.13), which uses an array of integers to store the grades of 10 students for a single exam. The array grades is declared as a data member in line 142 of Fig. 7.13—therefore, each GradeBook object maintains its own set of grades.

The size of the array in line 142 of Fig. 7.13 is specified by public static const data member students (declared in line 12), which is public so that it’s accessible to the class’s clients. We’ll soon see an example of a client program using this constant. Declaring students with the const qualifier indicates that this data member is constant—its value cannot be changed after being initialized. Keyword static in this variable declaration indicates that the data member is shared by all objects of the class—so in this particular implementation of class GradeBook, all GradeBook objects store grades for the same number of students. Recall from Section 3.3 that when each object of a class maintains its own copy of an attribute, the variable that represents the attribute is known as a data member—each object of the class has a separate copy of the variable in memory. There are variables for which each object of a class does not have a separate copy. That’s the case with static data members, which are also known as class variables. When objects of a class containing static data members are created, all the objects share one copy of the class’s static data members. A static data member can be accessed within the class definition and the member-function definitions like any other data member. As you’ll soon see, a public static data member can also be accessed outside of the class, even when no objects of the class exist, using the class name followed by the scope resolution operator (::) and the name of the data member. You’ll learn more about static data members in Chapter 9.

The class’s constructor (lines 15–18) has two parameters—the course name and a reference to an array of grades. When a program creates a GradeBook object (e.g., line 13 of Fig. 7.14), the program passes an existing int array to the constructor, which copies the array’s values into the data member grades (line 17 of Fig. 7.13). The grade values in the passed array could have been input from a user or read from a file on disk (as we discuss in Chapter 14, File Processing). In our test program, we simply initialize an array with a set of grade values (Fig. 7.14, lines 9–10). Once the grades are stored in data member grades of class GradeBook, all the class’s member functions can access the grades array as needed to perform various calculations. Note that the constructor receives both the string and the array by reference—this is more efficient than receiving copies of the original string and array. The constructor does not need to modify either the original string or array, so we also declared each parameter as const to ensure that the constructor does not accidentally modify the original data in the caller. We also declared function setCourseName to receives its string argument by reference.

Member function processGrades (lines 38–50 of Fig. 7.13) contains a series of member function calls that output a report summarizing the grades. Line 39 calls member function outputGrades to print the contents of the array grades. Lines 135–138 in member function outputGrades use a for statement to output each student’s grade. Although array indices start at 0, a professor would typically number students starting at 1. Thus, lines 136–137 output student + 1 as the student number to produce grade labels "Student 1: ", "Student 2: ", and so on.

Member function processGrades next calls member function getAverage (line 43) to obtain the average of the grades. Member function getAverage (lines 83–93) totals the values in array grades before calculating the average. The averaging calculation in line 92 uses grades.size() to determine the number of grades being averaged.

Lines 46–47 in processGrades call member functions getMinimum and getMaximum to determine the lowest and highest grades of any student on the exam, respectively. Let’s examine how member function getMinimum finds the lowest grade. Because the highest grade allowed is 100, we begin by assuming that 100 is the lowest grade (line 54). Then, we compare each of the elements in the array to the lowest grade, looking for smaller values. Lines 57–62 in member function getMinimum loop through the array, and line 59 compares each grade to lowGrade. If a grade is less than lowGrade, lowGrade is set to that grade. When line 64 executes, lowGrade contains the lowest grade in the array. Member function getMaximum (lines 68–80) works similarly to member function getMinimum.

Finally, line 49 in member function processGrades calls member function outputBarChart to print a distribution chart of the grade data using a technique similar to that in Fig. 7.7. In that example, we manually calculated the number of grades in each category (i.e., 0–9, 10–19, …, 90–99 and 100) by simply looking at a set of grades. In Fig. 7.13, lines 104–106 use a technique similar to that in Fig. 7.8 and Fig. 7.9 to calculate the frequency of grades in each category. Line 100 declares and creates array frequency of 11 unsigned ints to store the frequency of grades in each grade category. For each grade in array grades, lines 104–106 increment the appropriate element of the frequency array. To determine which element to increment, line 105 divides the current grade by 10 using integer division. For example, if grade is 85, line 105 increments frequency[8] to update the count of grades in the range 80–89. Lines 109–127 next print the bar chart (see Fig. 7.12) based on the values in array frequency. Like lines 28–30 of Fig. 7.7, lines 122–124 of Fig. 7.13 use a value in array frequency to determine the number of asterisks to display in each bar.

The program of Fig. 7.14 creates an object of class GradeBook using the int array grades (declared and initialized in lines 9–10). The scope resolution operator (::) is used in the expression GradeBook::students (line 9) to access class GradeBook’s static constant students. We use this constant here to create an array that’s the same size as the array stored as a data member in class GradeBook. Line 11 declares a string representing the course name. Line 13 passes the course name and the array of grades to the GradeBook constructor. Line 14 displays a welcome message, and line 15 invokes the GradeBook object’s processGrades member function.