Programming with
Microsoft Visual Basic 2010
5th Edition
CHAPTER FIVE
MORE ON THE SELECTION STRUCTURE
Previewing the Math Practice
Application
Open Math.exe file
The Math Practice application will be used by 1st and 2nd grade students
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Previewing the Math Practice
Application (cont’d.)
Figure 5-1 Math Practice application’s user interface
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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
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Lesson A Objectives (cont’d.)
Include a multiple-alternative selection structure in pseudocode and in
a flowchart
Code a multiple-alternative selection structure
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Nested Selection Structures
Selection structure
◦ Used to make a decision and select the appropriate path
Nested selection structure
◦ Selection structure that lies on true or false path of an outer selection structure
Two decisions are involved
◦ Primary decision: Made by outer structure
◦ Secondary decision: Made by inner structure
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Figure 5-3 Selection structures you might use today
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The Voter Eligibility
Application
Figure 5-4 Problem specification for the Voter Eligibility application
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The Voter Eligibility
Application (cont’d.)
Figure 5-5 Sample run of the Voter Eligibility application
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Figure 5-6 Flowchart showing the nested selection structure in the true path
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Figure 5-7 Flowchart showing the nested selection structure in the false path
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Figure 5-8 Code for the flowcharts in Figures 5-6 and 5-7 (continues)
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Figure 5-8
Code for the flowcharts in
Figures 5-6 and 5-7 (cont’d.)
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Logic Errors in Selection
Structures
Common logic errors
◦ Using compound condition instead of nested structure
◦ Reversing primary and secondary decisions
◦ Using unnecessary nested selection structure
Algorithm
◦ Set of step-by-step instructions for accomplishing a task
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Logic Errors in Selection
Structures (cont’d.)
Desk-checking (or hand-tracing)
◦ Using pencil and paper to follow algorithm’s steps
◦ Using sample data
◦ Goal is to verify that algorithm is correct
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Logic Errors in Selection
Structures (cont’d.)
Figure 5-9 A correct algorithm for the bonus procedure
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Logic Errors in Selection
Structures (cont’d.)
Figure 5-10 Results of desk-checking the correct algorithm
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Using a Compound Condition
Rather Than a Nested Selection
Structure
Bonus scenario
◦ Calculate bonus amount for all employee types
◦ Give extra bonus to X contingent on sales amount
◦ Display the bonus amount
Problem when using compound condition
◦ It does not indicate hierarchy in decision process
Nested selection structure does enforce hierarchy of decisions
◦ Extra bonus decision depends on sales type decision
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Figure 5-11 Correct algorithm and an incorrect algorithm containing the first logic error
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Figure 5-12 Results of desk-checking the incorrect algorithm shown in Figure 5-11
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Reversing the Primary and
Secondary Decisions
Incorrect algorithm reverses primary and secondary decisions
◦ All salespeople with sales >= 10000 get extra bonus
◦ Amount of bonus depends on sales type
Extra bonus decision should depend on sales type decision first
Reminder: Desk-check your algorithm with sample data to verify its
correctness
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Figure 5-13 Correct algorithm and an incorrect algorithm
containing the second logic error
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Reversing the Primary and
Secondary Decisions (cont’d.)
Figure 5-14 Results of desk-checking the incorrect
algorithm shown in Figure 5-13
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Using an Unnecessary Nested
Selection Structure
Problem with incorrect algorithm
◦ Adds extra, redundant selection structure on inner false path
Extra code reduces efficiency and readability
The redundant selection structure:
◦ Makes decision that was already decided in prior selection structure
Solution: Eliminate second nested structure
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Using an Unnecessary Nested
Selection Structure (cont’d.)
Figure 5-15 Correct algorithm and an incorrect
algorithm containing the third logic error
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Using an Unnecessary Nested
Selection Structure (cont’d.)
Figure 5-16 Results of desk-checking the inefficient algorithm shown in Figure 5-15
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Multiple-Alternative Selection
Structures
Multiple-alternative selection structures
◦ Designed to choose from several alternatives
Condition in multiple-alternative selection structure does not require
true/false answer
◦ Requires expression whose value determines which path is chosen
Example: Display message based on grades ranging from A to F
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Multiple-Alternative Selection
Structures (cont’d.)
Figure 5-17 Letter grades and messages
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Figure 5-18 Pseudocode containing a multiple-alternative selection structure
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Figure 5-19 Flowchart containing a multiple-alternative selection structure
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Multiple-Alternative Selection
Structures (cont’d.)
Figure 5-20
◦ Shows two versions of code for btnDisplay control’s Click event procedure
◦ Both versions use If…Then…Else statements
◦ Second version contains more convenient way of writing a multiplealternative selection structure
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Figure 5-20 Two versions of the code containing a
multiple-alternative selection structure (continues)
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Figure 5-20 Two versions of the code containing a
multiple-alternative selection structure (cont’d.)
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The
Select
Case
Statement
Simpler and clearer when there are many paths
Select Case statement
◦
◦
◦
◦
◦
Begins with Select Case, ends with End Select
Each case represents a different instruction path
Optional Case Else clause handles all values not covered by other Case paths
selectorExpression is evaluated to determine path
Each case, except Case Else, has expressionList that is compared to
selectorExpression
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Figure 5-22 Syntax and an example of the Select Case statement (continues)
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Figure 5-22 Syntax and an example of the Select Case statement (cont’d.)
