CSC430 - Chapter 3: Control Structures (Selection)

Chapter 3: Control Structures (Selection)

Introduction

• This chapter covers control structures, specifically focusing on selection.

• Topics include Boolean expressions, if statements, if...else statements, if...elseif...else statements, switch statements, and nested if statements.

Learning Outcome

• By the end of this class, students should be able to:

  • Interpret the concept of relational and logical operators.

  • Develop a program using if statements.

  • Differentiate between if and switch statements.

  • Produce programs using selection control.

Boolean Expression

• A Boolean expression is a sequence of operands and operators that combine to produce one of the Boolean values, true or false.

• Two types of Boolean expressions:

  • Simple Boolean Expression (SBE)

  • Compound Boolean Expression (CBE)

Selection Criteria with Boolean Expression

• Simple Boolean Expression (SBE):

  • Form: expression1 relational-operator expression2

  • Relational operators allow comparisons.

  • Require two operands.

  • Return 1 if the expression is true, 0 otherwise.

Relational/Comparison Operators (SBE)

• Supported by VBA (Visual Basic for Applications).

• Assume variable A holds 10 and variable B holds 20.

  • A = B (Equal To): Returns False (0)

  • A <> B (Not Equal To): Returns True (1)

  • A > B (Greater Than): Returns False (0)

  • A < B (Less Than): Returns True (1)

  • A >= B (Greater Than or Equal To): Returns False (0)

  • A <= B (Less Than or Equal To): Returns True (1)

Logical Operator (CBE)

• Supported by VBA.

• Assume variable A holds 10 and variable B holds 0.

  • And: Returns True only if both expressions are true.

  • Or: Returns True if either expression is true.

  • Not: Reverses the logical state of its operand.

The If Statement (One-Way Selection)

• Syntax:

  If (condition) Then
    statement 1
    ...
    statement n
  End If
  

• Example:

  Private Sub yourbuttonname_Click()
    Dim x As Integer
    Dim y As Integer
  
    x = 234
    y = 32
  
    If x > y Then
      MsgBox "X is Greater than Y"
    End If
  End Sub
  

The If…Else Statement (Two-Way Selection)

• Syntax:

  If (condition) Then
    statement 1
    ...
    statement n
  Else
    statement 1
    ...
    statement n
  End If
  

• Example:

  Private Sub yourbuttonname_Click()
    Dim x As Integer
    Dim y As Integer
  
    x = 234
    y = 324
  
    If x > y Then
      MsgBox "X is Greater than Y"
    Else
      Msgbox "Y is Greater than X"
    End If
  End Sub
  

The If…Elseif, Else Statement (Multi-Way Selection)

• Syntax:

  If (condition) Then
    statement n
  ElseIf (condition) Then
    statement n
  Else
    statement n
  End If
  

• Example:

  Private Sub yourbuttonname_Click()
    Dim x, y As Integer
  
    x = 234
    y = 234
  
    If x > y Then
      MsgBox "X is Greater than Y"
    ElseIf y > x Then
      Msgbox "Y is Greater than X"
    Else
      MsgBox "X and Y are EQUAL"
    End If
  End Sub
  

Exercise Example

• Illustrates using If statements to assign grades based on scores.

• Score >= 80: Grade A, Display "Excellent"

• Score >= 70: Grade B, Display "Good"

• Score >= 50: Grade C, Display "Fair"

• Score < 50: Grade F, Display "Fail"

• VBA Code Snippet Example:

  Private Sub Worksheet_SelectionChange(ByVal Target As Range)
  
  End Sub
  

Exercise: Hungry and Foods Logic

• Illustrates using nested IF statements

  START
  INPUT hungry, foods
  IF hungry is TRUE AND foods is TRUE Then
    DISPLAY EATS
  ELSEIF hungry is TRUE AND foods is FALSE Then
    DISPLAY FIND FOODS
  ELSEIF hungry is FALSE AND foods is TRUE then
    DISPLAY GIVE TO FRIEND
  ELSEIF hungry is FALSE AND foods is FALSE then
    DISPLAY SLEEP
  END IF
  END
  

• Implementation uses ActiveX Control with a command button (cmdHungry) and input/output via MsgBox.

Switch Statement

• Syntax:

  Select Case (var to test)
    Case Value
      statement n
    Case Value
      statement n
    Case Else
      statement n
  End Select
  

• Example:

  Private Sub switch_demo_Click()
    Dim MyVar As Integer
    MyVar = 1
  
    Select Case MyVar
      Case 1
        MsgBox "Number 1"
      Case 2
        MsgBox "Number 2"
      Case 3
        MsgBox "Number 3"
      Case Else
        MsgBox "Unknown Number"
    End Select
  End Sub
  

Exercise: ActiveX Control with Option Buttons

• Involves 4 Option Buttons (optG, optH, optA, optNone) and 1 Command Button (cmdDisplayApartment).

• range("B11") displays the value chosen from the option buttons.

• The command button displays the full apartment name and rate in MsgBox based on range("B11").

Nested IF

• An if statement inside another if statement.

• Syntax:

  If (condition) Then 'outer IF
    If (condition) Then 'inner IF
      statement n
    ElseIf (condition) Then
      statement n
    Else
      statement n
    End If 'END for inner IF
  Else 'when hungry is FALSE
    If (condition) Then 'inner IF
      statement n
    ElseIf (condition) Then
      statement n
    Else
      statement n
    End If 'END for inner IF
  End If 'END for outer IF
  

Exercise: Hungry and Foods Logic (Nested IF)

• Illustrates using nested IF statements based on hungry and foods inputs.

  START
  INPUT hungry, foods
  IF hungry is TRUE then 'outer IF
    IF foods is TRUE then 'inner IF
      DISPLAY EATS
    ELSE
      DISPLAY FIND FOODS
    END IF 'END for inner IF
  ELSE 'when hungry is FALSE
    IF foods is TRUE then 'inner IF
      DISPLAY GIVE TO FRIEND
    ELSE
      DISPLAY SLEEP
    END IF 'END for inner IF
  END IF 'END for outer IF
  END
  

• Implementation uses ActiveX Control with 2 Check Boxes (chkHungry, chkFoods).

• range("B8") displays the action based on the selection of chkHungry and chkFoods.