1. Chapter 4: The Selection Structure
Programming with Microsoft Visual Basic 2005, Third Edition

2. The If…Then…Else Statement Lesson A Objectives
Write pseudocode for the selection structure
Create a flowchart to help you plan an application’s code
Write an If...Then...Else statement
Write code that uses comparison operators and logical operators
Programming with Microsoft Visual Basic 2005, Third Edition

3. The If…Then…Else Statement Lesson A Objectives (continued)
Change the case of a string
Determine whether a text box contains data
Determine the success of the TryParse method
Programming with Microsoft Visual Basic 2005, Third Edition

4. The Selection Structure
Condition: expression evaluating to true or false
Selection (decision) structure
Chooses one of two paths based on a condition
Example
If employee works over 40 hours, add overtime pay
Four selection structures in Visual Basic
If, If/Else, If/ElseIf/Else, and Case
Programming with Microsoft Visual Basic 2005, Third Edition

5. Writing Pseudocode for If and If/Else Selection Structures
If selection structure
Contains only one set of instructions
Instructions are processed if the condition is true
If/Else selection
Contains two sets of instructions
True path: instruction set following true condition
False path: instruction set following false condition
Programming with Microsoft Visual Basic 2005, Third Edition

6. Writing Pseudocode for If and If/Else Selection Structures
Figure 4-4: Examples of the If and If/Else selection structures written in pseudocode
Programming with Microsoft Visual Basic 2005, Third Edition

7. Flowcharting the If and If/Else Selection Structures
Set of standardized symbols showing program flow
Oval: the start/stop symbol
Rectangle: the process symbol
Parallelogram: the input/output symbol
Diamond: selection/repetition symbol
Programming with Microsoft Visual Basic 2005, Third Edition

8. Flowcharting the If and If/Else Selection Structures (continued)
Figure 4-5: Examples of the If and If/Else selection structures drawn in flowchart form
Programming with Microsoft Visual Basic 2005, Third Edition

9. Coding the If and If/Else Selection Structures
Syntax
If condition Then statement block for true path [Else statement block for false path] End If
condition must be a Boolean expression
The Else clause is optional
Programming with Microsoft Visual Basic 2005, Third Edition

10. Comparison Operators Comparison (relational) operators:
Test two items for equality or types of non-equality
Rules for comparison operators
Cause an expression to evaluate to true or false
Have lower precedence than arithmetic operators
Are evaluated from left to right
Example: 5 -2 >  3 > 3  False
Programming with Microsoft Visual Basic 2005, Third Edition

11. Comparison Operators (continued)
Figure 4-7: Listing and examples of commonly used comparison operators (continued)
Programming with Microsoft Visual Basic 2005, Third Edition

12. Comparison Operators (continued)
Figure 4-7: Listing and examples of commonly used comparison operators
Programming with Microsoft Visual Basic 2005, Third Edition

13. Using Comparison Operators–Swapping Numeric Values
xDisplayButton Click event procedure – pseudocode
Store text box values in the num1 and num2 variables
If the value of num1 is greater than the value of num2
Swap numbers so num1 contains the smaller number
End if
Display message stating lowest and highest numbers
Block scope: restricts variable to a statement block
Swap variable in if clause will have block scope
Programming with Microsoft Visual Basic 2005, Third Edition

14. Using Comparison Operator–Swapping Numeric Values (continued)
Figure 4-11: The If selection structure shown in the xDisplayButton’s Click event procedure
Programming with Microsoft Visual Basic 2005, Third Edition

15. Using Comparison Operators—Displaying the Sum or Difference
xCalcButton Click event procedure – pseudocode
Store values in operation, number1, and number2
If the operation variable contains “1”
Calculate the sum of number1 and number2
Display the “Sum:” message along with the sum
Else
Subtract number2 from number1
Display “Difference:” message along with difference
End if
Programming with Microsoft Visual Basic 2005, Third Edition

16. Using Comparison Operators—Displaying the Sum or Difference (continued)
Figure 4-16: The If/Else selection structure shown in the xCalcButton’s Click event procedure
Programming with Microsoft Visual Basic 2005, Third Edition

17. Logical Operators Logical (Boolean) operators
Operators that create compound conditions
Types: And, Or, Not, AndAlso, OrElse, Xor
Example: If hours > 0 And hours <= 40 Then
Truth tables: show how operators are evaluated
Short circuit evaluation: bypasses a condition
Operators using technique: AndAlso, OrElse
Example: If state = "TN" AndAlso st > 50000D Then
If st is not TN, no need to evaluate sales > 50000D
Programming with Microsoft Visual Basic 2005, Third Edition

