1. IS 350 Decision-making

2. Objectives Use the Boolean data type in decision-making statements
Write If statements and Select Case statements to make decisions
Use logical operators to create complex conditions
Create message boxes and input boxes
Use decision-making statements to perform input validation

3. Objectives (continued)
Create structured exception handlers so that run-time errors will not cause an application to terminate
Create instances of check boxes, scroll bars, group boxes, and radio buttons
Group control instances

4. Introduction to Decision Making
Chapter 1 introduced the three control structures
Sequence structure
Decision-making structure
Repetition structure
This chapter discusses the decision-making structure
Statements execute conditionally based on the outcome of a decision

5. Introduction to Boolean Data
Boolean data operates similarly to an on/off switch
True signifies on
False signifies off
Many properties store Boolean data
Visible and Enabled for example

6. Declaring a Boolean Variable
Declare a Boolean variable
Uninitialized Boolean variables have a value of False
Dim Valid As Boolean
Declare and initialize a Boolean variable
Dim BrowseMode As Boolean = True
Declare multiple Boolean variables
Dim Test1, Test2 As Boolean

7. Boolean Assignment Statements
The keywords True and False are used in Boolean assignment statements
Example:
Dim Valid As Boolean
Valid = True
Valid = False

8. Introduction to Decision Making
Applications need the capability to execute one group of statements in certain circumstances and other statements in other circumstances
These statements are called decision-making statements
The If statement is used to make decisions

9. Decision-making (pseudocode)
If the input date is greater than today then
Display a message indicating the input is invalid.
End of If statement

10. Flowchart of a decision-making statement

11. Using If Statements and Comparison Operators
A conditional statement executes one group of statements when a condition is True and another group of statements when a condition is False
Comparison operators are used in conditional statements
Conditional operations always produce a Boolean result

12. Comparison Operators Equal to (=) Not equal to (<>)
Less than (<)
Greater than (>)
Less than or equal to (<=)
Greater than or equal to (>=)

13. Using Comparison Operators (example 1)
Dim Result As Boolean
Dim Value1 As Integer = 3, Value2 As Integer = 5
Result = Value1 < Value2 ' True
Result = Value1 + 2 < Value2 – 1 ' False

14. Using Comparison Operators (example 2)
Parentheses can clarify the order of evaluation
The following two statements are equivalent:
Result = Value1 + 2 < Value2 – 1
Result = (Value1 + 2) < (Value2 – 1)
Result = (3 + 2) < (5 – 1)
Result = (5) < (5 – 1)
Result = (5) < (4)
Result = False

15. Evaluating a condition

16. Comparison Operators and If Statements
Comparison operators are most commonly used with an If statement
A group of statements execute only when a condition is True
This form of If statement is called a one-way If statement
The statements that execute as a result of a condition are called a statement block

17. One-Way If Statement (Syntax)
If condition Then
statements
End If
statement
condition must evaluate to a Boolean value
If the condition is True, statements execute
If the condition is False, statements do not execute
Execution continues at the statement following the End If
statements make up a statement block

18. One-Way If Statement (example)
Dim CurrentValue As Boolean = True
If CurrentValue = True Then
' Statements that execute when
' CurrentValue is True
End If
' statements

19. One-Way If statement

20. Comparison Operations Involving Dates
Comparison operations can be performed on dates
Dates in the past are less than dates in the future
Example:
Dim StartDate As DateTime = #3/22/2010#
Dim EndDate As DateTime = #3/24/2010#
If StartDate < EndDate = True Then
Console.WriteLine( _
"StartDate is less than EndDate")
End If

21. Comparison Operations on Numeric Data Types
Comparison operations can be performed on numeric data
Example:
Dim Percentage As Integer = 90
If Percentage < 100 Then
Console.Writeline( _
"Percentage is less than 100")
End If
' statements

22. Comparison Operations on Strings
Comparison operations can be performed on strings
Strings are compared character-by-character from left to right
String comparisons are performed in two ways
Case sensitive (binary comparison)
1 < 9 < A < B < E < Z < a < b < e < z
Option Compare Binary
Case insensitive (text comparison)
1 < 9 < (A=a) < (B=b) < (E=e) < (Z=z)
Option Compare Text

