Chapter 4: The Selection Structure

1
Chapter 4 The Selection Structure
Programming with Microsoft Visual Basic .NET,
Second Edition
2
The IfThenElse Statement Lesson A Objectives

• Write pseudocode for the selection structure
• Create a flowchart to help you plan an
  applications code
• Write an If...Then...Else statement

3
The IfThenElse Statement Lesson A Objectives
(continued)

• Write code that uses comparison operators and
  logical operators
• Format numbers using the ToString method
• Change the case of a string

4
The Selection Structure

• Use the selection structure to make a decision or
  comparison and select a particular set of tasks
  to perform
• The selection structure is also called the
  decision structure
• The condition must result in either a true (yes)
  or false (no) answer

5
The Selection Structure (continued)

• If the condition is true, the program performs
  one set of tasks
• If the condition is false, there may or may not
  be a different set of tasks to perform
• Visual Basic .NET provides four forms of the
  selection structure If, If/Else, If/ElseIf/Else,
  and Case

6
Writing Pseudocode for If and If/Else Selection
Structures

• An If selection structure contains only one set
  of instructions, which are processed when the
  condition is true
• An If/Else selection structure contains two sets
  of instructions
• One set is processed when the condition is true
• The other set is processed when the condition is
  false

7
Flowcharting the If and If/Else Selection
Structures
8
Flowcharting the If and If/Else Selection
Structures (continued)
T
F
T
F
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Coding the If and If/Else Selection Structures

• If condition Then
• statement block containing one or more
  statements to be processed when the condition is
  true
• Else
• statement block containing one or more
  statements to be processed when the condition is
  false
• End If

10
Coding the If and If/Else Selection Structures
(continued)

• The items in square brackets ( ) in the syntax
  are optional
• You do not need to include the Else portion
• Words in bold are essential components of the
  statement

11
Coding the If and If/Else Selection Structures
(continued)

• Items in italic indicate where the programmer
  must supply information pertaining to the current
  application
• The set of statements contained in the true path,
  as well as the statements in the false path, are
  referred to as a statement block

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Comparison Operators
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Comparison Operators (continued)

• Comparison operators are also referred to as
  relational operators
• All expressions containing a relational operator
  will result in either a true or false answer only
• Comparison operators are evaluated from left to
  right, and are evaluated after any mathematical
  operators

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Comparison Operators (continued)

• 10 3 lt 5 2
• 5 2 is evaluated first, giving 10
• 10 3 is evaluated second, giving 13
• 13 lt 10 is evaluated last, giving false

• 7 gt 3 4 / 2
• 3 4 is evaluated first, giving 12
• 12 / 2 is evaluated second, giving 6
• 7 gt 6 is evaluated last, giving true

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Comparison Operators (continued)
16
Logical Operators
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Logical Operators (continued)

• Truth table for Not operator

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Logical Operators (continued)

• Truth table for And operator

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Logical Operators (continued)

• Truth table for AndAlso operator

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Logical Operators (continued)

• Truth table for Or operator

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Logical Operators (continued)

• Truth table for OrElse operator

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Logical Operators (continued)

• Truth table for Xor operator

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Logical Operators (continued)
Figure 4-19 Order of precedence for arithmetic,
comparison, and logical operators
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Using the ToString Method to Format Numbers

• Use the ToString method to format a number
• Syntax variablename.ToString(formatString)
• variablename is the name of a numeric variable

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Using the ToString Method to Format Numbers
(continued)

• formatString is a string that specifies the
  format
• Must be enclosed in double quotation marks
• Takes the form Axx
• A is an alphabetic character called the format
  specifier
• xx is a sequence of digits called the precision
  specifier

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Comparing Strings

• Example 1 Using the OrElse operator
• Dim letter As String
• letter Me.uiLetterTextBox.Text
• If letter P OrElse letter p Then
• Me.uiResultLabel.Text Pass
• Else
• Me.uiResultLabel.Text Fail
• End if

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Comparing Strings (continued)

• Example 2 Using the AndAlso operator
• Dim letter As String
• letter Me.uiLetterTextBox.Text
• If letter ltgt P AndAlso letter ltgt p Then
• Me.uiResultLabel.Text Fail
• Else
• Me.uiResultLabel.Text Pass
• End if

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Comparing Strings (continued)

• Example 3 Correct, but less efficient, solution
• Dim letter As String
• letter Me.uiLetterTextBox.Text
• If letter P OrElse letter p Then
• Me.uiResultLabel.Text Pass
• End If
• If letter ltgt P AndAlso letter ltgt p Then
• Me.uiResultLabel.Text Fail
• End if

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Comparing Strings (continued)

• Example 4 Using the ToUpper method
• Dim letter As String
• letter Me.uiLetterTextBox.Text
• If letter.ToUpper() P Then
• Me.uiResultLabel.Text Pass
• Else
• Me.uiResultLabel.Text Fail
• End if

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The Monthly Payment Calculator ApplicationLesson
B Objectives

• Group objects using a GroupBox control
• Calculate a periodic payment using the
  Financial.Pmt method
• Create a message box using the MessageBox.Show
  method
• Determine the value returned by a message box

