t u t o r i a l 2009 pearson education, inc. all rights reserved. 1 18 student grades application...

T U T O R I A L

2009 Pearson Education, Inc. All rights reserved.

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18Student Grades

ApplicationIntroducing Two-DimensionalArrays and RadioButtons


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Outline

18.1 Test-Driving the Student Grades Application

18.2 Two-Dimensional Rectangular Arrays18.3 Using RadioButtons18.4 Inserting Code into the Student Grades

Application


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In this tutorial you will learn: ■ Understand the similarities and differences

between one-dimensional and two-dimensional arrays.

■ Declare and manipulate two-dimensional arrays.■ Understand how to use two-dimensional arrays.■ Use nested For...Next loops.■ Use RadioButtons to enable users to select

exactly one option out of several.

Objectives

Application Requirements


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18.1 Test-Driving the Student Grades Application

A teacher issues three tests to a class of 10 students. The grades on these tests are integers in the range from 0 to 100. The teacher has asked you to develop an application to keep track of each student’s average and the class average. The teacher has also asked that there be a choice to view the grades as either number or letter grades.The application should allow a user to input the student’s three test grades, then compute each student’s average and the class average.


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Test-Driving the Student Grades Application

■ Run the completed application (Fig. 18.1).

Figure 18.1 | Running the completed Student Grades application.


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Test-Driving the Student Grades Application (Cont.)

Figure 18.2 | Inputting data to the Student Grades application.

■ Enter numbers in the Test 1:, Test 2: andTest 3: TextBoxes (Fig. 18.2).


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■ Click the Submit Grades Button to calculate the student’s average (Fig. 18.3).

Figure 18.3 | Displaying the student’s numerical grade.

Numeric RadioButtonselected by default


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■ Once 10 students have been entered, all the controls in the Input Grades GroupBox are disabled (Fig. 18.4).

Figure 18.4 | Input Grades GroupBox disabled after 10 students.

Input Grades GroupBox disabledListBox withnumeric gradesdisplayed

Label with numeric class average displayed


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■ Change the ListBox’s data to letter grades (Fig. 18.5).

Figure 18.5 | Displaying the students’ letter grades.

Select the Letter RadioButton

ListBox with letter grades displayed

Label with letter class average displayed


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■ Two-dimensional arrays require two indices to identify particular elements, in the form array(i, j).

– Rectangular arrays are often used to represent tables of values arranged in rows and columns.

■ Figure 18.6 illustrates a two-dimensional rectangular array.

18.2 Two-Dimensional Rectangular Arrays

Figure 18.6 | Two-dimensional rectangular array with three rows and four columns.


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■ Two-dimensional arrays are initialized much like one-dimensional arrays:Dim numbers As Integer(,) = New Integer(0 To 1, 0 To 1) {}numbers(0, 0) = 1numbers(0, 1) = 2numbers(1, 0) = 3numbers(1, 1) = 4

■ Note that a comma (,) is required inside the parentheses following the data type to indicate that the array is two-dimensional.

■ You can also write the declaration as:Dim numbers(0 To 1, 0 To 1) As Integer

18.2 Two-Dimensional Rectangular Arrays (Cont.)


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■ Two-dimensional arrays also may be initialized using an initializer list:

Dim numbers As Integer(,) = New Integer(,) {{1, 2}, {3, 4}}

■ The preceding declaration can also be written asDim numbers As Integer(,) = {{1, 2}, {3, 4}}

■ You can specify the lower bounds of an array implicitly:

Dim numbers As Integer(,) = New Integer(1, 1) {}Dim numbers(1, 1) As Integer

18.2 Two-Dimensional Rectangular Arrays (Cont.)


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■ A RadioButton is a small white circle that either is blank or contains a smaller dot.

– When a RadioButton is selected, a dot appears in the circle.

– RadioButtons represent a set of mutually exclusive options—only one can be selected at a time.

– By default, all RadioButtons added directly to a Form or GroupBox become part of the same group.

■ If the RadioButton is checked, the Checked property returns the Boolean value True.