23. String equality using text and binary comparison

24. Introduction to Two-way If Statements
One statement block executes when a condition is True and another statement block executes when the condition is False
This form of If statement is commonly referred to as an If Then Else statement

25. Two-way If Statements (syntax)
If condition Then
statements(True)
Else
statements(False)
End If
statements
Statements(True) execute if the condition is True
Statements(False) execute if the condition is False

26. Two-way If Statements (example)
If Grade is greater than 75, display “Passing grade”. Otherwise, display “Failing grade”
Dim Grade As Integer = 80
If Grade > 75 Then
Console.WriteLine("Passing grade")
Else
Console.WriteLine("Failing grade")
End If
' statements

27. Two-way If statement

28. Introduction to Multiway If Statements
Multiway If statements have three or more possible outcomes

29. Multiway If Statements (syntax)
If condition1 Then
[statements]
[ElseIf condition2 Then
[elseifStatements]]
[Else]
[elseStatements]]
End If
statements

30. Multiway If Statements (dissection)
condition1 is first tested
If True, then the first statement block executes
Execution continues at the statement following the decision-making statement
If False, condition2 is tested, and then the remaining conditions are tested
If no conditions are True, then the statements in the Else block execute
The Else block is optional

31. Multiway If statement

32. Multiway If Statement (example)
Dim NumericGrade As Integer = 84
Dim LetterGrade As String
If NumericGrade >= 90 Then
LetterGrade = "A"
ElseIf NumericGrade >= 80 Then
LetterGrade = "B"
ElseIf NumericGrade >= 70 Then
LetterGrade = "C"
ElseIf NumericGrade >= 60 Then
LetterGrade = "D"
Else
LetterGrade = "F"
End If
' statements

33. Notes About If Statements
If statements can be written in different ways
Chose the If statement that is most readable
This decision can be subjective
The Code Editor automatically indents blocks in an If statement
The Code Editor automatically inserts the End If
If statements can be nested
One If statement can contain another If statement

34. Introduction to Select Case Statements
Select Case statements are similar to multiway If statements
The same expression must be used in each condition
Select Case statements are faster than comparable multiway If statements
Select Case statements tend to be more readable than comparable multiway If statements

35. Select Case Statement (syntax)
Select Case testExpression
Case expressionList-1
statement-block1
[Case expressionList-2
statement-block2]
[Case expressionListn
statement-blockn]
[Case Else
statements]
End Select
' statements

36. Select Case Statement (dissection)
testExpression is evaluated once
Each expressionList is then tested. If True, the corresponding statement-block executes and the Select Case statement ends
Each expressionList is tested in-order
When an expressionList is found to be True, the statement block executes and the Select Case statement ends
If no expessionList is True, then the statements in the Case Else block execute

37. Select Case Statement (example)
Dim Quarter As Integer = 1
Dim QuarterString As String
Select Case Quarter
Case 1
QuarterString = "First"
Case 2
QuarterString = "Second"
Case 3
QuarterString = "Third"
Case 4
QuarterString = "Fourth"
Case Else
QuarterString = "Error"
End Select
' statements

38. Select Case statement

39. Select Case Statement (variations)
The To clause is used to test a range of values
Case 90 To 100
The Is clause is used with comparison operators
Case Is > 90
A list of values can be created with a comma separated list
Case 1, 3, 5

40. Logical Operators (introduction)
Logical operators are used in conjunction with comparison and arithmetic operators
Logical operators perform the same task as a conjunction (and) or a disjunction (or) in English
The logical operators are And, Or, Not, Xor
See Table 7-2 for examples

41. Logical Operators (precedence)
Logical operators have an order of precedence
Arithmetic operators are evaluated first
Comparison operators are evaluated second
Logical operators are evaluated last from left to right in the following order:
Not, And, Or, Xor

42. Logical Operators (example 1)
Evaluation of an expression:
Dim Result As Boolean
Result = (3 + 4) > 6 And (4 + 1) < 6
Result = 7 > 6 And 5 < 6
Result = True And True
Result = True

43. Logical Operators (example 2)
Evaluation of an expression using And and Or
Result = (7 > 9) Or (5 > 3) And (3 > 2)
Result = False Or True And True
Result = False Or True
Result = True

