1. Chapter 5 - Visual Basic Schneider
Decisions
Chapter 5 - Visual Basic Schneider

2. Outline and Objectives
Relational and Logical Operators
If Blocks
Select Case Blocks
This chapter introduces the control structure, powerful programming tool that will be used in virtually all programs from this point on.’
Control structures allow the programmer to determine whether or not a specific statements are executed.
Two types of control structures:
decision structure
loop structure
Decision structures are used to make comparisons
Chapter 5 - Visual Basic Schneider

3. Chapter 5 - Visual Basic Schneider
Relational Operators
The execution of an If block is controlled by a condition
A condition can be either true or false
Conditions can be formed by using the six Relational operators and the three Logical operators
Chapter 5 - Visual Basic Schneider

4. Chapter 5 - Visual Basic Schneider
Relational Operators
Chapter 5 - Visual Basic Schneider

5. Using Relational Operators on Strings
Computers use a special coding system to compare character strings called ANSI
The ANSI values of characters are given in Appendix A. A snippet included in Slide 34
You do not need to memorize the table
Just know the numbers, small and capital letters
“0” = 48
“A”= 65
“a” = 97
Punctuation < Capitals letters < Small letters
Chapter 5 - Visual Basic Schneider

6. Example of Comparing Character Strings
“Chase” < “Chaz”
True
“ Cat” < “Cat”
True
“Pay” < “Pay “
True
“Jones” <> “James”
True
“Hope” < “Hopeful”
True
Chapter 5 - Visual Basic Schneider

7. Chapter 5 - Visual Basic Schneider
Logical Operators
The result of a logical operator is also True or False
The three Logical Operators are:
Not
And Or
Precedence?!
Chapter 5 - Visual Basic Schneider

8. Chapter 5 - Visual Basic Schneider
Not
Not: Negates a single expression
Example: Suppose answ = “Y”
Not (answ = “y”) is True
True, because (“Y” = “y” I false.
Chapter 5 - Visual Basic Schneider

9. Chapter 5 - Visual Basic Schneider
And
Takes two expressions, returns True only if both expressions are evaluated as being true
Example: Suppose answ = “Y”
(answ = “Y”) And (answ = “y”) is False
False because the second condition I false.
Chapter 5 - Visual Basic Schneider

10. Chapter 5 - Visual Basic SchneiderOr
Takes two expressions, returns true if either one of the expressions is evaluated as being true.
Example: Suppose answ = “Y”
(answ = “Y”) Or (answ = “y”) is True
True, because the first condition becomes True (“Y”= “Y”
Chapter 5 - Visual Basic Schneider

11. Determine whether the following conditions are true or false?
(“Y” <> “X”) And ( < )
(0 = 14) Or (6 ^ <= 4 / 2 + 8)
(Not (6 = 7)) And (44 > 33)
A. False
B. False
C. True
Chapter 5 - Visual Basic Schneider

12. Chapter 5 - Visual Basic Schneider
Truth Tables X Y
X And Y
X Or Y
Not X F T
Not ( X And Y ) =
Not ( X Or Y ) =
Chapter 5 - Visual Basic Schneider

13. Determine if True or False
a = 2, b=3
a*a < b Or Not a*a <a
True
((a=b) Or Not (b<a)) And ((a<b) Or (b=a+1))
“9W” >= “9W” And “duck” < “Du” & ”ck”
False
Chapter 5 - Visual Basic Schneider

14. Are the conditions equivelent?
Not(a<m); n>m
(a=b) and (a<b); a<>b
Not ((a=b) or (a=c)) ; a<>b and a<>c
Not (a>=b); (a<b) and not (a=b)
Chapter 5 - Visual Basic Schneider

15. Write an Exp equal to the negation of
a>b
(a<b) and (c<>d)
(a=b) or (a=d)
Not ( (a=b) or (a>b) )
(a<> “”) and (a<b) and (len(a) < 5)
Chapter 5 - Visual Basic Schneider

16. Types of Decision Structures
If Block Statement
Single alternative: If...Then
Compound alternative: If Then...Else
Select Case Statement
Chapter 5 - Visual Basic Schneider

17. Single Alternative Decision
An action is taken if the condition is true, otherwise the control goes to the next statement.
Syntax: If condition Then action End If
If condition is true, action is executed. Otherwise, action is skipped
Chapter 5 - Visual Basic Schneider

18. Example of single-alternative decision
If (grade > = 90) Then
picOutput.Print “Excellent Student”
letterGrade = “A”
picOutput.Print “Your grade is “; letterGrade
End If
Chapter 5 - Visual Basic Schneider

19. Compound Alternative Decision
Syntax
If condition Then
action1
Else
action 2
End If
Chapter 5 - Visual Basic Schneider