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Specifying a Range of Values in
a Case Clause
To specify range of minimum and maximum values
◦ To keyword: Use if both upper and lower bounds are known
◦ Is keyword: Use if only upper or lower bound is known
Example with To: Case 1 To 5
Example with Is: Case Is > 10
Comparison operators used with Is
= , < , <= , > , >= , <>
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Figure 5-23 Syntax and an example of specifying a range of values
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Lesson A Summary
Nested selection structure lies on true or false path of outer selection
structure
Desk-check
◦ Validate algorithm by hand with sample data
Multiple-alternative selection structures
◦ Use If/ElseIf/Else statement or Select Case statement
Use To and Is keywords to specify range of values in Case clause of
Select Case statement
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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
Create and call an independent Sub procedure
Generate random numbers
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Creating the Math Practice
Application
Objective: Practice addition and subtraction
Specifications
◦
◦
◦
◦
◦
◦
1st grade students use numbers 1 through 10
2nd grade students use numbers 10 through 99
Should not allow problems whose answers will be negative numbers
Students should be able to check their answers
Extra attempts allowed when answer is incorrect
Application should track and display number of correct and incorrect
responses
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Adding a Radio Button to the
Form
Radio Button
◦ Limits user to one choice in group of options
◦ Should be labeled so its purpose is understood
◦ Should have unique keyboard access key
RadioButton tool: Used to add radio button
Each group of radio buttons must be placed within container such as
group box
Default radio button: Button initially selected
◦ Set Checked property to True
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Adding a Radio Button to the
Form (cont’d.)
Figure 5-29 Subtraction radio button added to the Operation group box
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Adding a Check Box Control to
the Form
Check box
◦
◦
◦
◦
Allows multiple selections among group of choices
Any number of check boxes can be selected at once
Does not limit choices like radio button control
Should be labeled to indicate its purpose
Checkbox tool: Used to add check box control
Lock controls, then set TabIndex after interface design is complete
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Adding a Check Box Control to
the Form (cont’d.)
Figure 5-30 Display summary check box added to the form
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Coding the Math Practice
Application
Procedures to code based on TOE chart
◦ Form’s load event procedure
◦ Click event procedures for seven controls
One task will be coded with an independent Sub procedure
◦ Generating and displaying two random numbers
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Creating an Independent Sub
Procedure
Event procedure
◦ Sub procedure associated with specific object and event
◦ Processed when specific event occurs
Independent Sub procedure
◦
◦
◦
◦
Processed only when called (invoked) from code
Can be invoked from one or more places in an application
Not associated with specific control
Reduces amount of code; promotes modularity
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Figure 5-33 Independent Sub procedure syntax, example, and steps
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Figure 5-34 Pseudocode for the GenerateAndDisplayIntegers procedure
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Generating Random Integers
Pseudo-random number generator
◦ Device that produces sequence of pseudo-random numbers
◦ Pseudo-random: Values statistically resemble randomness
To use pseudo-random number generator:
◦ Create Random object within procedure
◦ Generate random integers with Random.Next method
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Figure 5-35 Syntax and examples of generating random integers
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Figure 5-36 Random number generation code entered in the procedure
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Figure 5-37 Additional comments and code entered in the procedure
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Coding the Grade Radio Buttons’
Click Event Procedures
Radio button controls should invoke GenerateAndDisplayNumbers
procedure
Call statement: Invokes independent procedure
Syntax
Call procedureName([argumentList])
◦ procedureName: Name of procedure to be invoked
◦ argumentList: Optional list of arguments to pass
Keyword Call is optional
◦ Can simply use procedure name
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Figure 5-38 Syntax and examples of the Call statement
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Coding the Operation Radio
Buttons’ Click Event Procedures
Requirements for Click event procedures
◦ Display appropriate mathematical operator (+ or -)
◦ Generate and display two random integers
◦ Show addition problem by default when program starts
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Coding the Form’s Load Event
Procedure
 Form’s Load event procedure
 Processed when application is started
 Form is not displayed until its instructions are processed
 Two ways to initially display addition problem
 Call GenerateAndDisplayIntegers()
 radAddition.PerformClick()
 Enter either statement in Load event procedure
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Lesson B Summary
Radio button control
◦ Limits user to only one choice among group of choices
Check box control
◦ Allows user to select multiple choices in a group
Independent Sub procedures
◦ Not linked to controls and can be invoked anywhere in program
Use Random object to generate pseudo-random numbers
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Lesson B Summary (cont’d.)
Call statement
◦ Used to call (invoke) independent Sub procedure
Use object.PerformClick() to invoke object’s Click event
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Lesson C Objectives
After studying Lesson C, you should be able to:
 Code a check box’s Click event procedure
 Show and hide a control
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Coding the Check Answer
Button’s Click Event Procedure
•
Study procedure’s pseudocode to determine what variables and named
constants are needed
 Six integer variables and one named constant are needed
 Two variables have static storage type
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Figure 5-44 Pseudocode for the btnCheckAnswer
control’s Click event procedure
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Figure 5-45 Memory locations used by the
btnCheckAnswer control’s Click event procedure
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Coding the Display Summary
Check Box’s Click Event Procedure
chkSummary control’s Click event procedure
◦ Displays and hides grpSummary control
◦ When user selects check box, event is triggered, showing group box and its
contents
◦ When user deselects check box, event is triggered, hiding group box control
Checked property indicates whether check box is selected
Visible property indicates whether control is shown
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Lesson C Summary
To show or hide control, set control’s Visible property
Use selection structure to determine if check box was either selected or
deselected by user
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