18.3 Using RadioButtons


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When the user clicks the Submit Grades Button:Retrieve the student’s grades from the TextBoxesAdd the student’s test scores to the arrayDisplay the student’s test scores and average in the ListBoxDisplay the class’s average in the Class average: LabelClear the student’s test scores from the TextBoxes

If 10 students have been entered Disable the input controls

When the user selects the Numeric RadioButton:Display each student’s numeric test scores and average in the

ListBoxDisplay the class’s numeric average in the Class average:

Label

When the user selects the Letter RadioButton:Display each student’s letter test scores and average in the

ListBoxDisplay the class’s letter average in the Class average: Label

18.3 Using RadioButtons (Cont.)


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Action Control Event

Label the application’s components inputGroupBox, viewGroupBox, test1Label, test2Label, test3Label, classAverageLabel

Application is run

submitButton

Retrieve the student’s grades from the TextBoxes

test1TextBox, test2TextBox, test3TextBox

Add the student’s test scores to the array

Display the student’s test scores and average in the ListBox

gradesListBox

Display the class’s average in the Class average: Label

averageLabel

Click

Figure 18.7 | ACE table for the Student Grades application. (Part 1 of 2.)

■ Use an ACE table to convert the pseudocode into Visual Basic (Figure 18.7).

Action/Control/Event (ACE) Table for the Student Grades Application


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Action Control Event

Clear the student’s test scores from the TextBoxes

test1TextBox, test2TextBox, test3TextBox

I f 10 students have been entered

Click

Disable the input controls inputGroupBox

numericRadioButton CheckChanged

Display each student’s numeric test scores and average in the ListBox

gradesListBox

Display the class’s numeric average in the Class average: Label

averageLabel

letterRadioButton CheckChanged

Display each student’s letter test scores average in the ListBox

gradesListBox

Display the class’s letter average in the Class average: Label

averageLabel

Figure 18.7 | ACE table for the Student Grades application. (Part 2 of 2.)

Action/Control/Event (ACE) Table for the Student Grades Application (Cont.)


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■ Select the View GroupBox on the template application’s Form, and add two RadioButtons.

■ Name the left one numericRadioButton, and set its Text property to Numeric. Name the right one letterRadioButton, and set its Text property to Letter.

■ Set the Checked property of numericRadioButton to True (Fig. 18.8).

Adding RadioButtons to the View GroupBox

Figure 18.8 | RadioButtons placed in the GroupBox.


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GUI Design TipUse RadioButtons when the user must choose only one option from a group.


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GUI Design TipAlways place each group of RadioButtons in a separate container (such as a GroupBox).


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GUI Design TipAlign groups of RadioButtons either horizontally or vertically.


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Good Programming Practice

Append the RadioButton suffix to RadioButton control names.


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Error-Prevention Tip

To avoid subtle logic errors, one RadioButton in a group is often selected by default, by setting its Checked property to True. This can be done using code or by setting the value using the Properties window.


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Declaring a Two-Dimensional Array

■ Line 2 (Fig. 18.9) declares a 10-by-3 array of Integers to contain the test scores.

■ Each row in the array represents a student. Each column represents a test.

Figure 18.9 | Declaring a two-dimensional array.


24Finishing the Submit Grades Button’sClick Event Handler

■ Double click the Submit Grades Button to display its Click event handler (Fig. 18.10).

Figure 18.10 | Storing the student’s test scores.

Store the student’s testscores in the array


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■ The For...Next statement (Fig. 18.11) reviews each of the student’s test scores.

■ Retrieve the highest index of the array’s second dimension, indicated by passing 1 as the argument to GetUpperBound.

Finishing the Submit Grades Button’sClick Event Handler (Cont.)


26Finishing the Submit Grades Button’sClick Event Handler (Cont.)

Figure 18.11 | Displaying the output.

Iterating over the student’s test scores

Determining how todisplay the test scores

Calling the LetterGrade method

Displaying the student’s average

Update the number of students

Displaying the class average


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■ If grades array is full, line 51 (Fig. 18.12) disablesthe Input Grades GroupBox.

– Disabling a GroupBox disables all the controls it contains.■ The Length property of a two-dimensional array would

return the total number of elements (rows times columns).

Finishing the Submit Grades Button’sClick Event Handler (Cont.)

Figure 18.12 | Application does not allow more than 10 data entries.

Disable the input controls when the grades array is full


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■ The For...Next statement (Fig. 18.13) sums the grades contained in the specified student row of the grades array.

■ The format control string "{0:F}" formats the test average with exactly two digits following the decimal point.

Coding Methods to Average Test Grades


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Figure 18.13 | Calculating a student’s test average.

Calculate thestudent’s test average

Coding Methods to Average Test Grades (Cont.)

Sum the student’s test grades


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■ The CalculateStudentAverage method (Fig. 18.14) uses nested For...Next statements to iterate over the columns in each row.

Figure 18.14 | Calculating the class’s test average.

Coding Methods to Average Test Grades (Cont.)

Outer For...Next statement

iterates over each row

Inner For...Next statement iterates over each column

Calculate the class’s test average


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■ Double click the Numeric RadioButton to generate its CheckedChanged event handler (Fig. 18.15).

