1. Chapter 5: More on the Selection Structure

2. Previewing the Modified Covington Resort Application
Figure 5-1 Interface showing the calculated amounts
Figure 5-2 Recalculated amounts shown in the interface
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3. Lesson A Objectives After studying Lesson A, you should be able to:
Include a nested selection structure in pseudocode and in a flowchart
Code a nested selection structure
Desk-check an algorithm
Recognize common logic errors in selection structures
Include a multiple-alternative selection structure in pseudocode and in a flowchart
Code a multiple-alternative selection structure
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4. Nested Selection Structures
When either a selection structure’s true path or its false path contains another selection structure
The inner selection is called a nested selection
Figure 5-4 A problem that requires a nested selection structure
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5. Nested Selection Structures (cont.)
Figure 5-5 A problem that requires two nested selection structures
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6. Flowcharting a Nested Selection Structure
Figure 5-6 Problem specification and a correct solution for the voter eligibility problem
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7. Flowcharting a Nested Selection Structure (cont.)
Figure 5-7 Another correct solution for the voter eligibility problem
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8. Coding a Nested Selection Structure
Figure 5-8 Code for the flowcharts in Figures 5-6 and 5-7
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9. Logic Errors in Selection Structures
Four common errors:
Using a compound condition rather than a nested selection structure
Reversing the decisions in the outer and nested selection structures
Using an unnecessary nested selection structure
Including an unnecessary comparison in a condition
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10. Logic Errors in Selection Structures (cont.)
Algorithm
The step-by-step instructions for accomplishing a task
Desk-checking
The process of reviewing the algorithm while seated at your desk rather than in front of a computer
Also called hand-tracing
You use a pencil and paper to follow the algorithm’s instructions by hand
Desk-check to make sure you do not miss any instructions, and that existing instructions are correct and in the proper order
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11. Logic Errors in Selection Structures (cont.)
Figure 5-11 Sample data and expected results for the algorithm shown in Figure 5-10
Figure 5-10 A correct algorithm for the golf fee procedure
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12. Logic Errors in Selection Structures (cont.)
First Logic Error: Using a Compound Condition Rather Than a Nested Selection Structure
Figure 5-12 Correct algorithm and an incorrect algorithm containing the first logic error
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13. Logic Errors in Selection Structures (cont.)
First Logic Error: Using a Compound Condition Rather Than a Nested Selection Structure (cont.)
Figure 5-12 Correct algorithm and an incorrect algorithm containing the first logic error
Figure 5-13 Results of desk-checking the incorrect algorithm from Figure 5-12
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14. Logic Errors in Selection Structures (cont.)
Second Logic Error: Reversing the Outer and Nested Decisions
Figure 5-14 Correct algorithm and an incorrect algorithm containing the second logic error
Figure 5-15 Results of desk-checking the incorrect algorithm from Figure 5-14
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15. Logic Errors in Selection Structures (cont.)
Third Logic Error: Using an Unnecessary Nested Selection Structure
Figure 5-16 Correct algorithm and an inefficient algorithm containing the third logic error
Figure 5-17 Results of desk-checking the inefficient algorithm from Figure 5-16
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16. Logic Errors in Selection Structures (cont.)
Fourth Logic Error: Including an Unnecessary Comparison in a Condition
Figure 5-18 Problem specification, a correct algorithm, and an inefficient algorithm
Figure 5-19 Results of desk-checking the algorithms from Figure 5-18
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17. Multiple-Alternative Selection Structures
When you need to choose from several different options, use a multiple-alternative selection structure
Also called an extended selection structure
Figure 5-20 Problem specification for the Allen High School problem
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18. Multiple-Alternative Selection Structures (cont.)
Figure 5-21 Pseudocode and flowchart containing a multiple-alternative selection structure
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19. Multiple-Alternative Selection Structures (cont.)