18. Logical Operators (continued)
Figure 4-18: Listing and examples of logical operators (partial)
Programming with Microsoft Visual Basic 2005, Third Edition

19. Logical Operators (continued)
Figure 4-19: Truth tables for the logical operators (continued)
Programming with Microsoft Visual Basic 2005, Third Edition

20. Logical Operators (continued)
Figure 4-19: Truth tables for the logical operators
Programming with Microsoft Visual Basic 2005, Third Edition

21. Using the Truth Tables Scenario: calculate a bonus for a salesperson
Bonus condition: “A” rating and sales > $10,000
Appropriate operators: And, AndAlso (more efficient)
Both conditions must be true to receive bonus
Sample code: rating = "A" AndAlso sales > 10000
Precedence of logical operators
Lower than that of arithmetic or comparison operators
Programming with Microsoft Visual Basic 2005, Third Edition

22. Using Truth Tables (continued)
Figure 4-20: Order of precedence for arithmetic, comparison, and logical operators
Programming with Microsoft Visual Basic 2005, Third Edition

23. Using Logical Operators: Calculating Gross Pay
Data validation: verifying input data is within range
Scenario: calculate and display employee gross pay
Requirements for application implementing scenario
Verify hours are within range (>= 0.0 and <= 40.0)
If data is valid, calculate and display gross pay
If data is not valid, display error message
Compound condition can use AndAlso or OrElse
Programming with Microsoft Visual Basic 2005, Third Edition

24. Using Logical Operators: Calculating Gross Pay (continued)
Figure 4-22: AndAlso and OrElse logical operators in the If...Then...Else statement (continued)
Programming with Microsoft Visual Basic 2005, Third Edition

25. Using Logical Operators: Calculating Gross Pay (continued)
Figure 4-22: AndAlso and OrElse logical operators in the If...Then...Else statement
Programming with Microsoft Visual Basic 2005, Third Edition

26. Comparing Strings Containing Letters
Scenario
Display “Pass” if ‘P’ is entered in xLetterTextBox
Display “Fail” if ‘F’ is entered in xLetterTextBox
One of the possible implementations
Dim letter As String letter = Me.xLetterTextBox.Text If letter = "P" OrElse letter = "p“ Then Me.xResultLabel.Text = “Pass“ Else Me.xResultLabel.Text = "Fail“ End if
Programming with Microsoft Visual Basic 2005, Third Edition

27. Converting a String to Uppercase or Lowercase
String comparisons are case sensitive
CharacterCasing property
Three case values: Normal (default), Upper, Lower
ToUpper method: converts string to upper case
ToLower method: converts string to lower case
Example: If letter.ToUpper = "P" Then
Programming with Microsoft Visual Basic 2005, Third Edition

28. Using the ToUpper and ToLower Methods: Displaying a Message
Procedure requirements
Display message “We have a store in this state”
Valid states: IL, IN, KY
Account for case variations in state text entered
Choices for controlling case: ToLower or ToUpper
One way to enforce case for input
Dim state As String state = Me.xStateTextBox.Text.ToUpper
Use If/Else to test state value and display message
Programming with Microsoft Visual Basic 2005, Third Edition

29. Comparing Boolean Values
Boolean variable: contains either true or false
Naming convention: “is” denotes Boolean type
Example: isInsured
Determining whether a text box contains data
Compare Text property to String.empty value or “”
Alternative: use String.IsNullorEmpty method
Determining whether a string can be converted to a number
Use Boolean value returned by TryParse method
Programming with Microsoft Visual Basic 2005, Third Edition

30. Comparing Boolean Values (continued)
Figure 4-29: Syntax and examples of the String.IsNullOrEmpty method
Programming with Microsoft Visual Basic 2005, Third Edition

31. Comparing Boolean Values (continued)
Figure 4-31: Syntax and an example of using the Boolean value returned by the TryParse method
Programming with Microsoft Visual Basic 2005, Third Edition

32. Summary – Lesson A A Boolean condition evaluates to true or false
Selection structures choose an instruction path based on a condition
If...Then...Else statement: selection structure with a true path and a false path
Operator precedence: arithmetic, comparison, logical
ToUpper and ToLower modify case of input text
Programming with Microsoft Visual Basic 2005, Third Edition

33. The Monthly Payment Calculator Application Lesson B Objectives
Group objects using a GroupBox control
Calculate a periodic payment using the Financial.Pmt method
Create a message box using the MessageBox.Show method
Determine the value returned by a message box
Programming with Microsoft Visual Basic 2005, Third Edition

