Chapter 4Chapter 4Decisions and Decisions and

ConditionsConditions

Programming InProgramming In

Visual Basic .NETVisual Basic .NET

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If StatementsIf Statements

• Used to make decisionsUsed to make decisions• If true, only the Then clause is executed, if false, only Else If true, only the Then clause is executed, if false, only Else

clause, if present, is executedclause, if present, is executed• Block If…Then…Else must always conclude with End IfBlock If…Then…Else must always conclude with End If• Then must be on same line as If or ElseIfThen must be on same line as If or ElseIf• End If and Else must appear alone on a lineEnd If and Else must appear alone on a line• Note: ElseIf is 1 word, End If is 2 wordsNote: ElseIf is 1 word, End If is 2 words

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If…Then…Else – General FormIf…Then…Else – General Form

If (condition) Thenstatement(s)

[ElseIf (condition) Thenstatement(s)]

[Elsestatement(s)]

End If

ConditionCondition TrueTrue

FalseFalse

StatementStatement StatementStatement

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If…Then…Else - ExampleIf…Then…Else - Example

unitsDecimal = Decimal.Parse(unitsTextBox.Text)unitsDecimal = Decimal.Parse(unitsTextBox.Text)If unitsDecimal < 32D ThenIf unitsDecimal < 32D Then

freshmanRadioButton.Checked = TruefreshmanRadioButton.Checked = TrueElseElse

freshmanRadioButton.Checked = FalsefreshmanRadioButton.Checked = FalseEnd IfEnd If

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ConditionsConditions

• Test in an If statement is based on a conditionTest in an If statement is based on a condition• Six relational operators are used for comparisonSix relational operators are used for comparison• Negative numbers are less than positive numbersNegative numbers are less than positive numbers• An equal sign is used to test for equalityAn equal sign is used to test for equality• Strings can be compared, enclose strings in quotes (see Strings can be compared, enclose strings in quotes (see

Page 142 for ANSI Chart, case matters)Page 142 for ANSI Chart, case matters)– JOAN is less than JOHNJOAN is less than JOHN– HOPE is less than HOPELESSHOPE is less than HOPELESS

• Numbers are always less than lettersNumbers are always less than letters– 300ZX is less than Porsche300ZX is less than Porsche

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The Six Relational Operators The Six Relational Operators

• Greater ThanGreater Than >>

• Less ThanLess Than <<

• Equal ToEqual To ==

• Not Equal ToNot Equal To <><>

• Greater Than or Equal ToGreater Than or Equal To >=>=

• Less Than or Equal toLess Than or Equal to <=<=

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ToUpper and ToLower MethodsToUpper and ToLower Methods

• Use ToUpper and ToLower methods of the String class to Use ToUpper and ToLower methods of the String class to return the uppercase or lowercase equivalent of a string, return the uppercase or lowercase equivalent of a string, respectivelyrespectively

If nameTextBox.Text.ToUpper( ) = "Basic" Then ' Do something.

End If

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Compound ConditionsCompound Conditions

• Join conditions using logical operatorsJoin conditions using logical operators

– OrOr If one or both conditions True, If one or both conditions True, entire condition is Trueentire condition is True

– AndAnd Both conditions must be True Both conditions must be True for entire condition to be Truefor entire condition to be True

– NotNot Reverses the condition, a Reverses the condition, a True condition will evaluate False True condition will evaluate False

and vice versaand vice versa

OR T F

T

F

T T

T F

Condition 1

Con

diti

on 2

AND T F

T

F

T F

F F

Condition 1

Con

diti

on 2

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Compound Condition ExamplesCompound Condition Examples

If maleRadioButton.Checked And _ Integer.Parse(ageTextBox.Text) < 21 Then

minorMaleCountInteger += 1End If

If juniorRadioButton.Checked Or seniorRadioButton.Checked ThenupperClassmanInteger += 1

End If

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Combining And and Or ExampleCombining And and Or Example

If saleDecimal > 1000.0 Or discountRadioButton.Checked _ And stateTextBox.Text.ToUpper( ) <> "CA" Then

' Code here to calculate the discount.End If

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If tempInteger > 32 ThenIf tempInteger > 80 Then

commentLabel.Text = "Hot"Else

commentLabel.Text = "Moderate"End If

ElsecommentLabel.Text = "Freezing"

End If

Nested If StatementsNested If Statements

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Using If Statements with Radio Buttons Using If Statements with Radio Buttons & Check Boxes& Check Boxes

• Instead of coding the CheckedChanged events, use If Instead of coding the CheckedChanged events, use If statements to see which are selectedstatements to see which are selected

• Place the If statement in the Click event for a Button, such Place the If statement in the Click event for a Button, such as an OK or Apply buttonas an OK or Apply button

Private Sub testButton_Click(ByVal sender As System.Object, _ By Val e As System.EventArgs) Handles testButton.Click ' Test the value of the check box.

