chapter 4: the selection structure programming with microsoft visual basic.net, second edition

Chapter 4: The Selection Structure

Programming with Microsoft Visual Basic .NET, Second Edition

Programming with Microsoft Visual Basic .NET, Second Edition 2

The If…Then…Else Statement Lesson A Objectives

• Write pseudocode for the selection structure

• Create a flowchart to help you plan an application’s code

• Write an If...Then...Else statement

The If…Then…Else 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

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

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

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

Flowcharting the If and If/Else Selection Structures

start/stop oval

process rectangle

input/output parallelogram

selection/repetition diamond

symbols are connected by flowlines

Flowcharting the If and If/Else Selection Structures (continued)

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

statement block containing one or more statements to be processed when the condition is false]

End If

(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

(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

Comparison Operators

= Is equal to

> Is Greater Than

>= Is Greater Than or Equal to

< Is Less Than

<= Is Less Than or Equal to

<> Is Not Equal to

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

10 + 3 < 5 * 2

• 5 * 2 is evaluated first, giving 10

• 10 + 3 is evaluated second, giving 13

• 13 < 10 is evaluated last, giving false

7 > 3 * 4 / 2

• 3 * 4 is evaluated first, giving 12

• 12 / 2 is evaluated second, giving 6

• 7 > 6 is evaluated last, giving true

Using a Comparison Operator

Dim first, second As Integer

If (first > second) Then

Dim temp As Integer

temp = first

first = second

first = temp

End If

Logical OperatorsNot Reverses the truth value of condition; false becomes

true and true becomes false.1

And All conditions connected by the And operator must be true for the compound condition to be true.

AndAlso All conditions connected by the AndAlso operator must be true for the compound condition to be true.

Or Only one of the conditions connected by the Or operator needs to be true for the compound condition to be true.

OrElse Only one of the conditions connected by the OrElse operator needs to be true for the compound condition to be true.

Xor One of the conditions connected by Xor must be true for the compound condition to be true.

Logical Operators (continued)

• Truth table for Not operator

If condition is Value of Result is

True False

False True

Result = Not Condition

• Truth table for And operator

If condition1 is And condition2 is Value of Result is

True True True

True False False

False True False

False False False

Result = condition1 And Condition2

• Truth table for AndAlso operator

True True True

True False False

False (not evaluated) False

Result = condition1 AndAlso Condition2

• Truth table for Or operator

True True True

True False True

False True True

False False False

Result = condition1 Or Condition2

• Truth table for OrElse operator

True (not evaluated) True

False True True

False False False

Result = condition1 OrElse Condition2

• Truth table for Xor operator

True True True

True False False

False True True

False False False

Result = condition1 Xor Condition2

Figure 4-19: Order of precedence for arithmetic, comparison, and logical operators

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

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

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”

Me.uiResultLabel.Text = “Fail”

End if

Comparing Strings (continued)

• Example 2: Using the AndAlso operator

If letter <> “P” AndAlso letter <> “p” Then

End if

• Example 3: Correct, but less efficient, solution

If letter = “P” OrElse letter = “p” Then

End If

If letter <> “P” AndAlso letter <> “p” Then

End if

• Example 4: Using the ToUpper method

If letter.ToUpper() = “P” Then

End if

The Monthly Payment Calculator Application

Lesson 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

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

Completing the User Interface (continued)

Figure 4-31: Sketch of the Monthly Payment Calculator user interface

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

Coding the uiCalcPayButton Click Event Procedure

• The uiCalcPayButton’s 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 uiCalcPayButton’s Click event procedure

Coding the uiCalcPayButton Click Event Procedure (continued)

Figure 4-37: Pseudocode for the uiCalcPayButton Click event procedure

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)

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

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

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

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

Coding the TextChanged Event

• A control’s TextChanged event occurs when the contents of a control’s Text property have changed as a result of:

– The user entering data into the control, or

– The application’s code assigning data to the control’s Text property

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

Completing the Monthly Payment Calculator Application

Lesson 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

Coding the KeyPress Event

• Template

Private Sub uiPrincipalTextBox_KeyPress( _

ByVal sender As Object, _

ByVal e As System.Windows.Forms.KeyPressEventArgs) _

Handles uiPrincipalTextBox.KeyPress

• Setting e.Handled = True will cancel the key

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

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

Using a Try/Catch Block (continued)

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

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

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

Summary (continued)

• To allow a text box to accept only certain keys, code the text box’s KeyPress event

• To align the text in a control, set the control’s TextAlign property

• To catch an exception, and then have the computer take the appropriate action, use a Try/Catch block

chapter 4: the selection structure programming with microsoft visual basic.net, second edition

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