44. The Not Operator The Not operator is a unary operator Examples:
Result = Not (True) ' False
Result = Not (False) ' True
Result = Not (4 > 3) ' False

45. Using Logical Operators
Logical operators are typically combined with comparison and arithmetic operators in decision-making statements
Example:
If Input >= CurrentMin And _ Input <= CurrentMax Then
Valid = True
Else
Valid = False
End If
' statements

46. Introduction to the MessageBox Class
The MessageBox is a standard dialog box that displays
A message
A caption
An icon
One or more standard button groups

47. The MessageBox Show Method (syntax)
Public Shared Function Show(ByVal text As String, ByVal caption As String, ByVal buttons As MessageBoxButtons, ByVal icon As MessageBoxIcon) As DialogResult
text appears in the title bar
caption contains the message
buttons defines the button(s)
icon defines the icon

48. The MessageBox.Show Method (example)
Display a message box with Yes and No buttons
Dim Result As DialogResult
Result = MessageBox.Show( _
"Do you want to quit?", "Exit", _
MessageBoxButtons.YesNo, _
MessageBoxIcon.Question, _
MessageBoxDefaultButton.Button1)
If Result = DialogResult.Yes Then
Me.Close()
End If

49. Message box

50. Message box enumerations

51. The InputBox (introduction)
The InputBox method gets a text string from the end user
It's a standard dialog box
It is possible to supply a default textual value

52. The InputBox (syntax) prompt contains a descriptive prompt
Shared Function InputBox(ByVal prompt As String, Optional ByVal title As String, Optional ByVal defaultResponse As String, Optional ByVal xPos As Integer, Optional ByVal yPos As Integer) As String
prompt contains a descriptive prompt
title appears on the title bar
defaultResponse contains the default value
xPos and YPos contain the coordinate values where the input box will appear

53. The InputBox (example)
Display an input box and store the returned string in ResultString
Dim ResultString As String
ResultString = _
Microsoft.VisualBasic.InputBox( _
"Enter a value", "Title", _
"Default Value", 0, 0)

54. Input box

55. Decision Making and Input Validation (1)
Input validation is used to check input to make sure it is valid or at least plausible
Use the IsDate function to determine whether a string can be converted to a date
Use the IsNumeric function to determine whether a string can be converted to a number
These functions return True if the value can be converted and False otherwise
The methods do not actually convert the value

56. Decision Making and Input Validation (continued)
Use range checking to determine whether a value falls between a range of values
A person's age, for example
The format of some data can be validated
Social Security numbers
Telephone numbers
Zip codes

57. Input Validation Events
The Validating event fires just before a control instance loses focus
This event can be cancelled
The Validated event does not fire in this case
If the Validating event is not canceled, the Validated event fires

58. Focus and Validating event sequence

59. Validating Event (example)
Validate a text box and cancel the event if the contents are invalid
Private Sub txtDOB_Validating( _
ByVal sender As Object, _
ByVal e As _
System.ComponentModel.CancelEventArgs) _
Handles txtDOB.Validating
If Not (IsDate(txtDOB.Text)) Then
e.Cancel = True
End If
End Sub

60. Introduction to Structured Exception Handling
Run-time errors will cause a program to terminate because of an exception being thrown
Exceptions can be thrown for several reasons
Numeric overflow errors
Type conversion errors
Division by zero errors
Create structured exception handlers to prevent a program from terminating

61. Structured Exception Handlers (syntax)
Try
' Place executable statements that might throw
' an exception in this block.
Catch
' This code runs if the statements in the Try
' block throw an exception.
Finally
' This code always runs after
' the Try block or Catch block exits.
End Try

62. Structured Exception Handlers (syntax dissection)
The Try statement marks the beginning of an exception handler
Place the statement(s) that may cause an exception in the Try block
The Catch statement contains the code that executes if an exception is thrown
Multiple Catch blocks can be created
The statements in the optional Finally block always execute

63. Structured Exception Handler (example)
Handle all exceptions and display a message box, if necessary
Dim Value1 As Short = 100
Dim Value2 As Short = 0
Dim Result As Short
Try
Result = Value1 \ Value2
Catch
MessageBox.Show("Division by zero", "Error", _
MessageBoxButtons.OK, _
MessageBoxIcon.Information)
End Try

