1. Microsoft Visual Basic 2005: Reloaded Second Edition
Chapter 4
Making Decisions in a Program

2. Objectives After studying this chapter, you should be able to:
Include the selection structure in pseudocode and in a flowchart
Write an If…Then…Else statement
Write code that uses comparison operators and logical operators
Create a variable having block-scope
Concatenate strings
Microsoft Visual Basic 2005: Reloaded, Second Edition

3. Objectives (continued)
Use the ControlChars.NewLine constant
Change the case of a string
Determine whether a string contains data
Display a message in a message box
Include a nested selection structure in pseudocode, a flowchart, and code
Microsoft Visual Basic 2005: Reloaded, Second Edition

4. Objectives (continued)
Code an If/ElseIf/Else selection structure
Include a Case selection structure in pseudocode, a flowchart, and code
Generate random numbers
Microsoft Visual Basic 2005: Reloaded, Second Edition

5. The Selection Structure
Selection structure (or decision structure):
Used to select a path to take based on the outcome of a decision or comparison
Condition:
The decision to be made
Results in a Boolean (True or False) answer
Four forms of selection structure: If
If/Else
If/ElseIf/Else
Case
Microsoft Visual Basic 2005: Reloaded, Second Edition

6. The Selection Structure (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

7. Writing Pseudocode for the If and If/Else Selection Structures
If selection structure: contains one set of instructions to process when the condition is true
If/Else selection structure:
Contains two sets of instructions
One set is processed when the condition is true
The other set is processed when the condition is false
True path: path to follow when condition is true
False path: path to follow when condition is false
Microsoft Visual Basic 2005: Reloaded, Second Edition

8. Writing Pseudocode for the If and If/Else Selection Structures (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

9. Flowcharting the If and If/Else Selection Structures
Selection/repetition symbol:
Diamond shape
Represents both selection and repetition structures
One flowline entering and two flowlines leaving
Microsoft Visual Basic 2005: Reloaded, Second Edition

10. Flowcharting the If and If/Else Selection Structures (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

11. Coding the If and If/Else Selection Structures
Microsoft Visual Basic 2005: Reloaded, Second Edition

12. Coding the If and If/Else Selection Structures (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

13. Coding the If and If/Else Selection Structures (continued)
Else clause: an optional part of the If statement
Statement block: set of statements terminated by an Else or End If
Microsoft Visual Basic 2005: Reloaded, Second Edition

14. Comparison Operators Comparison operators (or relational operators):
Used as part of the condition in an If statement
Most commonly used comparison operators:
Equal to: =
Greater than: >
Greater than or equal to: >=
Less than: <
Less than or equal to: <=
Not equal to: <>
Microsoft Visual Basic 2005: Reloaded, Second Edition

15. Comparison Operators (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

16. Comparison Operators (continued)
Have no order of precedence
Are evaluated from left to right in an expression
Microsoft Visual Basic 2005: Reloaded, Second Edition

17. Comparison Operators (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

18. Using Comparison Operators – Swapping Numeric Values
Pseudocode for a procedure that displays highest and lowest of two numbers:
Microsoft Visual Basic 2005: Reloaded, Second Edition

19. Using Comparison Operators – Swapping Numeric Values (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

20. Using Comparison Operators – Swapping Numeric Values (continued)
Block scope: the scope of a variable created within a block
Block-scope variable: can only be used within the statement block in which it was declared
Concatenation operator (&): links two strings
ControlChars.NewLine constant:
Advances the insertion point to the next line
Microsoft Visual Basic 2005: Reloaded, Second Edition

21. Using Comparison Operators – Swapping Numeric Values (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

22. Using Comparison Operators – Swapping Numeric Values (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

23. Using Comparison Operators – Swapping Numeric Values (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

24. Using Comparison Operators – Swapping Numeric Values (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

25. Using Comparison Operators – Swapping Numeric Values (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

26. Using Comparison Operators – Example 2
Pseudocode for a procedure to allow the user to display the sum or difference of two numbers:
Microsoft Visual Basic 2005: Reloaded, Second Edition

27. Using Comparison Operators – Example 2 (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

28. Using Comparison Operators – Example 2 (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

29. Using Comparison Operators – Example 2 (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

30. Using Comparison Operators – Example 2 (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

31. Using the ToUpper and ToLower Methods
String comparisons in Visual Basic are case-sensitive
ToUpper method: converts a string to uppercase
ToLower method: converts a string to lowercase
ToUpper and ToLower can be used to permanently or temporarily convert a variable’s contents
Microsoft Visual Basic 2005: Reloaded, Second Edition

32. Using the ToUpper and ToLower Methods (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

33. Using the ToUpper and ToLower Methods (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

34. Using the ToUpper and ToLower Methods (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

35. Logical Operators Logical operators (or Boolean operators):
Used to combine one or more conditions
Compound condition: a combination of conditions using logical operator(s)
Microsoft Visual Basic 2005: Reloaded, Second Edition

36. Logical Operators (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

37. Logical Operators (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

38. Logical Operators (continued)
Truth tables: used to evaluate logical operators in an expression
Short-circuit evaluation: an evaluation in which the second condition may not be evaluated
And and Or operations always evaluate both conditions
AndAlso and OrElse operations do not evaluate second condition if the first condition is false
Microsoft Visual Basic 2005: Reloaded, Second Edition

