chapter 4 decisions and conditions

8/14/2019 Chapter 4 Decisions and Conditions

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Chapter 4Chapter 4

Decisions andDecisions and

ConditionsConditions

Programming InProgramming InVisual Basic .NETVisual Basic .NET


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2005 by The McGraw-Hill Companies, Inc. All rights reserved.4- 2

If StatementsIf Statements

Used to make decisionsUsed to make decisions

If true, only the Then clause is executed, if false, only ElseIf true, only the Then clause is executed, if false, only Elseclause, if present, is executedclause, if present, is executed

Block IfThenElse must always conclude with End IfBlock IfThenElse 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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IfThenElse General FormIfThenElse General Form

If(condition) Then

statement(s)

[ElseIf(condition) Then

statement(s)][Else

statement(s)]

End If

ConditionConditionTrueTrue

FalseFalse

StatementStatement StatementStatement


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IfThenElse - ExampleIfThenElse - Example

unitsDecimal = Decimal.Parse(unitsTextBox.Text)unitsDecimal = Decimal.Parse(unitsTextBox.Text)

If unitsDecimal < 32D ThenIf unitsDecimal < 32D Then

freshmanRadioButton.Checked = TruefreshmanRadioButton.Checked = True

ElseElse

freshmanRadioButton.Checked = FalsefreshmanRadioButton.Checked = False

End 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 (seeStrings can be compared, enclose strings in quotes (seePage 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 OperatorsThe Six Relational Operators

Greater ThanGreater Than >>

Less ThanLess Than =>=

Less Than or Equal toLess Than or Equal to


7/33 2005 by The McGraw-Hill Companies, Inc. All rights reserved.4- 7

ToUpper and ToLower MethodsToUpper and ToLower Methods

Use ToUpper and ToLower methods of the String class toUse 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

IfnameTextBox.Text.ToUpper( ) = "Basic" Then

' Do something.

EndIf



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 TrueBoth conditions must be True

for entire condition to be Truefor entire condition to be True

NotNot Reverses the condition, aReverses the condition, a

True condition will evaluate FalseTrue condition will evaluate False

and vice versaand vice versa

OR T F

T

F

T T

T F

Condition 1

Condi t

ion2

AND T F

T

F

T F

F F

Condition 1

C

ondi t

ion2



Compound Condition ExamplesCompound Condition Examples

IfmaleRadioButton.Checked And_

Integer.Parse(ageTextBox.Text) < 21 Then

minorMaleCountInteger += 1

End If

IfjuniorRadioButton.Checked Or seniorRadioButton.Checked Then

upperClassmanInteger += 1

End If



Combining And and Or ExampleCombining And and Or Example

IfsaleDecimal > 1000.0 Or discountRadioButton.Checked _

And stateTextBox.Text.ToUpper( ) "CA" Then

' Code here to calculate the discount.End If



IftempInteger > 32 Then

IftempInteger > 80 Then

commentLabel.Text = "Hot"

Else

commentLabel.Text = "Moderate"

EndIf

ElsecommentLabel.Text = "Freezing"

End If

Nested If StatementsNested If Statements



Using If Statements with Radio ButtonsUsing If Statements with Radio Buttons

& Check Boxes& Check Boxes

Instead of coding the CheckedChanged events, use IfInstead 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, suchPlace 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.IftestCheckBox.Checked Then

messageLabel.Text = "Check box is checked"

End If

End Sub



Enhancing Message BoxesEnhancing Message Boxes

For longer, more complex messages, store the messageFor longer, more complex messages, store the message

text in a String variable and use that variable as antext 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 andInclude 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 Description

CR Carriage Return

CRLF Carriage Return + Line Feed

NewLine Carriage Return + Line Feed

Tab Tab Character

NullChar 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 String

Dim formattedAvgString As String

Dim messageString As String

formattedTotalString = totalSalesDecimal.ToString("N")

formattedAvgString = averageSalesDecimal.ToString("N")

messageString = "Total Sales: " & formattedTotalString _

& ControlChars.NewLine & "Average Sale: " & _

formattedAvgString

MessageBox.Show(messageString, "Sales Summary", _

MessageBoxButtons.OK)


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Displaying Multiple ButtonsDisplaying Multiple Buttons

Use MessageBoxButtons constants to display more thanUse MessageBoxButtons constants to display more than

one button in the Message Boxone button in the Message Box

Message Box's Show method returns aMessage Box's Show method returns a DialogResultDialogResult

object that can be checked to see which button the userobject that can be checked to see which button the user

clickedclicked

Declare a variable to hold an instance of theDeclare 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 MethodDeclaring a Variable for the Method

ReturnReturn

Dim whichButtonDialogResult As DialogResult

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

MessageBoxButtons.YesNo, MessageBoxIcon.Question)

IfwhichButtonDialogResult = DialogResult.Yes Then

' Code to clear the order.End If


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Specifying a Default Button andSpecifying a Default Button and

OptionsOptions

Use a different signature for the Message Box ShowUse a different signature for the Message Box Show

method to specify a default buttonmethod to specify a default button

Add the MessageBoxDefaultButton argument after theAdd the MessageBoxDefaultButton argument after the

MessageBoxIcons argumentMessageBoxIcons argument Set message alignment with MessageBoxOptionsSet message alignment with MessageBoxOptions

argumentargument


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Input ValidationInput Validation

Check to see if valid values were entered by user beforeCheck 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 aUse 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 CaseNo limit to number of statements that follow a Case

statementstatement

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

Use the wordUse the word ToTo to indicate a range of constantsto 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 eventAdd events to the Handles clause at the top of an event

procedureprocedure

Allows the procedure to respond to events of otherAllows the procedure to respond to events of other

controlscontrols

Key to using a shared event procedure is theKey to using a shared event procedure is the sendersender

argumentargument

Cast (convert)Cast (convert)sendersenderto a specific object type using theto 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 leftToggle BreakPoints on/off by clicking Editor's gray leftmargin indicatormargin indicator

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

View the values of properties, variables, mathematicalView the values of properties, variables, mathematical

expressions, and conditionsexpressions, and conditions


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DebuggingDebugging (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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BreakpointsBreakpoints

Toggle Breakpoints On/Off by clickingToggle 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 ofChecking 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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