64. Execution flow of a structured exception handler

65. The System.Exception Class
All exceptions are derived from the System.Exception class
Properties
The Message property contains an informational message
The Source property is a string containing the name of the application causing the error
The StackTrace property returns a string containing the error location

66. Types of Exceptions
ArithmeticException can be thrown because of type conversion errors
DivideByZeroException is thrown when trying to divide a number by zero
OverflowExcpetion is thrown in cases of numeric overflow
Trying to reference an object that does not exist throws a NullReferenceException

67. .NET Framework exception hierarchy

68. Controls that Rely on Decision-Making
Three controls are commonly used with decision-making
CheckBox control allows the end user to select one of two possible values
HScrollBar control and VScrollBar controls allow the end user to select a value from a range of values

69. The CheckBox Control
The CheckBox control allows the end user to select from two possible values
(Checked and Unchecked)
CheckAlign property defines where the check box appears
Boolean Checked property indicates whether the box is checked
Text property contains the visible text
TextAlign property controls the alignment of the text
CheckedChanged event fires when the value of the Checked property changes

70. The CheckBox Control (example)
Determine whether the CheckBox named chkDemo is checked
If chkDemo.Checked = True Then
Console.WriteLine("Checked")
Else
Console.WriteLine("Not checked")
End If

71. The HScrollBar and VScrollBar Controls (ontroduction)
Use to select a value from a range of values
The two scroll bars work the same way
HScrollBar has a horizontal orientation
VScrollBar has a vertical orientation

72. The HScrollBar and VScrollBar Controls (syntax)
Minimum and Maximum properties define the range of values
Current value is stored in the Value property
SmallChange and LargeChange properties define the magnitude of change
Scroll event fires while scrolling
ValueChanged event fires when scrolling is complete and the value changes

73. Changing the Value property of a vertical scroll bar

74. The Maximum and Minimum properties of a vertical scroll bar

75. Scroll Event (example)
Display scroll bar values
Private Sub vsbDemo_Scroll( _ ByVal sender As System.Object, _
ByVal e As _
System.Windows.Forms.ScrollEventArgs) _
Handles vsbDemo.Scroll
txtValue.Text = vsbDemo.Value.ToString()
txtOldValue.Text = e.OldValue.ToString()
txtNewValue.Text = e.NewValue.ToString() End Sub

76. Control Groups
Container controls are used to group control instances together
The GroupBox control is a container control
Visual Studio supports several other container controls

77. The GroupBox Control (syntax)
The BackColor and ForeColor properties define the color
Visual text appears in the Text property

78. The RadioButton Control
The end user selects one button from a group of buttons
Members
The Text property contains the visible text
The Checked property defines whether the RadioButton is selected
The CheckedChanged event fires when the button is clicked

79. Multicast Event Handlers (introduction)
One event handler handles the same event for many control instances
The Handles clause contains a comma separated list of control instances and event names
A period separates the control instance and event name

80. Multicast Event Handlers (Example)
Handle the CheckChanged event for three radio buttons
Private Sub radChoices_CheckedChanged( _
ByVal sender As System.Object, _
ByVal e As System.EventArgs) _
Handles radFirstChoice.CheckedChanged, _
radSecondChoice.CheckedChanged, _
radThirdChoice.CheckedChanged
Dim CurrentRadioButton As RadioButton
CurrentRadioButton = CType(sender, RadioButton)
Select Case CurrentRadioButton.Name
Case "radFirstChoice"
Case "radSecondChoice"
Case "radThirdChoice"
End Select
End Sub

81. Chapter Summary The Boolean data type stores values of True and False
Decision-making statements are categorized into one-way, two-way and multiway If statements
Conditional statements are made up of arithmetic, comparison, and logical operators
In cases where an expression is compared with different values, a Select Case statement can be used in place of a multiway If statement

82. Chapter Summary (continued)
The MessageBox and InputBox classes are used to display standard dialog boxes
Structured exception handlers are a form of decision-making statement used to handle errors
The CheckBox, HScrollBar, and VScrollBar controls are used to work with Boolean data
Container controls, such as the GroupBox, group other control instances
A multicast event handler is often used with RadioButtons to handle events for multiple control instances