1. 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 and logical operators in a selection structure’s condition
Prevent the division by zero run time error
Swap two values
Create a block-level variable

2. CIS 16 Application Development Programming with Visual Basic
Chapter Four
The Selection Structure

3. Selection structures
All of the procedures in an application are written using 1 of 3 structures
Sequence
Instructions are processed sequentially
Selection
Instructions are dependent on a decision being made
Repetition
Instructions repeat until stopped

4. The Selection Structure
The Selection Structure - also called the decision structure – indicates that a decision needs to me made before the program continues
The decision is based on a condition that must be evaluated to determine the next instruction to process
The condition must evaluate to either True or False only

5. The Selection Structure
Three types of selection structures
A single-alternative selection structure has a specific set of instructions to follow only when its condition evaluates to True
A dual-alternative selection structure has one set of instructions to follow when the condition evaluates to True, but a different set of instructions to follow when the condition evaluates to False
A multiple-alternative selection structure has many sets of instructions to follow based on conditions evaluating to True or False

6. Flowcharting a Selection Structure
Decision symbol
Used to represent the condition (decision) in both the selection and repetition structures
Other symbols:
Oval: Start/stop symbol
Rectangle: Process symbol
Parallelogram: Input/output symbol

7. Selection Structures

8. Coding Single-Alternative and Dual-Alternative Selection Structures
STATEMENT BLOCK

9. Selection Structure (cont.)
To determine whether a procedure requires a selection structure, and which one, study the problem specification
Selection structure is: a) Sequence b) Dual c)not needed ?

10. Lets try flowcharting it

11. The Selection Structure (cont’d.)

12. Lets try flowcharting it

13. The Selection Structure (cont’d.)

14. NESTED SELECTION STRUCTURES

15. Nested Selection Structures
When a decision needs to be made within a decision
The inner selection structure is referred to as a nested selection structure.
It is contained (nested) entirely within the outer selection structure.

16. Nested Selection Structures

17. Nested Selection Structures

18. MULTIPLE ALTERNATIVE SELECTION STRUCTURES

19. Multiple-Alternative Selection Structures
Multiple-alternative selection structures or extended selection structures choose from several different alternatives.

20. Multiple-Alternative Selection Structures

21. Multiple-Alternative Selection Structures

22. Multiple-Alternative Selection Structures

23. Select Case Statement
Select Case statement is simple and clear to code multiple selection structures.

24. Select Case Statement
Specifying a Range of Values in a Case Clause

25. How to make comparisons

26. Comparison Operators
Each comparison operator (also referred to as relational operators) can be used to compare two values, and the comparison always results in a Boolean value: either True or False.
Rules for comparison operators:
They do not have an order of precedence
They are evaluated from left to right
They are evaluated after any arithmetic operators in the expression

27. Comparison Operators (cont’d.)

28. Comparing Strings
Visual Basic is case-sensitive, which means that the uppercase letters of the alphabet are not equal to their lowercase counterparts
These three string comparisons will evaluate to False: "A" = "a", "Yes" = "yes", and "Yes" = "YES".
Use the ToUpper method to temporarily convert a string to uppercase
Use the ToLower method to temporarily convert a string to lowercase
A text box’s CharacterCasing property indicates whether the text inside the control should remain as typed, or be converted to either uppercase or lowercase

29. Comparing Strings

30. Comparing Boolean Values
Check boxes provide one or more independent and nonexclusive items from which the user can choose
An interface can contain any number of check boxes, and any number of them can be selected at the same time
Each check box should be labeled to make its purpose obvious

31. Comparing Boolean Values

32. Logical Operators
Logical operators allow you to combine two or more sub-conditions into one compound condition
The compound condition will always evaluate to a Boolean value: either True or False

33. Using Logical Operators
When more than one condition is included in an If...Then...Else statement, the conditions are called a compound condition

34. Using the And Logical Operator

35. Using the Or Logical Operator

36. Using the Not Logical Operator

37. Logical Operators (cont’d.)

38. Logical Operators (cont’d.)

39. Microsoft Visual Basic 2012: Reloaded, Fifth Edition
CHAPTER APPLICATION