Case Study: Class GradeBook Using a Two-Dimensional Array

Case Study Class GradeBook Using a Two Dimensional Array

In Section 7.8, we presented class GradeBook (Fig. 7.14), which used a one-dimensional array to store student grades on a single exam. In most semesters, students take several exams. Professors are likely to want to analyze grades across the entire semester, both for a single student and for the class as a whole.

Storing Student Grades in a Two-Dimensional Array in Class GradeBook

Figure 7.18 contains a version of class GradeBook that uses a two-dimensional array grades to store the grades of a number of students on multiple exams. Each row of the array represents a single student's grades for the entire course, and each column represents a grade on one of the exams the students took during the course. An application such as GradeBookTest (Fig. 7.19) passes the array as an argument to the GradeBook constructor. In this example, we use a ten-by-three array containing ten students' grades on three exams. Five methods perform array manipulations to process the grades. Each method is similar to its counterpart in the earlier one-dimensional array version of class GradeBook (Fig. 7.14). Method getMinimum (lines 5270) determines the lowest grade of any student for the semester. Method getMaximum (lines 7391) determines the highest grade of any student for the semester. Method getAverage (lines 94104) determines a particular student's semester average. Method outputBarChart (lines 107137) outputs a bar chart of the distribution of all student grades for the semester. Method outputGrades (lines 140164) outputs the two-dimensional array in a tabular format, along with each student's semester average.

Figure 7.18. GradeBook class using a two-dimensional array to store grades.

(This item is displayed on pages 316 - 319 in the print version)

 1 // Fig. 7.18: GradeBook.java
 2 // Grade book using a two-dimensional array to store grades.
 3
 4 public class GradeBook
 5 {
 6 private String courseName; // name of course this grade book represents
 7 private int grades[][]; // two-dimensional array of student grades
 8
 9 // two-argument constructor initializes courseName and grades array
10 public GradeBook( String name, int gradesArray[][] )
11 {
12 courseName = name; // initialize courseName
13 grades = gradesArray; // store grades
14 } // end two-argument GradeBook constructor
15 
16 // method to set the course name
17 public void setCourseName( String name )
18 {
19 courseName = name; // store the course name
20 } // end method setCourseName
21 
22 // method to retrieve the course name
23 public String getCourseName()
24 {
25 return courseName;
26 } // end method getCourseName
27 
28 // display a welcome message to the GradeBook user
29 public void displayMessage()
30 {
31 // getCourseName gets the name of the course
32 System.out.printf( "Welcome to the grade book for
%s!

",
33 getCourseName() );
34 } // end method displayMessage
35 
36 // perform various operations on the data
37 public void processGrades()
38 {
39 // output grades array
40 outputGrades();
41 
42 // call methods getMinimum and getMaximum
43 System.out.printf( "
%s %d
%s %d