34. Completing the User Interface
Need: calculate monthly payment on a car loan
To make this calculation, the application needs:
The loan amount (principal)
The annual percentage rate (APR) of interest
The life of the loan (term) in years
Programming with Microsoft Visual Basic 2005, Third Edition

35. Completing the User Interface (continued)
Figure 4-34: Sketch of the Monthly Payment Calculator user interface
Programming with Microsoft Visual Basic 2005, Third Edition

36. Adding a Group Box to the Form
Group box: container for other controls
GroupBox tool
Located in the Toolbox window
Used to add a group box control to the interface
Purpose of a group box control
Visually separate related controls from other controls
Lock controls and set TabIndex after placement
Programming with Microsoft Visual Basic 2005, Third Edition

37. Coding the Monthly Payment Calculator Application
Procedures required according to TOE chart
Click event code for the two buttons
TextChanged, KeyPress, Enter code for text boxes
Procedures that are already coded
xExitButton’s Click event and TextChanged events
Procedure to code in Lesson B
xCalcButton’s Click event procedure
Programming with Microsoft Visual Basic 2005, Third Edition

38. Coding the xCalcButton’s Click Event Procedure
Tasks for xCalcPayButton’s Click event procedure
Calculating the monthly payment amount
Displaying the result in the xPaymentLabel control
Two selection structures needed: If and If/Else
Determining named constants and variables
Constants: items that do not change with each call
Variables: items will likely change with each call
Programming with Microsoft Visual Basic 2005, Third Edition

39. Coding the xCalcPayButton Click Event Procedure (continued)
Figure 4-39: Pseudocode for the xCalcButton’s Click event procedure
Programming with Microsoft Visual Basic 2005, Third Edition

40. Using the Financial.Pmt Method
Calculates periodic payment on loan or investment
Syntax: Financial.Pmt(Rate, NPer, PV[, FV, Due])
Rate: interest rate per period
NPer: total number of payment periods (the term)
PV: present value of the loan or investment
FV: future value of the loan or investment
Due: due date of payments
Programming with Microsoft Visual Basic 2005, Third Edition

41. The MessageBox.Show Method
Displays message box with text, button(s), icon
Syntax: MessageBox.Show(text, caption, buttons, icon[, defaultButton])
text: text to display in the message box
caption: text to display in title bar of message box
buttons: buttons to display in the message box
icon: icon to display in the message box
defaultButton: automatically selected if Enter pressed
Programming with Microsoft Visual Basic 2005, Third Edition

42. The MessageBox.Show Method (continued)
Figure 4-48: Completed xCalcButton’s Click event procedure
Programming with Microsoft Visual Basic 2005, Third Edition

43. The MessageBox.Show Method (continued)
Figure 4-50: Message box created by the MessageBox.Show method
Programming with Microsoft Visual Basic 2005, Third Edition

44. Summary – Lesson B Group box control treats components as one unit
Add a group box using the GroupBox tool
Financial.Pmt method calculates loan or investment payments
MessageBox.Show method displays a message box with text, one or more buttons, and an icon
Programming with Microsoft Visual Basic 2005, Third Edition

45. Completing the Monthly Payment Calculator Application Lesson C Objectives
Specify the keys that a text box will accept
Select the existing text in a text box
Programming with Microsoft Visual Basic 2005, Third Edition

46. Coding the KeyPress Event Procedures
Occurs when key pressed while a control has focus
Character corresponding to key is stored
Stored value sent to KeyPress event’s e parameter
One popular use of the KeyPress event
Prevents users from entering inappropriate characters
Selection structure used to test entered character
Programming with Microsoft Visual Basic 2005, Third Edition

47. Coding the KeyPress Event Procedures (continued)
Figure 4-54: Completed CancelKeys procedure
Programming with Microsoft Visual Basic 2005, Third Edition

48. Coding the Enter Event Procedure
Occurs when the text box receives the focus
Responsible for selecting contents of text box
User can replace existing text by pressing a key
SelectAll method syntax: Me.textbox.SelectAll()
Selects all text contained on a text box
Using the SelectAll method
Add to each text box’s Enter event procedure
Programming with Microsoft Visual Basic 2005, Third Edition

49. Coding the Enter Event Procedure (continued)
Figure 4-57: Existing text selected in the xPrincipalTextBox
Programming with Microsoft Visual Basic 2005, Third Edition

50. Summary – Lesson C
KeyPress event procedure: responds to the user pressing a key
One use of a KeyPress event: cancel a key entered by the user
Enter event: occurs when text box receives focus
SelectAll method: used to select all contents of a text box
Programming with Microsoft Visual Basic 2005, Third Edition