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Chapter 4: Control Structures: Selection

Chapter 4: Control Structures: Selection. 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:

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Chapter 4: Control Structures: Selection

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

  2. 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

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

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

  5. Example 4-1: Using the >= relational operator • An integervariable “examScore” contains the value 86 • Expression: examScore >= 90 • Mening: Is the value contained in exam-score greater than or equal to 90? • Answer: • False Visual Basic .NET Programming: From Problem Analysis to Program Design

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

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

  8. 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

  9. 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

  10. 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

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

  12. Using the VB .NET Logical Operators • Oroperator • Joins two expressions • Returns true if either or bothexpressions are true • Xoroperator • Joins two expressions • Returns true if one and only one expression is true • Otherwise, returns false Visual Basic .NET Programming: From Problem Analysis to Program Design

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

  14. 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 1 (group of) statement if it is true • Executes a different (group of) statements if it is false Visual Basic .NET Programming: From Problem Analysis to Program Design

  15. 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

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

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

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

  19. Writing One-way Selection Statements • Multi-line if statement: • 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

  20. Writing Two-way Selection Statements • Write a 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

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

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

  23. 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 (download example) Visual Basic .NET Programming: From Problem Analysis to Program Design

  24. Writing Multi-way Selection Statements • Acts like multi-way Ifstatement • 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

  25. 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

  26. 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

  27. 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

  28. 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

  29. Summary (continued) • Two-way selection: • If and Else statements • Multi-way selection structure • Keywords Select Case For in-class practice: p. 138, Ex. 3, 5 (use And instead of OrElse), 7 (use And instead of AndAlso) p. 139, PEx. 1 (optional), 2, 3 HOMEWORK: p. 139, PEx. 4; Due: 2/19/09 Visual Basic .NET Programming: From Problem Analysis to Program Design

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