If testCheckBox.Checked ThenmessageLabel.Text = "Check box is checked"

End IfEnd Sub

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Enhancing Message BoxesEnhancing Message Boxes

• For longer, more complex messages, store the message For longer, more complex messages, store the message text in a String variable and use that variable as an text in a String variable and use that variable as an argument of the Show methodargument of the Show method

• VB will wrap longer messages to a second lineVB will wrap longer messages to a second line

• Include ControlChars to control the line length and Include ControlChars to control the line length and position of the line breakposition of the line break

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ControlChars ConstantsControlChars Constants(p 152)(p 152)

Constant DescriptionCR Carriage ReturnCRLF Carriage Return + Line FeedNewLine Carriage Return + Line FeedTab Tab CharacterNullChar Character with a Value of ZeroQuote Quotation Mark Character

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Message Box - Multiple Lines of OutputMessage Box - Multiple Lines of Output

ControlChars.NewLineUsed to force to next line

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Message String ExampleMessage String Example

Dim formattedTotalString As StringDim formattedAvgString As StringDim messageString As String

formattedTotalString = totalSalesDecimal.ToString("N")formattedAvgString = averageSalesDecimal.ToString("N")messageString = "Total Sales: " & formattedTotalString _ & ControlChars.NewLine & "Average Sale: " & _ formattedAvgStringMessageBox.Show(messageString, "Sales Summary", _ MessageBoxButtons.OK)

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Displaying Multiple ButtonsDisplaying Multiple Buttons

• Use MessageBoxButtons constants to display more than Use MessageBoxButtons constants to display more than one button in the Message Boxone button in the Message Box

• Message Box's Show method returns a Message Box's Show method returns a DialogResultDialogResult object that can be checked to see which button the user object that can be checked to see which button the user clickedclicked

• Declare a variable to hold an instance of the Declare a variable to hold an instance of the DialogResultDialogResult type to capture the outcome of the Show methodtype to capture the outcome of the Show method

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Message Box - Multiple ButtonsMessage Box - Multiple Buttons

MessageBoxButtons.YesNo

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Declaring a Variable for the Method Declaring a Variable for the Method ReturnReturn

Dim whichButtonDialogResult As DialogResult

whichButtonDialogResult = MessageBox.Show _("Clear the current order figures?", "Clear Order", _

MessageBoxButtons.YesNo, MessageBoxIcon.Question)If whichButtonDialogResult = DialogResult.Yes Then

' Code to clear the order.End If

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Specifying a Default Button and Specifying a Default Button and OptionsOptions

• Use a different signature for the Message Box Show Use a different signature for the Message Box Show method to specify a default buttonmethod to specify a default button

• Add the MessageBoxDefaultButton argument after the Add the MessageBoxDefaultButton argument after the MessageBoxIcons argumentMessageBoxIcons argument

• Set message alignment with MessageBoxOptions Set message alignment with MessageBoxOptions argumentargument

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Input ValidationInput Validation

• Check to see if valid values were entered by user before Check to see if valid values were entered by user before beginning calculationsbeginning calculations

• Check for a range of values (reasonableness)Check for a range of values (reasonableness)

– If Integer.Parse(hoursTextBox.Text) > 10 ThenIf Integer.Parse(hoursTextBox.Text) > 10 Then

MessageBox.Show("Too many hours")MessageBox.Show("Too many hours")

• Check for a required field (not blank)Check for a required field (not blank)

– If nameTextBox.Text <> "" Then ...If nameTextBox.Text <> "" Then ...

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Performing Multiple ValidationsPerforming Multiple Validations

• Use nested If statement to validate multiple values on a Use nested If statement to validate multiple values on a formform

– Examine example on Page 156Examine example on Page 156

• Use Case structure to validate multiple valuesUse Case structure to validate multiple values

– Simpler and clearer than nested IfSimpler and clearer than nested If

– No limit to number of statements that follow a Case No limit to number of statements that follow a Case statementstatement

– When using a relational operator must use the word When using a relational operator must use the word IsIs

– Use the word Use the word ToTo to indicate a range of constants to indicate a range of constants

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Sharing an Event ProcedureSharing an Event Procedure

• Add events to the Handles clause at the top of an event Add events to the Handles clause at the top of an event procedureprocedure

– Allows the procedure to respond to events of other Allows the procedure to respond to events of other controlscontrols

• Key to using a shared event procedure is the Key to using a shared event procedure is the sendersender argumentargument

– Cast (convert) Cast (convert) sendersender to a specific object type using the to a specific object type using the CType functionCType function

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Calling Event ProceduresCalling Event Procedures

• Reusable codeReusable code• General FormGeneral Form

– [Call] ProcedureName ( )[Call] ProcedureName ( )– Keyword Call is optional and rarely usedKeyword Call is optional and rarely used

• ExamplesExamples– Call clearButton_Click (sender, e)Call clearButton_Click (sender, e)OROR– clearButton_Click (sender, e)clearButton_Click (sender, e)

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Debugging (p 169)Debugging (p 169)

• Debug MenuDebug Menu

• Debug ToolbarDebug Toolbar

• Toggle BreakPoints on/off by clicking Editor's gray left Toggle BreakPoints on/off by clicking Editor's gray left margin indicatormargin indicator

• Step through Code, Step Into, Step OverStep through Code, Step Into, Step Over

• View the values of properties, variables, mathematical View the values of properties, variables, mathematical expressions, and conditionsexpressions, and conditions

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Debugging Debugging (cont.)(cont.)

• Output WindowOutput Window

• Locals WindowLocals Window

• Autos WindowAutos Window

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Debug Menu and ToolbarDebug Menu and Toolbar

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Writing to the Output WindowWriting to the Output Window

• Debug.WriteLine(Debug.WriteLine(TextStringTextString))

• Debug.WriteLine(Debug.WriteLine(ObjectObject))

Debug.WriteLine("calculateButton procedure entered")Debug.WriteLine(quantityTextBox)

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BreakpointsBreakpoints

Toggle Breakpoints On/Off by clicking Toggle Breakpoints On/Off by clicking in Editor's gray left margin indicatorin Editor's gray left margin indicator

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Checking the Current Value of Checking the Current Value of ExpressionsExpressions

Place mouse pointer over variable or property to view current value

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Locals WindowLocals Window

Shows values of local variables that are within scope of current statement

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Autos WindowAutos Window

Automatically adjusts to show variables and properties that appear in previous and next few lines

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