1. Microsoft Visual Basic 2008: Reloaded Fourth Edition
Chapter Four
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
Explain the difference between single-alternative and dual-alternative selection structures
Code a selection structure using the If…Then…Else statement
Include comparison operators and logical operators in a selection structure’s condition
Create a block-level variable
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

3. Objectives (cont'd.) Concatenate strings
Use the ControlChars.NewLine constant
Change the case of a string
Include a check box in an interface
Generate random numbers
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

4. 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
Single-alternative selection structure: performs a set of tasks only when the condition is true
True path: the tasks to perform when the condition is true
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

5. The Selection Structure (cont’d.)
Dual-alternative selection structure: contains one set of tasks to perform when the condition is true and a different set of tasks to perform when the condition is false
False path: the tasks to perform when the condition is false
Pseudocode uses if…end if to denote a selection structure and else to denote the false path
Indent instructions within the selection structure
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

6. The Selection Structure (cont'd.)
Figure 4-1: Selection structures you might use today
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

7. The Selection Structure (cont'd.)
Figure 4-1: Selection structures you might use today (cont’d.)
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

8. The Selection Structure (cont'd.)
Figure 4-2: Problem specification for Mountain Biking
Figure 4-3: Interface for the Mountain Biking application
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

9. The Selection Structure (cont'd.)
Figure 4-4: Pseudocode containing only the sequence structure
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

10. The Selection Structure (cont'd.)
Figure 4-5: Modified problem specification and pseudocode containing a single-alternative selection structure
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

11. Figure 4-6: Single-alternative selection structure shown in a flowchart
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

12. Figure 4-7: Modified problem specification and pseudocode containing a dual-alternative selection structure
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

13. Figure 4-8: Dual-alternative selection structure shown in a flowchart
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

14. Coding Single-Alternative and Dual-Alternative Selection Structures
If…Then…Else statement: used to code single-alternative and dual-alternative selection structures
Else clause: an optional part of the If statement
Only used for the dual-alternative selection structure
Condition must be a Boolean expression that evaluates to either True or False
Can contain variables, literal constants, named constants, properties, methods, arithmetic operators, comparison operators, and logical operators
Statement block: set of statements in the true path or the false path
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

15. Figure 4-9: How to use the If…Then…Else statement
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

16. Figure 4-9: How to use the If…Then…Else statement (cont’d.)
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

17. Comparison Operators Comparison operators (or relational operators):
Used as part of the condition in an If…Then…Else statement to compare two values
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 2010: Reloaded, Fourth Edition

18. Figure 4-10: How to use comparison operators in a condition
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

19. Comparison Operators (cont’d.)
Figure 4-10: How to use comparison operators in a condition (cont’d.)
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

20. Comparison Operators (cont'd.)
Have no order of precedence
Are evaluated from left to right in an expression
Are evaluated after any arithmetic operators in the expression
All expressions containing comparison operators evaluate to True or False only
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

21. Comparison Operators (cont'd.)
Figure 4-11: Evaluation steps for an expression containing arithmetic and comparison operators
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

22. Comparing Numeric Values
Auction House application displays highest and lowest of two bids entered by the user
Figure 4-12: Sample run of the Auction House application
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

23. Comparing Numeric Values (cont'd.)
Figure 4-13: Pseudocode containing a single-alternative selection structure
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

24. Figure 4-14: Flowchart containing a single-alternative selection structure
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

25. Comparing Numeric Values (cont'd.)
Figure 4-15: Code entered in the Display button’s Click event procedure
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

26. Comparing Numeric Values (cont'd.)
Figure 4-15: Code entered in the Display button’s Click event procedure (cont’d.)
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

27. Comparing Numeric Values (cont'd.)
Block-level variables: declared within a statement block and remain in memory until the procedure ends
Block scope: A block-scope variable can only be used within the statement block in which it was declared
Concatenation operator (&): connects or links two strings together
ControlChars.NewLine constant:
Advances the insertion point to the next line
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

28. Comparing Numeric Values (cont'd.)
Figure 4-16: Illustration of the swapping concept
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

29. Comparing Numeric Values (cont'd.)
Figure 4-17: How to concatenate strings
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

30. Comparing Strings
Addition and Subtraction Calculator application: displays the sum or difference of two numbers
Figure 4-18: Sample run of the Addition and Subtraction Calculator application
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

31. Comparing Strings (cont'd.)
Figure 4-19: Pseudocode containing a dual-alternative selection structure
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

32. Figure 4-20: Flowchart containing a dual-alternative selection structure
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

33. Comparing Strings (cont'd.)
MaxLength property: text box property that specifies the maximum number of characters that can be entered
CharacterCasing property: text box property that indicates if text should remain as typed or be converted to uppercase or lowercase
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