",
44 "Lowest grade in the grade book is", getMinimum(),
45 "Highest grade in the grade book is", getMaximum() );
46 
47 // output grade distribution chart of all grades on all tests
48 outputBarChart();
49 } // end method processGrades
50 
51 // find minimum grade
52 public int getMinimum()
53 {
54 // assume first element of grades array is smallest
55 int lowGrade = grades[ 0 ][ 0 ];
56 
57 // loop through rows of grades array 
58 for ( int studentGrades[] : grades ) 
59 { 
60  // loop through columns of current row 
61  for ( int grade : studentGrades ) 
62  { 
63  // if grade less than lowGrade, assign it to lowGrade
64  if ( grade < lowGrade ) 
65  lowGrade = grade; 
66  } // end inner for 
67 } // end outer for 
68 
69 return lowGrade; // return lowest grade
70 } // end method getMinimum
71 
72 // find maximum grade
73 public int getMaximum()
74 {
75 // assume first element of grades array is largest
76 int highGrade = grades[ 0 ][ 0 ];
77 
78 // loop through rows of grades array
79 for ( int studentGrades[] : grades )
80 {
81 // loop through columns of current row
82 for ( int grade : studentGrades )
83 {
84 // if grade greater than highGrade, assign it to highGrade
85 if ( grade > highGrade )
86 highGrade = grade;
87 } // end inner for
88 } // end outer for
89 
90 return highGrade; // return highest grade
91 } // end method getMaximum
92 
93 // determine average grade for particular set of grades
94 public double getAverage( int setOfGrades[] ) 
95 { 
96  int total = 0; // initialize total 
97 
98  // sum grades for one student 
99  for ( int grade : setOfGrades ) 
100  total += grade; 
101 
102  // return average of grades 
103  return (double) total / setOfGrades.length; 
104 } // end method getAverage 
105
106 // output bar chart displaying overall grade distribution
107 public void outputBarChart()
108 {
109 System.out.println( "Overall grade distribution:" );
110
111 // stores frequency of grades in each range of 10 grades
112 int frequency[] = new int [ 11 ];
113
114 // for each grade in GradeBook, increment the appropriate frequency
115 for ( int studentGrades[] : grades ) 
116 { 
117  for ( int grade : studentGrades ) 
118  ++frequency[ grade / 10 ]; 
119 } // end outer for 
120
121 // for each grade frequency, print bar in chart
122 for ( int count = 0; count < frequency.length; count++ )
123 {
124 // output bar label ( "00-09: ", ..., "90-99: ", "100: " )
125 if ( count == 10 )
126 System.out.printf( "%5d: ", 100 );
127 else
128 System.out.printf( "%02d-%02d: ",
129 count * 10, count * 10 + 9 );
130
131 // print bar of asterisks
132 for ( int stars = 0; stars < frequency[ count ]; stars++ )
133 System.out.print( "*" );
134
135 System.out.println(); // start a new line of output
136 } // end outer for
137 } // end method outputBarChart
138
139 // output the contents of the grades array
140 public void outputGrades()
141 {
142 System.out.println( "The grades are:
" );
143 System.out.print( " " ); // align column heads
144
145 // create a column heading for each of the tests
146 for ( int test = 0; test < grades[ 0 ].length; test++ )
147 System.out.printf( "Test %d ", test + 1 );
148
149 System.out.println( "Average" ); // student average column heading
150
151 // create rows/columns of text representing array grades
152 for ( int student = 0; student < grades.length; student++ )
153 {
154 System.out.printf( "Student %2d", student + 1 );
155
156 for ( int test : grades[ student ] ) // output student's grades
157 System.out.printf( "%8d", test );
158
159 // call method getAverage to calculate student's average grade;
160 // pass row of grades as the argument to getAverage
161 double average = getAverage( grades[ student ] );
162 System.out.printf( "%9.2f
", average );
163 } // end outer for
164 } // end method outputGrades
165 } // end class GradeBook

Figure 7.19. Creates GradeBook object using a two-dimensional array of grades, then invokes method processGrades to analyze them.

(This item is displayed on pages 321 - 322 in the print version)

 1 // Fig. 7.19: GradeBookTest.java
 2 // Creates GradeBook object using a two-dimensional array of grades.
 3
 4 public class GradeBookTest
 5 {
 6 // main method begins program execution
 7 public static void main( String args[] )
 8 {
 9 // two-dimensional array of student grades
10 int gradesArray[][] = { { 87, 96, 70 }, 
11  { 68, 87, 90 }, 
12  { 94, 100, 90 }, 
13  { 100, 81, 82 }, 
14  { 83, 65, 85 }, 
15  { 78, 87, 65 }, 
16  { 85, 75, 83 }, 
17  { 91, 94, 100 }, 
18  { 76, 72, 84 }, 
19  { 87, 93, 73 } }; 
20
21 GradeBook myGradeBook = new GradeBook(
22 "CS101 Introduction to Java Programming", gradesArray );
23 myGradeBook.displayMessage();
24 myGradeBook.processGrades();
25 } // end main
26 } // end class GradeBookTest

 

Welcome to the grade book for
CS101 Introduction to Java Programming!

