Chapter 05

CHAPTER FIVEDecision Structures

Chapter 5: Decision Structures2ObjectivesUse the GroupBox objectPlace RadioButton objects in applicationsDisplay a message boxMake decisions using If…Then statementsMake decisions using If…Then…Else statementsMake decisions using nested If statements

Chapter 5: Decision Structures3ObjectivesMake decisions using logical operatorsMake decisions using Case statementsInsert code snippetsTest input to ensure a value is numeric

Chapter 5: Decision Structures4Using the GroupBox ObjectDrag the GroupBox object in the Containers category of the Toolbox over the Form object to the approximate location where you want to place the GroupBox objectWhen the mouse pointer is in the correct location, release the left mouse button. With the GroupBoxobject selected, scroll in the Properties window to the (Name) property. Double-click in the right column of the (Name) property and then enter the name grpWoodTypeClick to the right of the Size property of the GroupBox object and enter 125,100 as the size. Change the Font property to Goudy Old Style, Regular, Size 12. Change the BackColor property to White

Chapter 5: Decision Structures5Using the GroupBox Object

Chapter 5: Decision Structures6Adding the RadioButton ObjectsDrag and drop one RadioButton object from the Toolbox into the GroupBox object on the Form object.Drag a second RadioButton object from the Toolbox into the GroupBox object, using blue snap lines to align and separate the RadioButton objects verticallyRelease the left mouse button to place the RadioButton object on the Form object within the GroupBox object. Using the same technique, add a third RadioButton object

Chapter 5: Decision Structures7Adding the RadioButton ObjectsName the RadioButton objects by selecting a RadioButton object, double-clicking in the right column of the (Name) property in the Properties window, and entering the name. The names for the radio buttons, from top to bottom, should be radPine, radOak, and radCherryChange the Text property for each RadioButton by double-clicking in the right column of the Text property and typing Pine for the first RadioButton, Oak for the second RadioButton and Cherry for the third RadioButton

Chapter 5: Decision Structures8Adding the RadioButton Objects

Chapter 5: Decision Structures9Windows Application Container Objects

Chapter 5: Decision Structures10Displaying a Message Box

Displaying a Message BoxChapter 5: Decision Structures14

Chapter 5: Decision Structures15Message Box IntelliSenseIn the code editing window, inside the event handler you are coding, type msgto display MsgBox in the IntelliSense listPress the Tab key to select MsgBox from the IntelliSense list. Type the following text: (“You have been disconnected from the Internet”, m)Select the MsgBoxStyle.AbortRetryIgnore argument by pressing the UP ARROW until the correct argument is highlighted.Type a comma.Then type "ISP” and a right parenthesis Click the Start Debugging button on the Standard toolbar

Chapter 5: Decision Structures17Making Decisions with Conditional Statements: Using an If…Then StatementA decision structure is one of the three fundamental control structures used in computer programmingWhen a condition is tested in a Visual Basic program, the condition either is true or false

Chapter 5: Decision Structures18Relational Operators

Chapter 5: Decision Structures19Relational OperatorsWith the insertion point located in the correct location in the code, type ifand then press the SPACEBARType inta to select the variable named intAge in the IntelliSense list. Then, type >=18as the condition to be tested. Press the ENTER keyOn the blank line, enter the statement that should be executed when the condition is true. To place the message, “You are old enough to vote” in the Text property of the lblVotingEligibility Label object, insert the code shown in Figure 5-33 on page 315. Remember to use IntelliSense to reference the lblVotingEligibility Label object

Chapter 5: Decision Structures20Comparing StringsA string value comparison compares each character in two strings, starting with the first character in each string

Chapter 5: Decision Structures21Comparing Different Data TypesEvery type of data available in Visual Basic can be comparedDifferent numeric types can be compared to each otherA single string character can be compared to a Char data type

Using the If…Then…Else StatementChapter 5: Decision Structures22

Chapter 5: Decision Structures23Using the If…Then…ElseIf Statement

Nested If StatementsChapter 5: Decision Structures24

Nested If StatementsChapter 5: Decision Structures25

Chapter 5: Decision Structures26Matching If, Else, and End If EntriesIf statements must be fully contained within the outer If statementPlace the correct statements with the correct If and Else statements within the nested If statementThis illustration shows incorrect logic

Testing the Status of a RadioButton Object in CodeChapter 5: Decision Structures27

Chapter 5: Decision Structures28Block-Level ScopeScope is defined by where the variable is declared within a programWithin an event handler, an If…Then…Else statement is considered a block of codeVariables can be declared within a block of codeThe variable can be referenced only within the block of code where it is declared

Chapter 5: Decision Structures29Using Logical OperatorsWhen more than one condition is included in an If...Then...Else statement, the conditions are called a compound condition

Using the And Logical OperatorChapter 5: Decision Structures30

Using the Or Logical OperatorChapter 5: Decision Structures31

Using the Not Logical OperatorChapter 5: Decision Structures32

Chapter 5: Decision Structures33Other Logical Operators

Chapter 5: Decision Structures34Order of Operations for Logical Operators

Chapter 5: Decision Structures35Select Case StatementIn some programming applications, different operations can occur based upon the value in a single field

Select Case StatementChapter 5: Decision Structures36

Chapter 5: Decision Structures37Select Case Test Expressions

Chapter 5: Decision Structures38Using Relational Operators in a Select Case Statement

Using Ranges in Select Case StatementsChapter 5: Decision Structures39

Chapter 5: Decision Structures40Selecting Which Decision Structure to UseYou might be faced with determining if you should use the Select Case statement or the If...Then...ElseIf statement to solve a problemGenerally, the Select Case statement is most useful when more than two or three values must be tested for a given variableThe If...Then...ElseIf statement is more flexibleMore than one variable can be used in the comparisonCompound conditions with the And, Or, and Not logical operators can be used

Chapter 5: Decision Structures41Code SnippetsRight-click the line in the code editing window where you want to insert the snippetClick Insert Snippet on the shortcut menuDouble-click the folder Code Patterns - If, For Each,Try Catch, Property, etc, which contains commonly used code such as the If . . . Then . . . Else statement Double-click the Conditionals and Loops folder because an If...Then...Else statement is a conditional statementDouble-click the If...Else...End If Statement code snippet

Chapter 5: Decision Structures42Code Snippets

Chapter 5: Decision Structures43Validating DataDevelopers should anticipate that users will enter invalid dataDevelopers must write code that will prevent the invalid data from being used in the program to produce invalid output

Chapter 5: Decision Structures44Testing Input to Determine If the Value Is NumericThe Visual Basic IsNumeric function can check the input value to determine if the value can be converted into a numeric value such as an Integer or Decimal data type

Chapter 5: Decision Structures45Checking for a Positive Number

Program DesignChapter 5: Decision Structures46

Chapter 5: Decision Structures49Chapter SummaryUse the GroupBox objectPlace RadioButton objects in applicationsDisplay a message boxMake decisions using If…Then statementsMake decisions using If…Then…Else statementsMake decisions using nested If statements

Chapter 5: Decision Structures50Chapter SummaryMake decisions using logical operatorsMake decisions using Case statementsInsert code snippetsTest input to ensure a value is numeric

CHAPTER FIVE COMPLETEDecision Structures

Chapter 05

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