39. Logical Operators (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

40. Logical Operators (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

41. Using the Truth Tables
Use And or AndAlso when both conditions must be true to give a true result
Use Or or OrElse when one or both conditions must be true to give a true result
Use XOr when exactly one condition must be true to give a true result
Logical operators are evaluated after arithmetic or comparison operators in an expression
Microsoft Visual Basic 2005: Reloaded, Second Edition

42. Using the Truth Tables (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

43. Using the Truth Tables (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

44. Using Logical Operators in an If…Then…Else Statement
Data validation:
Verifying that the input data is within the expected range
Use an If…Then…Else statement to validate input data
Microsoft Visual Basic 2005: Reloaded, Second Edition

45. Using Logical Operators in an If…Then…Else Statement (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

46. Using Logical Operators in an If…Then…Else Statement (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

47. Using Logical Operators in an If…Then…Else Statement (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

48. The String.IsNullOrEmpty Method
String.IsNullOrEmpty method: determine if a control’s Text property or String variable contains data
Microsoft Visual Basic 2005: Reloaded, Second Edition

49. The String.IsNullOrEmpty Method (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

50. Modifying the Skate-Away Sales Application
Microsoft Visual Basic 2005: Reloaded, Second Edition

51. Modifying the Skate-Away Sales Application (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

52. Modifying the Skate-Away Sales Application (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

53. Modifying the Skate-Away Sales Application (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

54. The MessageBox.Show Method
Display message box with text, buttons and an icon
When a message box is displayed, the program waits until the user selects a button
MessageBox.Show returns an integer value indicating which button the user selected
Microsoft Visual Basic 2005: Reloaded, Second Edition

55. The MessageBox.Show Method (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

56. The MessageBox.Show Method (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

57. The MessageBox.Show Method (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

58. The MessageBox.Show Method (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

59. The MessageBox.Show Method (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

60. The MessageBox.Show Method (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

61. Nested Selection Structures
Nested selection structure: a selection structure that is completely contained within another selection structure
Primary decision: decision made by the outer selection structure
Secondary decision: decision made by the inner selection structure
Microsoft Visual Basic 2005: Reloaded, Second Edition

62. Nested Selection Structures (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

63. Nested Selection Structures (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

64. Nested Selection Structures (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

65. Nested Selection Structures (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

66. Nested Selection Structures (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

67. Nested Selection Structures (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

68. Nested Selection Structures (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

69. The If/ElseIf/Else Selection Structure
Need a procedure to display a message based on a letter grade:
Letter grade
Message A
Excellent B
Above Average C
Average D
Below Average F
Microsoft Visual Basic 2005: Reloaded, Second Edition

70. The If/ElseIf/Else Selection Structure (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

71. The If/ElseIf/Else Selection Structure (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

72. The Case Selection Structure
Used when there are many paths from which to choose
Simpler and clearer than using If/ElseIf/Else
Microsoft Visual Basic 2005: Reloaded, Second Edition

73. The Case Selection Structure (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

74. The Case Selection Structure (continued)
Case selection structure in a flowchart:
Uses the diamond symbol
One flowline into the diamond, but many flowlines out of the diamond
Case selection structure evaluates an expression to determine which path to take
Case selection structure:
Begins with Select Case
Ends with End Select
Has one Case clause for each possible value
Microsoft Visual Basic 2005: Reloaded, Second Edition

75. The Case Selection Structure (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

76. The Case Selection Structure (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

77. Using To and Is in an ExpressionList
TO and IS keywords: specify a range of values in a Case clause’s expression list
TO:
When you know both the upper and lower bounds of the range
IS:
When you know only one end of the range
Used with a comparison operator
Microsoft Visual Basic 2005: Reloaded, Second Edition

78. Using To and Is in an Expression List (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

79. Generating Random Integers
Pseudo-random number generator: produces a sequence of numbers that meets certain statistical requirements for randomness
Random.Next method:
Generates a random integer
Can specify a minimum and maximum value
Microsoft Visual Basic 2005: Reloaded, Second Edition

80. Generating Random Integers (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

81. Generating Random Integers (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

82. Generating Random Integers (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

83. Programming Tutorial
Microsoft Visual Basic 2005: Reloaded, Second Edition

84. Programming Example
Microsoft Visual Basic 2005: Reloaded, Second Edition

85. Summary Selection structure is used for decisions
Four forms of selection structures: If, If/Else, If/ElseIf/Else, and Case
Diamond symbol represents a decision in a flowchart
Expressions with comparison operators will result in an answer of True or False
Variables declared within a selection expression have block-level scope
Microsoft Visual Basic 2005: Reloaded, Second Edition

86. Summary (continued) Concatenation: linking two strings together
Use logical operators to create compound conditions
String.IsNullOrEmpty method will determine if a string contains data
MessageBox.Show method returns an integer indicating which button was chosen
Selection structures can be nested
Microsoft Visual Basic 2005: Reloaded, Second Edition

87. Summary (continued)
Use If/ElseIf/Else or Case structures when there are several possible alternative outcomes
Use TO keyword to specify a range of valid values when both the lower and upper bound are known
Use IS keyword with a comparison operator to specify a lower or upper bound but not both
Use the pseudo-random number generator to generate random numbers
Microsoft Visual Basic 2005: Reloaded, Second Edition