31
Completing the User Interface

• Herman Juarez has asked you to create an
  application that he can use to calculate the
  monthly payment on a car loan
• To make this calculation, the application needs
• The loan amount (principal)
• The annual percentage rate (APR) of interest
• The life of the loan (term) in years

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Completing the User Interface (continued)
Figure 4-31 Sketch of the Monthly Payment
Calculator user interface
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Adding a Group Box Control to the Form

• Use the GroupBox tool in the Toolbox window to
  add a group box control to the interface
• A group box control serves as a container for
  other controls
• Use a group box control to visually separate
  related controls from other controls on the form

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Coding the uiCalcPayButton Click Event Procedure

• The uiCalcPayButtons Click event procedure is
  responsible for
• Calculating the monthly payment amount
• Displaying the result in the uiPaymentLabel
  control
• Figure 4-37 shows the pseudocode for the
  uiCalcPayButtons Click event procedure

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Coding the uiCalcPayButton Click Event Procedure
(continued)
Figure 4-37 Pseudocode for the uiCalcPayButton
Click event procedure
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Using the Financial.Pmt Method

• Use the Visual Basic .NET Financial.Pmt method to
  calculate a periodic payment on either a loan or
  an investment
• Syntax Financial.Pmt(Rate, NPer, PV, FV, Due)
• Rate interest rate per period
• NPer total number of payment periods (the term)

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Using the Financial.Pmt Method (continued)

• PV present value of the loan or investment the
  present value of a loan is the loan amount,
  whereas the present value of an investment is
  zero
• FV future value of the loan or investment the
  future value of a loan is zero, whereas the
  future value of an investment is the amount you
  want to accumulate if omitted, the number 0 is
  assumed

38
Using the Financial.Pmt Method (continued)

• Due due date of payments can be either the
  constant DueDate.EndOfPeriod or the constant
  DueDate.BegOfPeriod if omitted,
  DueDate.EndOfPeriod is assumed

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The MessageBox.Show Method

• Use the MessageBox.Show method to display a
  message box that contains text, one or more
  buttons, and an icon
• Syntax MessageBox.Show(text, caption, buttons,
  icon, defaultButton)
• text text to display in the message box
• caption text to display in the title bar of the
  message box

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The MessageBox.Show Method (continued)

• buttons buttons to display in the message box
• icon icon to display in the message box
• defaultButton button automatically selected when
  the user presses Enter

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Coding the TextChanged Event

• A controls TextChanged event occurs when the
  contents of a controls Text property have
  changed as a result of
• The user entering data into the control, or
• The applications code assigning data to the
  controls Text property

42
Coding the TextChanged Event (continued)

• When the user makes a change to the information
  entered in the three text box controls, the
  Monthly Payment Calculator application should
  delete the monthly payment displayed in the
  uiPaymentLabel control

43
Completing the Monthly Payment Calculator
ApplicationLesson C Objectives

• Specify the keys that a text box will accept
• Align the text in a label control
• Handle exceptions using a Try/Catch block

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Coding the KeyPress Event

• Template
• Private Sub uiPrincipalTextBox_KeyPress( _
• ByVal sender As Object, _
• ByVal e As System.Windows.Forms.KeyPressEve
  ntArgs) _
• Handles uiPrincipalTextBox.KeyPress
• Setting e.Handled True will cancel the key

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Aligning the Text in a Label Control

• The TextAlign property controls the placement of
  the text in a label control
• The TextAlign property can be set to TopLeft (the
  default), TopCenter, TopRight, MiddleLeft,
  MiddleCenter, MiddleRight, BottomLeft,
  BottomCenter, or BottomRight

46
Using a Try/Catch Block

• An exception is an error that occurs while a
  program is running
• Use the Try statement to catch (or trap) an
  exception when it occurs in a program
• Use a Catch statement to take the appropriate
  action to resolve the problem
• A block of code that uses both the Try and Catch
  statements is referred to as a Try/Catch block

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Using a Try/Catch Block (continued)

• Try
• one or more statements that might generate an
  exception
• Catch variablename As exceptionType
• one or more statements that will execute when
  an exceptionType exception occurs
• Catch variablename As exceptionType
• one or more statements that will execute when
  an exceptionType exception occurs
• End Try

48
Summary

• To evaluate an expression containing arithmetic,
  comparison, and logical operators, evaluate
  arithmetic operators first, then comparison
  operators, and then logical operators
• To code a selection structure, use the
  If...Then...Else statement
• To create a compound condition, use the logical
  operators

49
Summary (continued)

• Use the GroupBox tool to add a group box control
  to the form drag controls from the form or the
  Toolbox window into the group box control
• To calculate a periodic payment on either a loan
  or an investment, use the Financial.Pmt method
• To display a message box that contains text, one
  or more buttons, and an icon, use the
  MessageBox.Show method

50
Summary (continued)

• To allow a text box to accept only certain keys,
  code the text boxs KeyPress event
• To align the text in a control, set the controls
  TextAlign property
• To catch an exception, and then have the computer
  take the appropriate action, use a Try/Catch block