Transcript Chapter 4 Decisions and Conditions Programming In Visual Basic .NET If Statements • Used to make decisions • If true, only the Then clause is executed,

Chapter 4
Decisions and
Conditions
Programming In
Visual Basic .NET
If Statements
• Used to make decisions
• If true, only the Then clause is executed, if false, only Else
clause, if present, is executed
• Block If…Then…Else must always conclude with End If
• Then must be on same line as If or ElseIf
• End If and Else must appear alone on a line
• Note: ElseIf is 1 word, End If is 2 words
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If…Then…Else – General Form
If (condition) Then
statement(s)
[ElseIf (condition) Then
statement(s)]
[Else
statement(s)]
End If
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Condition True
False
Statement
Statement
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If…Then…Else - Example
unitsDecimal = Decimal.Parse(unitsTextBox.Text)
If unitsDecimal < 32D Then
freshmanRadioButton.Checked = True
Else
freshmanRadioButton.Checked = False
End If
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Conditions
•
•
•
•
•
Test in an If statement is based on a condition
Six relational operators are used for comparison
Negative numbers are less than positive numbers
An equal sign is used to test for equality
Strings can be compared, enclose strings in quotes (see
Page 142 for ANSI Chart, case matters)
– JOAN is less than JOHN
– HOPE is less than HOPELESS
• Numbers are always less than letters
– 300ZX is less than Porsche
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The Six Relational Operators
•
•
•
•
•
•
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Greater Than
Less Than
Equal To
Not Equal To
Greater Than or Equal To
Less Than or Equal to
>
<
=
<>
>=
<=
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ToUpper and ToLower Methods
• Use ToUpper and ToLower methods of the String class to
return the uppercase or lowercase equivalent of a string,
respectively
If nameTextBox.Text.ToUpper( ) = "Basic" Then
' Do something.
End If
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Compound Conditions
Condition 1
Condition 2
Condition 2
• Join conditions using logical operators
OR T
F
– Or
If one or both conditions True,
T
T
T
entire condition is True
– And Both conditions must be True
F
T
F
for entire condition to be True
– Not
Reverses the condition, a
Condition 1
True condition will evaluate False
AND T
F
and vice versa
T
T
F
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F
F
F
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Compound Condition Examples
If maleRadioButton.Checked And _
Integer.Parse(ageTextBox.Text) < 21 Then
minorMaleCountInteger += 1
End If
If juniorRadioButton.Checked Or seniorRadioButton.Checked Then
upperClassmanInteger += 1
End If
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Combining 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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Nested If Statements
If tempInteger > 32 Then
If tempInteger > 80 Then
commentLabel.Text = "Hot"
Else
commentLabel.Text = "Moderate"
End If
Else
commentLabel.Text = "Freezing"
End If
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Using If Statements with Radio Buttons
& Check Boxes
• Instead of coding the CheckedChanged events, use If
statements to see which are selected
• Place the If statement in the Click event for a Button, such
as 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 Then
messageLabel.Text = "Check box is checked"
End If
End Sub
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Enhancing Message Boxes
• For longer, more complex messages, store the message text
in a String variable and use that variable as an argument of
the Show method
• VB will wrap longer messages to a second line
• Include ControlChars to control the line length and
position of the line break
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ControlChars Constants
(p 152)
Constant
CR
CRLF
NewLine
Tab
NullChar
Quote
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Description
Carriage Return
Carriage Return + Line Feed
Carriage Return + Line Feed
Tab Character
Character with a Value of Zero
Quotation Mark Character
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Message Box - Multiple Lines of Output
ControlChars.NewLine
Used to force to next line
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Message 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 Buttons
• Use MessageBoxButtons constants to display more than
one button in the Message Box
• Message Box's Show method returns a DialogResult
object that can be checked to see which button the user
clicked
• Declare a variable to hold an instance of the DialogResult
type to capture the outcome of the Show method
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Message Box - Multiple Buttons
MessageBoxButtons.YesNo
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Declaring a Variable for the Method
Return
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
Options
• Use a different signature for the Message Box Show
method to specify a default button
• Add the MessageBoxDefaultButton argument after the
MessageBoxIcons argument
• Set message alignment with MessageBoxOptions argument
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Input Validation
• Check to see if valid values were entered by user before
beginning calculations
• Check for a range of values (reasonableness)
– If Integer.Parse(hoursTextBox.Text) > 10 Then
MessageBox.Show("Too many hours")
• Check for a required field (not blank)
– If nameTextBox.Text <> "" Then ...
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Performing Multiple Validations
• Use nested If statement to validate multiple values on a
form
– Examine example on Page 156
• Use Case structure to validate multiple values
– Simpler and clearer than nested If
– No limit to number of statements that follow a Case
statement
– When using a relational operator must use the word Is
– Use the word To to indicate a range of constants
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Sharing an Event Procedure
• Add events to the Handles clause at the top of an event
procedure
– Allows the procedure to respond to events of other
controls
• Key to using a shared event procedure is the sender
argument
– Cast (convert) sender to a specific object type using the
CType function
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Calling Event Procedures
• Reusable code
• General Form
– [Call] ProcedureName ( )
– Keyword Call is optional and rarely used
• Examples
– Call clearButton_Click (sender, e)
OR
– clearButton_Click (sender, e)
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Debugging (p 169)
• Debug Menu
• Debug Toolbar
• Toggle BreakPoints on/off by clicking Editor's gray left
margin indicator
• Step through Code, Step Into, Step Over
• View the values of properties, variables, mathematical
expressions, and conditions
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Debugging (cont.)
• Output Window
• Locals Window
• Autos Window
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Debug Menu and Toolbar
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Writing to the Output Window
• Debug.WriteLine(TextString)
• Debug.WriteLine(Object)
Debug.WriteLine("calculateButton procedure entered")
Debug.WriteLine(quantityTextBox)
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Breakpoints
Toggle Breakpoints On/Off by clicking
in Editor's gray left margin indicator
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Checking the Current Value of
Expressions
Place mouse pointer over variable or
property to view current value
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Locals Window
Shows values of local variables that are
within scope of current statement
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Autos Window
Automatically adjusts to show variables
and properties that appear in previous
and next few lines
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