20. Example of Compound If statement
Private Sub cmdFindLarger_Click()
Dim largerNum As Single
If Val(txtFirstNum.Text) > Val(txtSecondNum.Text) Then
largerNum = Val(txtFirstNum.Text)
Else
largerNum = Val(txtSecondNum.Text)
End If
picResult.Print "The larger number is"; largerNum
End Sub
Chapter 5 - Visual Basic Schneider

21. 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 read and understand
Chapter 5 - Visual Basic Schneider

22. This is easier to understand
Example
If cond1 Then
If cond2 Then
action(s)
End If
If cond1 And cond2 Then
action(s)
End If
This is easier to understand
A confusing If Block
Chapter 5 - Visual Basic Schneider

23. Compound Alternative Decision
Syntax
If condition1 Then
action1
ElseIf condition2 Then
action 2
ElseIf condittion3 Then
action3
Else
action4
End If
This block statement searches for the first True condition, carries out its action, and then skips o the statement following end if.
If none of the conditions is true, then else’s action is carried out.
In general, an IF block can contain any numer of ElsIf clauses.
Chapter 5 - Visual Basic Schneider

24. Chapter 5 - Visual Basic Schneider
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) > Val(txtSecondNum.Text) Then
picResult.Print "The larger number is"; txtFirstNum.Text
ElseIf Val(txtSecondNum.Text) > Val(txtFirstNum.Text) Then
picResult.Print "The larger number is "; txtSecondNum.Text
Else
picResult.Print "The two numbers are equal."
End If
End Sub
Chapter 5 - Visual Basic Schneider

25. Example (find cost of phone call from NY to LA)
Private Sub DisplayCost (length As Single)
PicOutput.Print "Cost of call: "; FormatCurrency(Cost(length))
End Sub
Private Function Cost (length As Single) As Single
If length < 1 Then
Cost = .46
Else
Cost = (length –1) * .36
End If
End Function
Chapter 5 - Visual Basic Schneider

26. Single Line If Statement
Syntax
If condition Then action
There is no End If or Else blocks
If the condition is true, the action will be executed
If the condition is false, no action will be taken and the next statement will be executed
This block statement searches for the first True condition, carries out its action, and then skips o the statement following end if.
If none of the conditions is true, then else’s action is carried out.
In general, an IF block can contain any numer of ElsIf clauses.
Chapter 5 - Visual Basic Schneider

27. Chapter 5 - Visual Basic Schneider
Examples
If 5 Then
action(s)
End If
If n Then
action(s)
End If
If Not(n < m) Then
action(s)
End If
Equivalent to
If (n >= m) Then
action2
If 2 < n < 5 Then
action(s)
End If
Wrong!
(n > 2) And (n < 5)
Chapter 5 - Visual Basic Schneider

28. Chapter 5 - Visual Basic Schneider
The Select Case Block
Similar to If statement
Used instead of compound If statements
Action is selected from a list of alternatives
Avoids confusion of deeply nested If blocks
Chapter 5 - Visual Basic Schneider

29. 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
Select case choices are determined by the value of an expression called a SELECTOR. . Each of the possible actions is preceded by statement of the form CASE valulist., where valuelist itemizes the values of the selector for which the action should be taken
Chapter 5 - Visual Basic Schneider

30. Chapter 5 - Visual Basic Schneider
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.
Chapter 5 - Visual Basic Schneider

31. Chapter 5 - Visual Basic Schneider
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
Chapter 5 - Visual Basic Schneider

32. Chapter 5 - Visual Basic Schneider
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
letterGrade = txtGrade.text
If(letterGrade = “A” or letterGrade=“B”) Then
picOutput.Print “ Good Work”
Elseif (letterGrade=“C”) Then
picOutput.Print “Average Work”
Else
picOutputPrint “Poor Work”
End if
Chapter 5 - Visual Basic Schneider

33. 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
Chapter 5 - Visual Basic Schneider

34. Several different types of value-list
Private Sub cmdInterpret_Click()
Dim x As Integer, y As Integer, num As Integer
x = 2
num = Val(txtNumber.Text)
Select Case num
Case 3 - x, x picPhrase.Print "Buckle my shoe." Case Is <= picPhrase.Print "Shut the door." Case x + 3 To x * picPhrase.Print "Pick up sticks." Case Else picPhrase.Print "Start all over again.“
End Select
End Sub
Chapter 5 - Visual Basic Schneider

35. Selector Is a String Variable
Private Sub cmdInterpret_Click()
Select Case txtFirstName.Text
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
Chapter 5 - Visual Basic Schneider

36. Chapter 5 - Visual Basic Schneider