InputBox
Welcome message from author
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
InputBox
Format of the InputBox Function
• Prompt - message to the user
• Title - text for the box's title bar
• Default - default text for user's input
• Xpos - X coordinate for the box's position
• Ypos - Y coordinate for the box's position
• Title and beyond are optional arguments
InputBox(Prompt [,Title] [,Default] [,Xpos] [,Ypos])
Sample InputBox Usage
• userInput = InputBox("Enter the distance.", "Provide a Value", "150")
Xpos, Ypos, and Twips
• Xpos specifies the distance from the left of the screen to the left side of the box
• Ypos, from the top of the screen to the top of the box
• Both are specified in twips
• One twip is 1/440 inch
The Decision Structure
The Decision Structure Allows a Program to Have More Than One
Path of Execution
Order of Statement Execution
• Thus far, all of our code statements have been executed sequentially
• To write meaningful programs our code needs to be able to• Execute code conditionally (this chapter)• Repeat sections of code (next chapter)
The Decision Structure
• Flowchart of atypical decisionstructure
• Evaluate thecondition
• Execute, or notsome code
Condition
ConditionalCode
True
False
The If…Then Statement
The If…Then Statement Causes Other Statements to Execute Only
Under a Certain Condition
If…Then Statement Syntax
• New keywords used above:• If• Then• End
If condition Thenstatement(more statements as needed)
End If
Relational Operators
• Usually the condition is formed with a relational operator• > Greater than• < Less than• = Equal to• <> Not equal to• >= Greater than or equal to• <= Less than or equal to
Binary Operators
• Operators are classified as to how many operands each takes
• Relational operators are binary operators --each has two operands, e.g.• length > width• size <= 10
• Relational operators yield a True or False result
If…Then Examples
If sales > 50000 ThengetsBonus = True
End If
If sales > 50000 ThengetsBonus = TruecommissionRate = 0.12daysOff = daysOff + 1
End If
If…Then Rules
• The 'If' and the 'Then' must be on the same line
• Only a remark may follow the 'Then'
• The 'End If' must be on a separate line
• Only a remark may follow the 'End If'
If…Then Conventions
• The code between the 'If…Then' and the 'End If' is indented
• Visual Basic .NET does not require this• It is a convention among programmers to
aid in the readability of programs• By default, the Visual Basic .NET editor
will automatically do this indentation as you enter your program
Relational Operators with Math Operators
• Either or both of the relational operator operands may themselves be expressions
• Math operators are done before the relational operators
If x + y > a - b ThenlblMessage.Text = "It is true!"
End If
Relational Operators with Function Calls
• Either or both of the relational operator operands may themselves be function calls
If Val(txtInput.Text) < 100 ThenlblMessage.Text = "It is true!"
End If
Boolean Variables as Flags
• A flag is a Boolean variable that signals when some condition exists in the program
• They can be used as the condition
If quotaMet ThenlblMessage.Text = "You have met your sales quota"
End If
What an 'If…Then' Really Tests For
• Visual Basic .NET first evaluates the conditional expression
• If the result is 0, the condition is regarded as being False
• If the result is anything else, the condition is regarded as being True
The If…Then…Else Statement
The If...Then...Else Statement Executes One Group of Statements If the Condition Is
True and Another Group of Statements If the Condition Is False
'If…Then' vs. 'If…Then…Else'
• The 'If…Then' construct will execute or not a group of statements (do something or do nothing)
• The 'If…Then…Else' construct will execute one group of statements or another group (do this or do that)
The 'If…Then…Else' Structure
Condition
ExecuteIf True
TrueFalse
ExecuteIf False
'If…Then…Else' Example
If temperature < 40 ThenlblMesage.Text = “A little cold, isn’t it?”
ElselblMesage.Text = “Nice weather we’re having!”