Figure 5-23 Excellent message shown in the interface
Figure 5-22 Two versions of the code containing a multiple-alternative selection structure
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20. The Select Case Statement
Used when a multiple-alternative selection structure has many paths from which to choose
It’s simpler and clearer to code the selection structure rather than several If…Then…Else statements
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21. The Select Case Statement (cont.)
Figure 5-24 Syntax and an example of the Select Case statement
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22. The Select Case Statement (cont.)
Specifying a Range of Values in a Case Clause
Figure 5-25 Syntax and an example of specifying a range of values
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23. Lesson A Summary
To create a selection structure that evaluates both a primary and a secondary decision:
Place (nest) the secondary decision’s selection structure within either the true or false path of the primary decision’s selection structure
To verify that an algorithm works correctly:
Desk-check (hand-trace) the algorithm
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24. Lesson A Summary (cont.)
To code a multiple-alternative selection structure:
Use either If…Then…Else statements or the Select Case statement
To specify a range of values in a Select Case statement’s Case clause:
Use the To keyword when you know both the upper and lower values in the range
Use the Is keyword when you know only one end of the range
The Is keyword is used in combination with one of the following comparison operators: =, <, <=, >, >=, <>
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25. Lesson B Objectives After studying Lesson B, you should be able to:
Include a group of radio buttons in an interface
Designate a default radio button
Include a check box in an interface
Compare Boolean values
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26. Modifying the Covington Resort Application
Figure 5-36 Revised TOE chart for the Covington Resort application (continues)
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27. Modifying the Covington Resort Application (cont.)
(continued)
Figure 5-36 Revised TOE chart for the Covington Resort application
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28. Modifying the Covington Resort Application (cont.)
Figure 5-37 Partially completed interface for the Covington Resort application
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29. Modifying the Covington Resort Application (cont.)
Adding a Radio Button to the Interface
Radio button
Limits the user to only one choice from a group of two or more related but mutually exclusive choices
The three-character ID for a radio button’s name is rad
Figure 5-38 Atrium radio button added to the View group box
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30. Modifying the Covington Resort Application (cont.)
Adding a Radio Button to the Interface (cont.)
The automatically selected radio button is called the default radio button
Represents the user’s most likely choice
The first button in the group
Checked property
Set to True to designate the default radio button
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31. Modifying the Covington Resort Application (cont.)
Adding a Check Box to the Interface
Unlike radio buttons, check boxes provide one or more independent and nonexclusive items from which the user can choose
Any number of check boxes on a form can be selected at the same time
The three-character ID for a check box’s name is chk
Figure 5-39 Vehicle parking fee check box added to the interface
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32. Modifying the Covington Resort Application (cont.)
Figure 5-40 Correct TabIndex values
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33. Modifying the Calculate Button’s Code
Figure 5-41 Modified pseudocode for the btnCalc_Click procedure
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34. Modifying the Calculate Button’s Code (cont.)
Figure 5-42 Modified list of named constants and variables
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35. Modifying the Calculate Button’s Code (cont.)
Figure 5-43 Named constants added to the procedure
Figure 5-44 Variables added to the procedure
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36. Modifying the Calculate Button’s Code (cont.)
Comparing Boolean Values
Figure 5-45 Examples of comparing Boolean values (continues)
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37. Modifying the Calculate Button’s Code (cont.)
(continued)
Figure 5-45 Examples of comparing Boolean values
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38. Modifying the ClearLabels Procedure
CheckedChanged event
Occurs when the value in a control’s Checked property changes
Figure 5-50 ClearLabels procedure
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39. Modifying the ClearLabels Procedure (cont.)
Figure 5-51 Covington Resort application’s code at the end of Lesson B (continues)
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40. Modifying the ClearLabels Procedure (cont.)
(continued)
Figure 5-51 Covington Resort application’s code at the end of Lesson B (continues)
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41. Modifying the ClearLabels Procedure (cont.)
