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4.11 Formulating Algorithms: Nested Control Statements

We've seen that control statements can be stacked on top of one another (in sequence). We now examine the only other structured way that control statements can be combined—by nesting one control statement inside another.

Problem Statement and Notes

Consider the following problem statement:

  • A college offers a course that prepares students for the state real estate broker licensing exam. Last year, 10 of the students who completed this course took the exam. The college wants to know how well its students did. You've been asked to write a program to summarize the results. You've been given a list of the 10 students. Next to each name is written a "P" if the student passed the exam and an "F" if the student failed the exam.
  • Your program should analyze the results of the exam as follows:
  1. Input each exam result (that is, a "P" or an "F").
  2. Count the number of passes and the number of failures.
  3. Display a summary of the exam results, indicating the number of students who passed and the number who failed.
  4. If more than eight students passed the exam, display the message "Bonus to Instructor."

After reading the problem statement carefully, we make the following observations:

  1. The program must process exam results for 10 students. A counter-controlled loop can be used because the number of test results is known in advance.
  2. Each exam result is a string—either a "P" or an "F." When the program reads an exam result, it must determine whether the result is a "P" or an "F." We test for a "P" in our algorithm. If the input is not a "P," we assume it's an "F." An exercise at the end of the chapter considers the consequences of this assumption. For instance, consider what happens when the user enters a lowercase "p."
  3. Two counters store the exam results—one counts the number of students who passed the exam and the other counts the number of students who failed.
  4. After the program has processed all the exam results, it must determine whether more than eight students passed the exam, and, if so, bonus the instructor.

GUI for the Licensing-Exam Analysis Application

Figure 4.13 shows this application's GUI. Each result is placed in the ListBox when the user presses the Submit Result Button. We process all 10 results when the user presses the Analyze Results Button. We also provide a Clear Results Button to reset the GUI so the user can enter a new set of results. As in the previous example, we used the Form's Accept-Button property to set the Submit Result Button as the default Button. We disabled the Analyze Results Button initially by setting its Enabled property to False.

Figure 4.13

Fig. 4.13 GUI for the licensing-exam analysis problem.

Developing the Pseudocode Algorithm with Top-Down, Stepwise Refinement: The Top and the First Refinement

We approach this program with top-down, stepwise refinement, a technique for developing well-structured algorithms. We begin with a pseudocode representation of the top—a single statement that conveys the overall function of the program:

   Analyze exam results and decide if the instructor should receive a bonus

The top is a complete representation of a program. Unfortunately, the top rarely conveys enough detail from which to write a program, so we conduct the refinement process. We divide the top into a series of smaller tasks and list them in the order in which they must be performed, resulting in the following first refinement:

   Initialize variables

   Input the 10 exam results, and count passes and failures

   Display a summary of the exam results and decide if the instructor should
   receive a bonus

Proceeding to the Second Refinement

To proceed to the second refinement, we commit to specific variables. In this example, counters are needed to record the passes and failures. A counter controls the looping process and a variable stores the user input. The pseudocode statement

   Initialize variables

can be refined as follows:

   Initialize passes to zero

   Initialize failures to zero

   Initialize student to zero

Only the counters for the number of passes, number of failures and number of students need to be initialized before they're used. The variable we use to store each value the user enters does not need to be initialized because the assignment of its value does not depend on its previous value, as is the case for the counter variables.

The pseudocode statement

   Input the 10 exam results, and count passes and failures

requires a repetition statement (that is, a loop) that successively inputs the result of each exam. We know in advance that there are 10 exam results, so counter-controlled looping is appropriate. Inside the loop (that is, nested inside the loop), a double-selection statement will determine whether each exam result is a pass or a failure and will increment the appropriate counter. The refinement of the preceding pseudocode statement is then

   While student counter is less than or equal to 10
   Get the next exam result

   If the student passed then
   Add one to passes
   Add one to failures

   Add one to student counter

Note the use of blank lines to set off the If...Else control statement to improve readability.

The pseudocode statement

   Display a summary of the exam results and decide if the instructor should
   receive a bonus

may be refined as follows:

   Display the number of passes

   Display the number of failures

   If more than eight students passed then
   Display "Bonus to instructor!"

Complete Second Refinement of Pseudocode

The complete second refinement of the pseudocode appears in Fig. 4.14. Blank lines are also used to set off the While loop (lines 5–13) for readability. This pseudocode is now sufficiently refined for conversion to Visual Basic.

Fig 4.14. Pseudocode for examination-results problem.

1   Initialize passes to zero
2   Initialize failures to zero
3   Initialize student to zero
5   While student counter is less than or equal to 10
6      Get the next exam result
8      If the student passed then
9        Add one to passes
10      Else

11         Add one to failures
13      Add one to student counter

15  Display the number of passes

16  Display the number of failures

18 If more than eight students passed then

19 Display "Bonus to instructor!"

Class Analysis

The program that implements the pseudocode algorithm and the sample outputs are shown in Fig. 4.15. We defined the Click event handlers for the submitResultButton, analyzeResultsButton and clearResultsButton. The analyzeResultsButton_Click method (lines 24–55) implements the class-averaging algorithm described by the pseudo-code in Fig. 4.14.

Fig 4.15. Nested control statements: Examination-results problem.