Figure 18.15 | Method numericRadioButton_CheckedChanged.

Coding Event Handlers for the RadioButtons

Displaying the class’s grades


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■ Double click the Letter RadioButton to generate its event handler (Fig. 18.16).

Figure 18.16 | Method letterRadioButton_CheckedChanged.

Coding Event Handlers for theRadioButtons (Cont.)

Displaying the class’s grades


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■ Method DisplayClassGrades (Fig. 18.17) displays each student’s grades and test average, as well as the class average, in the format selected by the user.



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Figure 18.17 | Method DisplayClassGrades.


Displaying the class’s average

Inner For...Next statement adds the grades in each column of the

current row to the output String

Outer For...Next statementiterates over each row of the array


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1 Public Class StudentGradesForm 2 Dim grades As Integer(,) = New Integer(0 To 9, 0 To 2) {} 3 Dim studentCount As Integer = 0 ' number of students entered 4 5 ' handles submit button click event 6 Private Sub submitButton_Click(ByVal sender As System.Object, _ 7 ByVal e As System.EventArgs) Handles submitButton.Click 8 9 ' retrieve the student's grades 10 grades(studentCount, 0) = Convert.ToInt32(test1TextBox.Text) 11 grades(studentCount, 1) = Convert.ToInt32(test2TextBox.Text) 12 grades(studentCount, 2) = Convert.ToInt32(test3TextBox.Text) 13 14 ' output string containing the student's grades and average 15 Dim output As String = _ 16 "Student " & studentCount & ControlChars.Tab

■ Figure 18.18 presents the source code for the Student Grades application.

Outline

(1 of 11 )

Creating a two-dimensional array

Assigning values to elements of a two-dimensional array


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17 18 ' add each test grade to the output 19 For column As Integer = 0 To grades.GetUpperBound(1) 20 ' if the Letter RadioButton is checked 21 If letterRadioButton.Checked = True Then 22 ' add letter grade to the output 23 output &= ControlChars.Tab & _ 24 LetterGrade(grades(studentCount, column)) 25 Else 26 ' add number grade to the output 27 output &= ControlChars.Tab & _ 28 grades(studentCount, column) 29 End If 30 Next 31 Next 32 ' add the student's test average to the output 33 output &= ControlChars.Tab & _ 34 CalculateStudentAverage(studentCount)

Outline

(2 of 11 )

Using the Checked property of a RadioButton control

Calling the LetterGrade method with an element of a two-dimensional array

Accessing an element of a two-dimensional array

Iterating over the columns in a row of a two-dimensional array


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35 36 gradesListBox.Items.Add(output) ' add output to the ListBox 37 38 studentCount += 1 ' update number of students entered 39 40 ' display the class average 41 averageLabel.Text = CalculateClassAverage() 42 43 ' clear the input TextBoxes and set focus to first TextBox 44 test1TextBox.Clear() 45 test2TextBox.Clear() 46 test3TextBox.Clear() 47 test1TextBox.Focus() 48 49 ' limit number of students 50 If studentCount = grades.GetUpperBound(0) + 1 Then 51 inputGroupBox.Enabled = False ' disable GroupBox's controls 52 End If 53 End Sub ' submitButton_Click

Outline

(3 of 11 )

Determining the number of rows in a two-dimensional array

Disabling all the controls in a GroupBox


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54 55 ' calculates a student's test average 56 Function CalculateStudentAverage(ByVal row As Integer) As String 57 Dim gradeTotal As Integer = 0 ' student's total grade 58 59 ' sum the grades for the student 60 For column As Integer = 0 To grades.GetUpperBound(1) 61 gradeTotal += grades(row, column) 62 Next 63 64 Dim studentAverage As String = "" ' output string 65 66 ' calculate the student's test average 67 If letterRadioButton.Checked = True Then 68 studentAverage = _ 69 LetterGrade(gradeTotal / (grades.GetUpperBound(1) + 1)) 70 Else 71 studentAverage = String.Format("{0:F}", _ 72 (gradeTotal / (grades.GetUpperBound(1) + 1))) 73 End If

Outline

(4 of 11 )

Iterating over the columns in a row of a two-dimensional array


Calculating the student’s average and formatting it as a number with two digits after the decimal point


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74 75 Return studentAverage ' return the student's average 76 End Function ' CalculateStudentAverage 77 78 ' calculates the class average 79 Function CalculateClassAverage() As String 80 Dim classTotal As Integer = 0 ' class's total grade 81 82 ' loop through all rows 83 For row As Integer = 0 To studentCount - 1 84 ' loop through all columns 85 For column As Integer = 0 To grades.GetUpperBound(1) 86 classTotal += grades(row, column) ' add grade to total 87 Next column 88 Next row 89 90 Dim classAverage As String = "" ' output string 91