34. Figure 4-21: Calculate button’s Click event procedure
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

35. 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 2010: Reloaded, Fourth Edition

36. The ToUpper and ToLower Methods (cont’d.)
Figure 4-22: How to use the ToUpper and ToLower methods
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

37. The ToUpper and ToLower Methods (cont’d.)
Figure 4-22: How to use the ToUpper and ToLower methods (cont'd.)
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

38. The ToUpper and ToLower Methods (cont’d.)
Figure 4-22: How to use the ToUpper and ToLower methods (cont'd.)
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

39. Figure 4-23: Examples of using the ToUpper method in the calcButton Click event procedure
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

40. Figure 4-23: Examples of using the ToUpper method in the calcButton Click event procedure (cont’d.)
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

41. Comparing Boolean Values
Check boxes: used to offer the user one or more independent and nonexclusive items from which to choose
Figure 4-24: A different interface for the Addition and Subtraction Calculator application
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

42. Comparing Boolean Values (cont’d.)
Figure 4-25: Click event procedures for the subtractionCheckBox and calcButton
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

43. Comparing Boolean Values (cont’d.)
Figure 4-25: Click event procedures for the subtractionCheckBox and calcButton (cont’d.)
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

44. Logical Operators Logical operators (or Boolean operators):
Used to combine two or more conditions into one compound condition
Compound condition: a combination of conditions using logical operator(s)
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

45. Logical Operators (cont'd.)
Figure 4-26: How to use logical operators in a condition
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

46. Logical Operators (cont'd.)
Figure 4-26: How to use logical operators in a condition (cont'd.)
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

47. Figure 4-26: How to use logical operators in a condition (cont'd.)
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

48. Logical Operators (cont'd.)
Truth tables: used to evaluate logical operators in an expression
Short-circuit evaluation: an evaluation in which the second condition may not be evaluated
AndAlso evaluates to True only when both sub-conditions are True
OrElse evaluates to False only when both sub-conditions are False
AndAlso and OrElse operations do not evaluate the second condition if the first condition is false
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

49. Logical Operators (cont'd.)
Figure 4-27: Truth tables for the AndAlso and OrElse logical operators
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

50. 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
Remember: logical operators are evaluated after arithmetic or comparison operators in an expression
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

51. The Carroll Company Application
Data validation:
Process of verifying that the input data is within the expected range
Figure 4-28: Two ways of writing the calcButton Click event procedure
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

52. Figure 4-28: Two ways of writing the calcButton Click event procedure (cont’d.)
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

53. The Carroll Company Application (cont'd.)
Figure 4-29: Sample run of the Carroll Company application using valid data
Figure 4-30: Sample run of the Carroll Company application using invalid data
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

54. Summary of Operators
Figure 4-31: Listing of arithmetic, concatenation, comparison, and logical operators
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

55. Summary of Operators (cont’d.)
Figure 4-31: Listing of arithmetic, concatenation, comparison, and logical operators (cont’d.)
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

56. Generating Random Integers
Pseudo-random number generator: a device that produces a sequence of numbers that meets certain statistical requirements for randomness
Random object: represents a pseudo-random number generator
Random.Next method:
Generates a random integer
Can specify a minimum and maximum value
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

57. Figure 4-32: How to generate random integers
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

58. Generating Random Integers (cont'd.)
Figure 4-33: Sample run of the Random Integers application
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

59. Generating Random Integers (cont'd.)
Figure 4-34: Generate button’s Click event procedure
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

60. Programming Tutorial 1
Figure 4-36: MainForm for the Find the Mouse Game application
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

61. Programming Tutorial 2
Figure 4-44: MainForm for the Greenview Health Club application
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

62. Programming Example
Figure 4-50: MainForm in the Fat Calculator application
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

63. Summary
Selection structure allows a procedure to make a decision and then take the appropriate action
Three types of selection structures: single-alternative, dual-alternative, and multiple-alternative
Diamond symbol represents a decision in a flowchart
Expressions with comparison operators will result in an answer of True or False
Comparison operators are evaluated from left to right in expressions, after arithmetic operators
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

64. Summary (cont'd.)
Variables declared within a selection expression have block-level scope
Concatenation: connecting or linking two strings together with the concatenation operator (&)
ControlChars.Newline advances the insertion point to the next line in a control
String comparisons are case-sensitive
Use ToUpper and ToLower methods to temporarily convert the case of a string
Microsoft Visual Basic 2010: Reloaded, Fourth Edition

65. Summary (cont'd.)
Use check boxes to provide the user with one or more independent and nonexclusive choices
Use logical operators to create compound conditions
An expression containing a logical operator will evaluate to either True or False
Logical operators have an order of precedence and are evaluated after arithmetic and comparison operators
Use the pseudo-random number generator to generate random numbers
Microsoft Visual Basic 2010: Reloaded, Fourth Edition