The grades are:

 Test 1 Test 2 Test 3 Average
Student 1 87 96 70 84.33
Student 2 68 87 90 81.67
Student 3 94 100 90 94.67
Student 4 100 81 82 87.67
Student 5 83 65 85 77.67
Student 6 78 87 65 76.67
Student 7 85 75 83 81.00
Student 8 91 94 100 95.00
Student 9 76 72 84 77.33
Student 10 87 93 73 84.33

Lowest grade in the grade book is 65
Highest grade in the grade book is 100

Overall grade distribution:
00-09:
10-19:
20-29:
30-39:
40-49:
50-59:
60-69: ***
70-79: ******
80-89: ***********
90-99: *******
 100: ***

 

Methods getMinimum, getMaximum, outputBarChart and outputGrades each loop through array grades by using nested for statementsfor example, the nested enhanced for statement from the declaration of method getMinimum (lines 5867). The outer enhanced for statement iterates through the two-dimensional array grades, assigning successive rows to parameter studentGrades on successive iterations. The square brackets following the parameter name indicate that studentGrades refers to a one-dimensional int arraynamely, a row in array grades containing one student's grades. To find the lowest overall grade, the inner for statement compares the elements of the current one-dimensional array studentGrades to variable lowGrade. For example, on the first iteration of the outer for, row 0 of grades is assigned to parameter studentGrades. The inner enhanced for statement then loops through studentGrades and compares each grade value with lowGrade. If a grade is less than lowGrade, lowGrade is set to that grade. On the second iteration of the outer enhanced for statement, row 1 of grades is assigned to studentGrades, and the elements of this row are compared with variable lowGrade. This repeats until all rows of grades have been traversed. When execution of the nested statement is complete, lowGrade contains the lowest grade in the two-dimensional array. Method getMaximum works similarly to method getMinimum.

Method outputBarChart in Fig. 7.18 is nearly identical to the one in Fig. 7.14. However, to output the overall grade distribution for a whole semester, the method here uses a nested enhanced for statement (lines 115119) to create the one-dimensional array frequency based on all the grades in the two-dimensional array. The rest of the code in each of the two outputBarChart methods that displays the chart is identical.

Method outputGrades (lines 140164) also uses nested for statements to output values of the array grades, in addition to each student's semester average. The output in Fig. 7.19 shows the result, which resembles the tabular format of a professor's physical grade book. Lines 146147 print the column headings for each test. We use a counter-controlled for statement here so that we can identify each test with a number. Similarly, the for statement in lines 152163 first outputs a row label using a counter variable to identify each student (line 154). Although array indices start at 0, note that lines 147 and 154 output test + 1 and student + 1, respectively, to produce test and student numbers starting at 1 (see Fig. 7.19). The inner for statement in lines 156157 uses the outer for statement's counter variable student to loop through a specific row of array grades and output each student's test grade. Note that an enhanced for statement can be nested in a counter-controlled for statement, and vice versa. Finally, line 161 obtains each student's semester average by passing the current row of grades (i.e., grades[ student ]) to method getAverage.

Method getAverage (lines 94104) takes one argumenta one-dimensional array of test results for a particular student. When line 161 calls getAverage, the argument is grades[ student ], which specifies that a particular row of the two-dimensional array grades should be passed to getAverage. For example, based on the array created in Fig. 7.19, the argument grades[ 1 ] represents the three values (a one-dimensional array of grades) stored in row 1 of the two-dimensional array grades. Remember that a two-dimensional array is an array whose elements are one-dimensional arrays. Method getAverage calculates the sum of the array elements, divides the total by the number of test results and returns the floating-point result as a double value (line 103).

Class GradeBookTest That Demonstrates Class GradeBook

The application in Fig. 7.19 creates an object of class GradeBook (Fig. 7.18) using the two-dimensional array of ints named gradesArray (declared and initialized in lines 1019). Lines 2122 pass a course name and gradesArray to the GradeBook constructor. Lines 2324 then invoke myGradeBook's displayMessage and processGrades methods to display a welcome message and obtain a report summarizing the students' grades for the semester, respectively.