Outline

(5 of 11 )

Using nested For... Next statements to iterate over the columns in each row of a two-dimensional array


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92 ' if the Letter RadioButton is checked, return letter grade 93 If letterRadioButton.Checked = True Then 94 classAverage = LetterGrade(classTotal / _ 95 (studentCount * (grades.GetUpperBound(1) + 1))) 96 Else ' return number grade 97 classAverage = String.Format("{0:F}", (classTotal / _ 98 (studentCount * (grades.GetUpperBound(1) + 1)))) 99 End If 100 101 Return classAverage ' return the class average 102 End Function ' CalculateClassAverage 103 104 ' determines a letter grade corresponding to a numeric grade 105 Function LetterGrade(ByVal grade As Double) As String 106 Dim output As String ' the letter grade to return 107

Outline

(6 of 11 )


Calculating the class’s average and formatting it as a number with two digits after the decimal point


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108 ' determine the correct letter grade 109 Select Case grade 110 Case Is >= 90 111 output = "A" 112 Case Is >= 80 113 output = "B" 114 Case Is >= 70 115 output = "C" 116 Case Is >= 60 117 output = "D" 118 Case Else 119 output = "F" 120 End Select 121 122 Return output ' return the letter grade 123 End Function ' LetterGrade 124

Outline

(7 of 11 )


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125 ' handles Numeric RadioButton's CheckChanged event 126 Private Sub numericRadioButton_CheckedChanged(ByVal sender As _ 127 System.Object, ByVal e As System.EventArgs) _ 128 Handles numericRadioButton.CheckedChanged 129 130 ' if the Numeric RadioButton is checked, display number grades 131 If numericRadioButton.Checked = True AndAlso _ 132 studentCount > 0 Then 133 134 DisplayClassGrades() 135 End If 136 End Sub ' numericRadioButton_CheckedChanged 137 138 ' handles Letter RadioButton's CheckChanged event 139 Private Sub letterRadioButton_CheckedChanged(ByVal sender As _ 140 System.Object, ByVal e As System.EventArgs) _ 141 Handles letterRadioButton.CheckedChanged 142

Outline

(8 of 11 )

Handling the CheckedChanged event of a RadioButton control

Handling the CheckedChanged event of a RadioButton control


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143 ' if the Letter RadioButton is checked, display letter grades 144 If letterRadioButton.Checked = True AndAlso _ 145 studentCount > 0 Then 146 147 DisplayClassGrades() 148 End If 149 End Sub ' letterRadioButton_CheckedChanged 150 151 ' display the grades for all students entered 152 Sub DisplayClassGrades() 153 gradesListBox.Items.Clear() ' clear the ListBox 154 155 ' add the header to the ListBox 156 gradesListBox.Items.Add(ControlChars.Tab & ControlChars.Tab _ 157 & "Test 1" & ControlChars.Tab _ 158 & "Test 2" & ControlChars.Tab _ 159 & "Test 3" & ControlChars.Tab & "Average") 160

Outline

(9 of 11 )


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161 ' loop through all the rows 162 For row As Integer = 0 To studentCount - 1 163 Dim output As String = "Student " & row & ControlChars.Tab 164 165 ' loop through all the columns 166 For column As Integer = 0 To grades.GetUpperBound(1) 167 If letterRadioButton.Checked = True Then 168 ' add letter grade to output string 169 output &= ControlChars.Tab & _ 170 LetterGrade(grades(row, column)) 171 Else 172 ' add number grade to output string 173 output &= ControlChars.Tab & (grades(row, column)) 174 End If 175 Next column 176

Outline

(10 of 11 )

Outer For...Next statement iterates over the rows of a two-dimensional array

Inner For...Next statement iterates over the columns of a row in a two-dimensional array

Using the control variable’s name to improve readability of nested For...Next statements


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177 ' add the student's average to the output 178 output &= ControlChars.Tab & CalculateStudentAverage(row) 179 180 ' add the output to the ListBox 181 gradesListBox.Items.Add(output) 182 Next row 183 184 ' update the class average 185 averageLabel.Text = CalculateClassAverage() 186 End Sub ' DisplayClassGrades 187 End Class ' StudentGradesForm

Outline

(11 of 11 )

Using the control variable’s name to improve readability of nested For...Next statements

t u t o r i a l 2009 pearson education, inc. all rights reserved. 1 18 student grades application...

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