(continued)
Figure 5-51 Covington Resort application’s code at the end of Lesson B (continues)
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42. Modifying the ClearLabels Procedure (cont.)
(continued)
Figure 5-51 Covington Resort application’s code at the end of Lesson B (continues)
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43. Modifying the ClearLabels Procedure (cont.)
(continued)
Figure 5-51 Covington Resort application’s code at the end of Lesson B (continues)
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44. Modifying the ClearLabels Procedure (cont.)
(continued)
Figure 5-51 Covington Resort application’s code at the end of Lesson B
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45. Lesson B Summary
To limit the user to only one choice in a group of two or more related but mutually exclusive choices:
Use the RadioButton tool to add two or more radio buttons to the form
To include two groups of radio buttons on a form, at least one of the groups must be placed within a container, such as a group box
To allow the user to select any number of choices from a group of one or more independent and nonexclusive choices:
Use the CheckBox tool to add one or more check box controls to the form
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46. Lesson B Summary (cont.)
To determine whether a radio button or check box is selected or unselected:
Use the Checked property of the radio button or check box
The property will contain the Boolean value True if the control is selected; otherwise, it will contain the Boolean value False
To process code when the value in the Checked property of a radio button or check box changes:
Enter the code in the radio button’s or check box’s CheckedChanged event procedure
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47. Lesson C Objectives After studying Lesson C, you should be able to:
Determine the success of the TryParse method
Generate random numbers
Show and hide a control while an application is running
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48. Using the TryParse Method for Data Validation
Converts a string to a number of a specific data type
With successful conversion:
The TryParse method stores the number in the variable specified in the method’s NumericVariableName argument
With unsuccessful conversion:
TryParse stores the number 0 in the variable
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49. Using the TryParse Method for Data Validation (cont.)
Figure 5-58 Syntax and an example of using the Boolean value returned by the TryParse method
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50. Using the TryParse Method for Data Validation (cont.)
Figure 5-59 Sample run of the original Click event procedure
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51. Using the TryParse Method for Data Validation (cont.)
Figure 5-60 Modified btnCalc_Click procedure
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52. Generating Random Integers
Pseudo-random number generator
A device that produces a sequence of numbers that meet certain statistical requirements for randomness
Create a Random object by declaring it in a Dim statement
Use the Random.Next method to generate random integers
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53. Generating Random Integers (cont.)
Figure 5-61 Syntax and examples of generating random integers
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54. Generating Random Integers (cont.)
Figure 5-62 Roll ‘Em Game application’s interface
Figure 5-63 Pseudocode for the Roll the Dice button’s Click event procedure
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55. Generating Random Integers (cont.)
Figure 5-64 Result of clicking the Roll the Dice button
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56. Showing and Hiding a Control
Hide objects by changing their Visible property from True to False
Figure 5-65 Resized form
Figure 5-66 Interface with six of the picture boxes hidden
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57. Showing and Hiding a Control (cont.)
Figure 5-67 Roll the Dice button’s Click event procedure
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58. Lesson C Summary
To determine whether the TryParse method converted a string to a number of the specified data type:
Use the syntax booleanVariable = dataType.TryParse(string, numericVariableName)
The TryParse method returns the Boolean value True when the string can be converted to the numeric dataType
Otherwise, it returns the Boolean value False
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59. Lesson C Summary (cont.)
To generate random integers:
Create a Random object to represent the pseudo-random number generator
The syntax for creating a Random object is Dim randomObjectName As New Random
Use the Random.Next method to generate a random integer
The method’s syntax is randomObjectName.Next(minValue, maxValue)
The Random.Next method returns an integer that is greater than or equal to minValue, but less than maxValue
The Random.Next method’s return value is assigned to a variable
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60. Lesson C Summary (cont.)
To show or hide a control while an application is running:
Set the control’s Visible property to the Boolean value True to show the control during runtime
Set the control’s Visible property to the Boolean value False to hide the control during runtime
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