End If
4.5The If…Then…ElseIf Statement
The If...Then…Elseif Statement Is Like a Chain of If...Then...Else Statements
They Perform Their Tests, One After the Other, Until One of Them Is Found to Be True
Two Mutually Exclusive Choices('If…Then…Else')
• The 'If…Then…Else' statement has two choices
• The conditional statement will either be True or False
• Hence, exactly one of the two choices will be selected
• They are mutually exclusive
More Than TwoMutually Exclusive Choices
• The 'If…Then…ElseIf' statement provides a
series of mutually exclusive choices
• In pseudo code:If it is very cold Then
Wear a coatElseif it is chilly
Wear a light jacketElseif it is windy
Wear a windbreakerElesif it is hot
Wear no jacket
In Visual Basic .NET Syntax
If condition1 ThenStatement(s)1
Elseif condition2 ThenStatements(s)2
Elseif condition3 ThenStatements3
…End If
In Flowchart Form
C1
C2
C3
Statement(s)1
True
Statement(s)2
True
Statement(s)3
True
False
False
False
Example of Usage
If average < 60 ThenlblGrade.Text = "F"
ElseIf average < 70 ThenlblGrade.Text = "D"
ElseIf average < 80 ThenlblGrade.Text = "C"
ElseIf average < 90 ThenlblGrade.Text = "B"
ElseIf average <= 100 ThenlblGrade.Text = "A"
End If
Contrast the Preceding CodeTo This Code Without the Elses
If average < 60 ThenlblGrade.Text = "F"
End IfIf average < 70 Then
lblGrade.Text = "D"End IfIf average < 80 Then
lblGrade.Text = "C"End IfIf average < 90 Then
lblGrade.Text = "B"End IfIf average <= 100 Then
lblGrade.Text = "A"End If
The (Optional) Trailing Else
• A sequence of ElseIfs may end with a plain else (called a trailing Else)
• If none of the conditions were True, the statement(s) after this Else will be executed
• Continuing with the preceding example on the next slide:
Use of a Trailing Else
If average < 60 ThenlblGrade.Text = "F"
ElseIf average < 70 ThenlblGrade.Text = "D"
ElseIf average < 80 ThenlblGrade.Text = "C"
ElseIf average < 90 ThenlblGrade.Text = "B"
ElseIf average <= 100 ThenlblGrade.Text = "A"
ElselblGrade.Text = "Invalid"
End If
Nested If Statements
A Nested If Statement Is an If Statement in the Conditionally Executed Code of Another If
Statement
'If' Statements Within 'If' Statements
• Inside of any of the styles of 'If' statements what we have seen, the statement(s) portion can be any combination of statements
• This includes other 'If' statements
• Hence more complex decision structures can be created (by nesting 'Ifs' within 'Ifs')
Consider This Code
If Val(txtSalary.Text) > 30000 ThenIf Val(txtYearsOnJob.Text) > 2 Then
lblMessage.Text = "The applicant qualifies."Else
lblMessage.Text = "The applicant does not qualify."End If
ElseIf Val(txtYearsOnJob.Text) > 5 Then
lblMessage.Text = "The applicant qualifies."Else
lblMessage.Text = "The applicant does not qualify."End If
End If
Note how the convention of indentationsemphasizes the structure of nested Ifs.
Flowchart Version
Val(txtSalary.Text) > 30000
Val(txtSalary.Text) > 30000
Val(txtSalary.Text) > 30000
lblMessage.Text = "The applicant
qualifies."
lblMessage.Text = "The applicantdoes
not qualify."
lblMessage.Text = "The applicant
qualifies."
lblMessage.Text = "The applicantdoes
not qualify."
False
False False
True
TrueTrue
Logical Operators
Logical Operators Connect Two or More Relational Expressions Into One, or Reverse the Logic of an
Expression
Visual Basic .NET Logical Operators
Operator EffectAnd Both operands must be true for the overall
expression to be true, otherwise it is falseOr One or both operands must be true for the overall
expression to be true, otherwise it is falseNot Reverses the logical value of an expression
The 'And' Operator
Truth Table for 'And' OperatorExpression 1 Expression 2 Expression 1 And Expression 2
True False FalseFalse True FalseFalse False FalseTrue True True
If temperature < 20 And minutes > 12 ThenlblMessage.Text = "The temperature is in the danger zone."
End If
The 'Or' Operator
Truth Table for 'Or' OperatorExpression 1 Expression 2 Expression 1 Or Expression 2
True False TrueFalse True TrueFalse False FalseTrue True True
If temperature < 20 Or temperature > 100 ThenlblMessage.Text = "The temperature is in the danger zone."
