Decisions

1
Chapter 5

• Decisions

2
Outline and Objective

• Relational and Logical Operators
• If Blocks
• Select Case Blocks

3
Relational Operators

• The execution of the block If is controlled by a
  Boolean Expression
• A Boolean Expression can be either True or False
• Boolean Expressions are formed by using six
  Relational operators and three Logical operators

4
Relational Operators
5
Using Relational Operators on Strings

• Computer uses a special coding system to compare
  character strings
• Usually ANSI (also called ASCII) coding system is
  used on PC.
• The association between characters and values of
  characters are given in Appendix A.

6
Example of Comparing Character Strings

• Chase lt Chaz True
• Cat lt Cat True
• Pay lt Pay True
• Jones ltgt James True
• Hope lt Hopeful True

7
Logical Operators

• The result of a logical operator is also True or
  False
• The three Logical Operators are
• Not
• And
• Or

8
Not

• Not Negates a single expression
• Example Suppose answ Y
• Not ( answ y) is True

9
And

• Combines two expressions each expression must
  be True for the entire expression to be True.
  
• Example Suppose answ Y
• (answ Y) And (answ y) is False

10
Or

• Combines two expressions either expression (or
  both expressions) must be True for the entire
  expression to be True.
• Example Suppose answ Y
• (answ Y) Or (answ y) is True

11
Determine whether the following conditions are
true or false?

• (Y ltgt X) And (143.55 lt 143.55)
• ( 0 14) Or (6 2 - 3 lt 4 / 2 8)
• Not ( 6 7) And ( 44 gt 33)

12
Control Structures

• Allows the programmer to alter the normal flow of
  statement execution.
• Types of control structures
• Loop
• Decision

13
Types of Decision Structures

• The Block If Statement
• Simple alternative if
• Compound alternative if.else
• Select Case Statement

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Single Alternative Decision

• An action is taken if the condition is true,
  otherwise the control goes to the next statement.

15
Single Alternative Decision

• Syntax
• If condition Then
• action(s)
• End If
• If condition is true, action(s) is
  executedotherwise action(s) is skipped

16
Example of single-alternative decision

• If (grade gt 90) Then
• picOutput.Print Excellent Student
• letterGrade A
• picOutput.Print Your grade is letterGrade
• End If

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Compound Alternative Decision

• Syntax
• If condition Then
• action1
• Else
• action 2
• End If

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Example of Compound If statement (find the
larger of two numbers)

• Private Sub cmdFindLarger_Click()
• Dim largerNum As Single
• picResult.Cls
• If Val(txtFirstNum.Text) gt Val(txtSecondNum.Tex
  t) Then
• largerNum Val(txtFirstNum.Text)
• Else
• largerNum Val(txtSecondNum.Text)
• End If
• picResult.Print "The larger number is"
  largerNum
• End Sub

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Important Points in Using If Statement

• Use indentation
• In the nested If statement, each If must have its
  own End If statement
• Care should be taken to make If blocks easy to
  understand

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Example

• If cond1 Then
• If cond2 Then
• action(s)
• End If
• End If

• If cond1 And cond2 Then
• action(s)
• End If

This is easier to understand
A confusing If Block
21
Compound Alternative Decision

• Syntax
• If condition1 Then
• action1
• ElseIf condition2 Then
• action 2
• ElseIf condittion3 Then
• action3
• Else
• action4
• End If

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Example (find the larger of two numbers, and
report if the two numbers are equal)

• Private Sub cmdFindLarger_Click()
• picResult.Cls
• If Val(txtFirstNum.Text) gt
  Val(txtSecondNum.Text) Then
• picResult.Print "The larger number is"
  txtFirstNum.Text
• ElseIf Val(txtSecondNum.Text) gt
  Val(txtFirstNum.Text) Then
• picResult.Print "The larger number is "
  txtSecondNum.Text
• Else
• picResult.Print "The two numbers are
  equal."
• End If
• End Sub

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Example (find cost of phone call from NY to LA)

• Private Sub cmdDisplay_Click()
• Dim length As Single
• Call InputLength (length)
• Call DisplayCost(length)
• End Sub

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Sub DisplayCost InputLenght

• Private Sub DisplayCost (length As Single)
• ' Display the cost of a call
• PicOutput.Print "Cost of call "
  FormatCurrency ( Cost (length))
• End Sub
• Private Sub InputLength(length As Single)
• ' Request the length of a phone call
• length Val (InputBox("Duration of the call
  in minutes? "))
• End Sub

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Function Cost

• Private Function Cost (length As Single) As
  Single
• If length lt 1 Then
• Cost .46
• Else
• Cost .46 (length 1) .36
• End If
• End Function

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The Select Case Block

• Similar to If statement
• Used instead of nested If statement
• An action to be selected from a list of
  alternatives
• Avoids confusion of deeply nested If blocks

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Select Case Block (Syntax)

• Select Case selector
• Case value-list-1
• action1
• Case value-list-2
• action2
• ..
• Case Else
• action of last resort
• End Select

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Select Case Block

• Each value-list contains one or more of the
  following types of items separated by a comma
• a constant
• a variable
• an expression
• an inequality sign preceded by Is and followed by
  a constant, variable, or expression
• a range expressed in the form a To b , where a
  and b are constants, variables, or expressions.

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Example of Select Case

• letterGrade txtGrade.text
• Select Case letterGrade
• Case A, B
• picOutput.Print Good Work
• Case C
• picOutput.Print Average Work
• Case Else
• picOutputPrint Poor Work
• End Select

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Example of If statement

• letterGrade txtGrade.Text
• If ( letterGrade A) Or (letterGrade B)
  Then
• picOutput.print Good Work
• ElseIf (letterGrade C) Then
• picOutput.Print Average Work
• Else
• picOutput.Print Poor Work
• End If

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Several different types of value-list

• Private Sub cmdInterpret_Click() Dim x As
  Integer, y As Integer, num As Integer ' One,
  Two, Buckle My Shoe picPhrase.Cls x 2
  y 3 num Val(txtNumber.Text) Select Case
  num Case y - x, x
  picPhrase.Print "Buckle my shoe." Case
  Is lt 4 picPhrase.Print "Shut the
  door." Case x y To x y
  picPhrase.Print "Pick up sticks." Case
  7, 8 picPhrase.Print "Lay them
  straight." Case Else
  picPhrase.Print "Start all over again." End
  SelectEnd Sub

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Selector Is a String Variable

• Private Sub cmdInterpret_Click()
• Dim firstName As String
• firstName txtAnswer.Text
• Select Case firstName
• Case "Thomas"
• picSolution.Print "Correct."
• Case "Woodrow"
• picSolution.Print "Sorry, his full
  name was"
• picSolution.Print "Thomas Woodrow
  Wilson."
• Case "President"
• picSolution.Print "Are you for real?"
• Case Else
• picSolution.Print "Nice try, but no
  cigar."
• End Select
• End Sub

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Summary Points

• How to use If statements
• Select Case statements
• How to use relational operators
• How to use Not, And, and Or in Boolean expressions