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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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