End If
The 'Not' Operator
Truth Table for 'Not' OperatorExpression 1 Not Expression 1
True FalseFalse True
If Not temperature > 100 ThenlblMessage.Text = "You are below the maximum temperature."
End If
Checking Numerical Ranges
• Checking for a value inside of a range:
• Checking for a value outside of a range:
If x >= 20 And x <= 40 ThenlblMessage.Text = "The value is in the acceptable range."
End If
If x < 20 Or x > 40 ThenlblMessage.Text = "The value is outside the acceptable range."
End If
The Message Box
Sometimes You Need a Convenient Way to Display a Message to the User
This Section Introduces the Messagebox.Show Method, Which Allows You to Display a
Message in a Dialog Box
Message Box Arguments
• Message - text to display within the box
• Caption - title for the top bar of the box
• Buttons - indicates which buttons to display
• Icon - indicates icon to display
• DefaultButton - indicates which button corresponds to the Return Key
• Arguments are optional bottom to top
Buttons Argument
Value - DescriptionMessageBoxButtons.AbortRetryIgnore -
Displays Abort, Retry, and Ignore buttons.MessageBoxButtons.OK - Displays only an OK button.MessageBoxButtons.OKCancel - Displays OK and Cancel buttons.MessageBoxButtons.RetryCancel -
Display Retry and Cancel buttons.MessageBoxButtons.YesNo- -Displays Yes and No buttons.MessageBoxButtons.YesNoCancel -
Displays Yes, No, and Cancel buttons.
Example Message Box
MessageBox.Show("Do you wish to continue?", _"Please Confirm", _
MessageBoxButtons.YesNo)
The Select Case Statement
In a Select Case Statement, One of Several Possible Actions Is Taken, Depending on the
Value of an Expression
Select Case Statement Example, ISelect Case Val(txtInput.Text)
Case 1MessageBox.Show("Day 1 is Monday.")
Case 2MessageBox.Show("Day 2 is Tuesday.")
Case 3MessageBox.Show("Day 3 is Wednesday.")
Case 4MessageBox.Show("Day 4 is Thursday.")
Case 5MessageBox.Show("Day 5 is Friday.")
Case 6MessageBox.Show("Day 6 is Saturday.")
Case 7MessageBox.Show("Day 7 is Sunday.")
Case ElseMessageBox.Show("That value is invalid.")
End Select
Select Case Statement Example, II
Select Case animalCase "Dogs", "Cats"
MessageBox.Show ("House Pets")Case "Cows", "Pigs", "Goats"
MessageBox.Show ("Farm Animals")Case "Lions", "Tigers", "Bears"
MessageBox.Show ("Oh My!")End Select
Input Validation
Input Validation Is the Process of Inspecting Input Values and Determining
Whether They Are Valid
Validation Example
' Validate the input to ensure that' no negative numbers were entered.
If sales < 0 Or advance < 0 ThenMessageBox.Show("Please enter positive numbers for " & _" sales and/or advance pay.")
EndIf
4.12Radio Buttons and
Check Boxes
Radio Buttons Appear in Groups of Two or More, and Allow the User to Select One of Several Possible Options
Check Boxes, Which May Appear Alone or in Groups, Allow the User to Make Yes/no or On/off Selections
Checking Radio Buttons in Code
If radChoice1.Checked = True ThenMessageBox.Show("You selected Choice 1")
ElseIf radChoice2.Checked = True ThenMessageBox.Show("You selected Choice 2")
ElseIf radChoice3.Checked = True ThenMessageBox.Show("You selected Choice 3")
End If
Checking Check Boxes in Code
If chkChoice4.Checked = True Thenmessage = message & " and Choice 4"
End If
Class-level Variables
Class-level Variables Are Not Local to Any Procedure
In a Form File They Are Declared Outside of Any Procedure, and May Be Accessed by Statements in
Any Procedure in the Same Form
Advantages of Class-level Variables
• The scope of class-level variables include all of the procedures below the declaration in the file
• Hence, with the use of class-level variables, communication of information between modules is very easy
Disadvantages of Class-level Variables
• Class-level variables should be used sparingly - only when really needed
• Why?
• In larger programs, the uses of these variables will be more difficult to keep track of and hence tend to introduce bugs into the procedures
Related Documents
