Chapter 4: Control Structures:
Selection
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
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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
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Using the VB .NET Relational
Operators
• Relational operator
– Use to make comparison in logical expression
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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
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Using the VB .NET Logical
Operators
• Logical operators
– Use to combine logical expressions
• Frequently used logical operators:
– Not
• Negates an expression
– And
• Joins two expressions
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Visual Basic .NET Programming: From Problem Analysis to Program Design
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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
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Example 4-4: Using the And
logical operator
• Variables:
– examScore
• Integer
• Value: 86
– engineeringStudent
• Boolean
• Value: True
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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
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Visual Basic .NET Programming: From Problem Analysis to Program Design
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Using the VB .NET Logical
Operators
• Or operator
– Joins two expressions
– Returns true if either or both expressions are true
• Xor operator
– Joins two expressions
– Returns true if one and only one expression is true
– Otherwise, returns false
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Using the VB .NET Logical
Operators
• AndAlso, OrElse
– Correspond to And and Or
– Employ short-circuit evaluation technique
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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
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Writing One-way Selection
Statements
• One-way selection statement:
– Evaluates logical expression
– Executes statements only if expression is true
• Two-way selection statement:
– Evaluates logical expression
– Executes statements if it is true
– Executes different statements if it is false
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Writing One-way Selection
Statements
• Flowchart
– Graphical representation of logic
– Use symbols to represent logical components of
algorithm
– Symbols:
• Diamond
• Rectangle
• Circle
• Flow lines
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Visual Basic .NET Programming: From Problem Analysis to Program Design
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Writing One-way Selection
Statements
• Single-line If syntax:
If (logical expression) Then statement
• Multi-line If syntax:
If (logical expression) Then
statement
.
statement
End If
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Writing One-way Selection
Statements
• Multi-line if:
– Statements written on separate lines
– Keyword End If must be used to terminate If
statement
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Visual Basic .NET Programming: From Problem Analysis to Program Design
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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
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Visual Basic .NET Programming: From Problem Analysis to Program Design
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Writing Two-way Selection
Statements
• Syntax:
If (logical expression) Then
statement(s)
Else
statement(s)
End If
• ElseIf
– Combines Else and If
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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
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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
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Example 4-18: Determine a
Grade Using Select Case
Statements
1. Select Case examScore
2. Case Is >= 90
3. grade = “A”
4. Case 80 To 89
5.
grade = “B”
6. Case 70 To 79
7.
grade = “C”
…
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Example 4-18: Determine a
Grade Using Select Case
Statements
…
8. Case 60 To 69
9. grade = “D”
10. Case Else
11. grade = “F”
12. End Select
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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
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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
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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
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Summary (continued)
• Two-way selection:
– If and Else statements
• Multi-way selection structure
– Keywords Select Case
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