Chapter 4: The Selection Structure 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

Programming with Microsoft Visual Basic.NET, Second Edition 3 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

Programming with Microsoft Visual Basic.NET, Second Edition 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

Programming with Microsoft Visual Basic.NET, Second Edition 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

Programming with Microsoft Visual Basic.NET, Second Edition 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

Programming with Microsoft Visual Basic.NET, Second Edition 7 Flowcharting the If and If/Else Selection Structures start/stop oval process rectangle input/output parallelogram selection/repetition diamond symbols are connected by flowlines

Programming with Microsoft Visual Basic.NET, Second Edition 8 Flowcharting the If and If/Else Selection Structures (continued) T F TF

Programming with Microsoft Visual Basic.NET, Second Edition 9 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

Programming with Microsoft Visual Basic.NET, Second Edition 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

Programming with Microsoft Visual Basic.NET, Second Edition 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

Programming with Microsoft Visual Basic.NET, Second Edition 12 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

Programming with Microsoft Visual Basic.NET, Second Edition 13 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

Programming with Microsoft Visual Basic.NET, Second Edition 14 Comparison Operators (continued) < 5 * 2 5 * 2 is evaluated first, giving is evaluated second, giving < 10 is evaluated last, giving false 7 > 3 * 4 / 2 3 * 4 is evaluated first, giving / 2 is evaluated second, giving 6 7 > 6 is evaluated last, giving true

Programming with Microsoft Visual Basic.NET, Second Edition 15 Comparison Operators (continued) 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

Programming with Microsoft Visual Basic.NET, Second Edition 16 Logical Operators NotReverses the truth value of condition; false becomes true and true becomes false. 1 AndAll conditions connected by the And operator must be true for the compound condition to be true. 2 AndAlsoAll conditions connected by the AndAlso operator must be true for the compound condition to be true. 2 OrOnly one of the conditions connected by the Or operator needs to be true for the compound condition to be true. 3 OrElseOnly one of the conditions connected by the OrElse operator needs to be true for the compound condition to be true. 3 XorOne of the conditions connected by Xor must be true for the compound condition to be true. 4

Programming with Microsoft Visual Basic.NET, Second Edition 17 Logical Operators (continued) Truth table for Not operator If condition isValue of Result is TrueFalse True Result = Not Condition

Programming with Microsoft Visual Basic.NET, Second Edition 18 Logical Operators (continued) Truth table for And operator If condition1 isAnd condition2 isValue of Result is True False TrueFalse Result = condition1 And Condition2

Programming with Microsoft Visual Basic.NET, Second Edition 19 Logical Operators (continued) Truth table for AndAlso operator If condition1 isAnd condition2 isValue of Result is True False (not evaluated)False Result = condition1 AndAlso Condition2

Programming with Microsoft Visual Basic.NET, Second Edition 20 Logical Operators (continued) Truth table for Or operator If condition1 isAnd condition2 isValue of Result is True FalseTrue FalseTrue False Result = condition1 Or Condition2

Programming with Microsoft Visual Basic.NET, Second Edition 21 Logical Operators (continued) Truth table for OrElse operator If condition1 isAnd condition2 isValue of Result is True(not evaluated)True FalseTrue False Result = condition1 OrElse Condition2

Programming with Microsoft Visual Basic.NET, Second Edition 22 Logical Operators (continued) Truth table for Xor operator If condition1 isAnd condition2 isValue of Result is True False True False Result = condition1 Xor Condition2

Programming with Microsoft Visual Basic.NET, Second Edition 23 Logical Operators (continued) Figure 4-19: Order of precedence for arithmetic, comparison, and logical operators

Programming with Microsoft Visual Basic.NET, Second Edition 24 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

Programming with Microsoft Visual Basic.NET, Second Edition 25 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

Programming with Microsoft Visual Basic.NET, Second Edition 26 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

Programming with Microsoft Visual Basic.NET, Second Edition 27 Comparing Strings (continued) Example 2: Using the AndAlso operator Dim letter As String letter = Me.uiLetterTextBox.Text If letter <> “P” AndAlso letter <> “p” Then Me.uiResultLabel.Text = “Fail” Else Me.uiResultLabel.Text = “Pass” End if

Programming with Microsoft Visual Basic.NET, Second Edition 28 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 <> “P” AndAlso letter <> “p” Then Me.uiResultLabel.Text = “Fail” End if

Programming with Microsoft Visual Basic.NET, Second Edition 29 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

Programming with Microsoft Visual Basic.NET, Second Edition 30 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

Programming with Microsoft Visual Basic.NET, Second Edition 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

Programming with Microsoft Visual Basic.NET, Second Edition 32 Completing the User Interface (continued) Figure 4-31: Sketch of the Monthly Payment Calculator user interface

Programming with Microsoft Visual Basic.NET, Second Edition 33 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

Programming with Microsoft Visual Basic.NET, Second Edition 34 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

Programming with Microsoft Visual Basic.NET, Second Edition 35 Coding the uiCalcPayButton Click Event Procedure (continued) Figure 4-37: Pseudocode for the uiCalcPayButton Click event procedure

Programming with Microsoft Visual Basic.NET, Second Edition 36 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)

Programming with Microsoft Visual Basic.NET, Second Edition 37 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

Programming with Microsoft Visual Basic.NET, Second Edition 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

Programming with Microsoft Visual Basic.NET, Second Edition 39 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

Programming with Microsoft Visual Basic.NET, Second Edition 40 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

Programming with Microsoft Visual Basic.NET, Second Edition 41 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

Programming with Microsoft Visual Basic.NET, Second Edition 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

Programming with Microsoft Visual Basic.NET, Second Edition 43 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

Programming with Microsoft Visual Basic.NET, Second Edition 44 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

Programming with Microsoft Visual Basic.NET, Second Edition 45 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

Programming with Microsoft Visual Basic.NET, Second Edition 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

Programming with Microsoft Visual Basic.NET, Second Edition 47 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

Programming with Microsoft Visual Basic.NET, Second Edition 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

Programming with Microsoft Visual Basic.NET, Second Edition 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

Programming with Microsoft Visual Basic.NET, Second Edition 50 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