1  ' Fig. 4.15: Analysis.vb
2  ' Nested control statements: Examination-results problem.
3  Public Class Analysis
4     ' place a result in the resultsListBox
5  Private Sub submitResultButton_Click(ByVal sender As System.Object,
6  ByVal e As System.EventArgs) Handles submitResultButton.Click
8  If resultsListBox.Items.Count < 10 Then
9          ' add the grade to the end of the resultsListbox
10           resultsListBox.Items.Add(resultTextBox.Text)
11           resultTextBox.Clear() ' clear the resultTextBox
12           resultTextBox.Focus() ' select the resultTextBox
13   End If
15        ' determine whether to prevent the user from entering more results
16        If resultsListBox.Items.Count = 10 Then
17         submitResultButton.Enabled = False ' disables submitResultButton 
18         resultTextBox.Enabled = False ' disables resultTextBox           
19         analyzeResultsButton.Enabled = True ' enable analyzeResultsButton
20        End If
21     End Sub  ' submitResultButton_Click
23     ' analyze the results
24     Private Sub analyzeResultsButton_Click(ByVal sender As System.Object,
25        ByVal e As System.EventArgs) Handles analyzeResultsButton.Click
27        ' initializing variables in declarations        
28        Dim passes As Integer = 0 ' number of passes    
29        Dim failures As Integer = 0 ' number of failures
30        Dim student As Integer = 0 ' student counter    
31        Dim result As String ' one exam result
33        ' process 10 students using counter-controlled loop
34        Do While student < 10
35           result = resultsListBox.Items(student) ' get a result
37           ' nested control statement                     
38           If result = "P" Then                           

39              passes += 1 ' increment number of passes    
40           Else                                           

41              failures += 1 ' increment number of failures
42           End If                                         

44           student += 1 ' increment student counter
45        Loop
47        ' display exam results
48        analysisResultsLabel.Text =
49           "Passed: " & passes & vbCrLf & "Failed: " & failures & vbCrLf
51        ' raise tuition if more than 8 students passed
52       If passes > 8 Then
53           analysisResultsLabel.Text &= "Bonus to instructor!"
54        End If
55     End Sub ' analyzeResultsButton_Click
57     ' clears the resultsListBox and analysisResultsLabel
58     Private Sub clearResultsButton_Click(ByVal sender As System.Object,
59       ByVal e As System.EventArgs) Handles clearResultsButton.Click
61         resultsListBox.Items.Clear() ' removes all items
62         analysisResultsLabel.Text = String.Empty ' clears the text
63         submitResultButton.Enabled = True ' enables submitResultButton      
64         resultTextBox.Enabled = True ' enables resultTextBox                
65         analyzeResultsButton.Enabled = False ' disables analyzeResultsButton
66         resultTextBox.Focus() ' select the resultTextBox
67      End Sub ' clearResultsButton_Click
68   End Class ' Analysis

Method submitResultButton_Click

When the user presses the Submit Result button, method submitResultButton_Click (lines 5–21) executes. The problem statement specified that the user can enter only 10 results, so we need to keep track of the number of results that have been entered. Since the ListBox property Items keeps track of the ListBox's total number of items, we use its Count property in line 8 to determine whether 10 results have been entered. If not, line 10 adds the next result to the ListBox, line 11 clears the TextBox and line 12 gives the Text-Box the focus so the user can enter the next result. The last result entered could have been the 10th result (line 16). If so, lines 17–18 disable the submitResultButton and the resultTextBox by setting their Enabled properties to False. This prevents the user from interacting with these two controls, so the user cannot enter more than 10 results. Line 19 enables analyzeResultsButton by setting its Enabled property to True so the user can tell the program to analyze the results.

Method analyzeResultsButton_Click

Method analyzeResultsButton_Click (lines 24–55) uses the algorithm in Fig. 4.14 to calculate the class average and display the results. Lines 28–31 declare the variables the algorithm uses to process the examination results. Several of these declarations incorporate variable initialization—passes, failures and student are all initialized to 0.

The Do While...Loop statement (lines 34–45) loops 10 times. During each iteration, the loop inputs and processes one exam result from the ListBox. The If...Then...Else statement (lines 38–42) for processing each result is nested in the Do While...Loop statement. If the result is "P", the If...Then...Else statement increments passes; otherwise, it assumes the result is "F" and increments failures. Strings are case sensitive by default—uppercase and lowercase letters are different. Only "P" represents a passing grade. In Exercise 4.13, we ask you to enhance the program by processing lowercase inputs such as "p" and "f".

Line 44 increments student before the loop condition is tested again at line 34. After 10 values have been processed, the loop terminates and lines 48–49 display the number of passes and failures. The If...Then statement at lines 52–54 determines whether more than eight students passed the exam and, if so, displays "Bonus to instructor!". In the first sample execution, the condition at line 52 is True—more than eight students passed the exam, so the program outputs the bonus message.

Method clearResultsButton_Click

When the user presses the Clear Results Button, method clearResultsButton_Click (lines 58–67) prepares for the user to enter a new set of results. Line 61 removes the results from the resultsListBox by calling the Item property's Clear method. Line 62 clears the text on the analysisResultsLabel. Lines 63 and 64 enable the submitResultButton and the resultTextBox by setting their Enabled properties to True—the user can now interact with these controls again. Line 65 disables the analyzeResultsButton by settingits Enabled property to False so the user cannot click it. Line 66 gives the focus to the result-TextBox so the user can begin typing the next result immediately.

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