1. Chapter 4: Control Structures: Selection
Visual Basic .NET Programming:
From Problem Analysis to Program Design

2. Visual Basic .NET Programming: From Problem Analysis to Program Design
Objectives
Write and interpret logical expressions
Write one-way selection statements
Write two-way selection statements
Write multi-way selection statements
Visual Basic .NET Programming: From Problem Analysis to Program Design

3. Writing And Interpreting Logical Expressions
Make decisions in selection statement by writing logical expressions
Logical expression
Specifies condition that evaluates to:
True
Or False
Use to compare two values
Visual Basic .NET Programming: From Problem Analysis to Program Design

4. Using the VB .NET Relational Operators
Use to make comparison in logical expression
Visual Basic .NET Programming: From Problem Analysis to Program Design

5. Visual Basic .NET Programming: From Problem Analysis to Program Design

6. Example 4-1: Using the >= relational operator
examScore
Integer variable
Contains value 86
Expression:
examScore >= 90
Is value contained in exam-score greater than or equal to 90?
Answer:
False
Visual Basic .NET Programming: From Problem Analysis to Program Design

7. Using the VB .NET Logical Operators
Use to combine logical expressions
Frequently used logical operators:
Not
Negates an expression
And
Joins two expressions
Visual Basic .NET Programming: From Problem Analysis to Program Design

8. Visual Basic .NET Programming: From Problem Analysis to Program Design

9. Using the VB .NET Logical Operators
And operator
Joins two expressions
Forms compound expression
If both expressions evaluate to true
Then compound expression is true
Otherwise, it is false
Visual Basic .NET Programming: From Problem Analysis to Program Design

10. Example 4-4: Using the And logical operator
Variables:
examScore
Integer
Value: 86
engineeringStudent
Boolean
Value: True
Visual Basic .NET Programming: From Problem Analysis to Program Design

11. Example 4-4: Using the And logical operator
Expression:
examScore >= 90 And engineeringStudent
First expression (examScore >= 90)
Evaluates to false
Second (engineeringStudent)
Evaluates to true
Result is false
Visual Basic .NET Programming: From Problem Analysis to Program Design

12. Visual Basic .NET Programming: From Problem Analysis to Program Design

13. Using the VB .NET Logical Operators
Or operator
Joins two expressions
Returns true if either or both expressions are true
Xor operator
Returns true if one and only one expression is true
Otherwise, returns false
Visual Basic .NET Programming: From Problem Analysis to Program Design

14. Visual Basic .NET Programming: From Problem Analysis to Program Design

15. Visual Basic .NET Programming: From Problem Analysis to Program Design

16. Using the VB .NET Logical Operators
AndAlso, OrElse
Correspond to And and Or
Employ short-circuit evaluation technique
Visual Basic .NET Programming: From Problem Analysis to Program Design

17. Example 4-8: Using the OrElse logical operator
Statement:
1 < 2 OrElse 2 < 3
If first expression is true
Compound expression true
Eliminates need to evaluate second expression
Visual Basic .NET Programming: From Problem Analysis to Program Design

18. Writing One-way Selection Statements
Evaluates logical expression
Executes statements only if expression is true
Two-way selection statement:
Executes statements if it is true
Executes different statements if it is false
Visual Basic .NET Programming: From Problem Analysis to Program Design

19. Writing One-way Selection Statements
Flowchart
Graphical representation of logic
Use symbols to represent logical components of algorithm
Symbols:
Diamond
Rectangle
Circle
Flow lines
Visual Basic .NET Programming: From Problem Analysis to Program Design

20. Visual Basic .NET Programming: From Problem Analysis to Program Design

21. Writing One-way Selection Statements
Single-line If syntax:
If (logical expression) Then statement
Multi-line If syntax:
If (logical expression) Then
statement .
End If
Visual Basic .NET Programming: From Problem Analysis to Program Design

22. Writing One-way Selection Statements
Multi-line if:
Statements written on separate lines
Keyword End If must be used to terminate If statement
Visual Basic .NET Programming: From Problem Analysis to Program Design

23. Visual Basic .NET Programming: From Problem Analysis to Program Design

24. Writing Two-way Selection Statements
Write two-way selection statement
When you want to execute one or more statements if logical expression is true
But also want to execute one or more different statements if it is false
Nested If
If statement written inside another If statement
Can replace compound expression with nested If
Visual Basic .NET Programming: From Problem Analysis to Program Design

25. Visual Basic .NET Programming: From Problem Analysis to Program Design

26. Writing Two-way Selection Statements
Syntax:
If (logical expression) Then
statement(s)
Else
End If
ElseIf
Combines Else and If
Visual Basic .NET Programming: From Problem Analysis to Program Design

27. Example 4-17: Determining a grade using ElseIf statements
1. If examScore >= 90 Then
2. grade = “A”
3. ElseIf examScore >= 80 Then
4. grade = “B”
5. ElseIf examScore >= 70 Then
6. grade = “C”
7. ElseIf examScore >= 60 Then
8. grade = “D”
9. Else grade = “F”
10. End If
Visual Basic .NET Programming: From Problem Analysis to Program Design

28. Writing Multi-way Selection Statements
Acts like multi-way If statement
By transferring control to one or more statements
Depending on value of a variable
Sometimes called case structure
Uses keywords:
Select
Case
Visual Basic .NET Programming: From Problem Analysis to Program Design

29. Example 4-18: Determine a Grade Using Select Case Statements
1. Select Case examScore
2. Case Is >= 90
grade = “A”
4. Case 80 To 89
grade = “B”
6. Case 70 To 79
grade = “C” …
Visual Basic .NET Programming: From Problem Analysis to Program Design

30. Example 4-18: Determine a Grade Using Select Case Statements…
8. Case 60 To 69
grade = “D”
10. Case Else
grade = “F”
12. End Select
Visual Basic .NET Programming: From Problem Analysis to Program Design

31. Programming Example: Payroll Calculation
Input
Overtime exempt “Y” or “N”
Hours worked
Hourly pay rate
Output
The employee’s
Regular pay
Overtime pay
Total pay
Visual Basic .NET Programming: From Problem Analysis to Program Design

32. Visual Basic .NET Programming: From Problem Analysis to Program Design
Summary
Make decisions in selection statement by writing logical expression
Evaluates to either true or false
Logical operators join two logical expressions to form compound expression
One-way selection statement
Evaluates logical expression
Executes one or more statements only if expression is true
Visual Basic .NET Programming: From Problem Analysis to Program Design

33. Visual Basic .NET Programming: From Problem Analysis to Program Design
Summary (continued)
Two-way selection statement
Evaluates logical expression
Executes one or more statements if it is true
Executes one or more different statements if it is false
One-way selection:
One-line and multi-line If statements
Visual Basic .NET Programming: From Problem Analysis to Program Design

34. Visual Basic .NET Programming: From Problem Analysis to Program Design
Summary (continued)
Two-way selection:
If and Else statements
Multi-way selection structure
Keywords Select Case
Visual Basic .NET Programming: From Problem Analysis to Program Design