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 a typical decision structure Evaluate the condition Execute, or not some code Condition Conditional Code 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 Then statement (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 > Then getsBonus = True End If If sales > Then getsBonus = True commissionRate = 0.12 daysOff = 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 Then lblMessage.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 Then lblMessage.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 Then lblMessage.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 Execute If True True False Execute If False

'If…Then…Else' Example If temperature < 40 Then lblMesage.Text = “A little cold, isn’t it?” Else lblMesage.Text = “Nice weather we’re having!” End If

4.5 The 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 Two Mutually 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 coat Elseif it is chilly Wear a light jacket Elseif it is windy Wear a windbreaker Elesif it is hot Wear no jacket

In Visual Basic.NET Syntax If condition 1 Then Statement(s) 1 Elseif condition 2 Then Statements(s) 2 Elseif condition 3 Then Statements 3 … End If

In Flowchart Form C1C1 C2C2 C3C3 Statement(s) 1 True Statement(s) 2 True Statement(s) 3 True False

Example of Usage If average < 60 Then lblGrade.Text = "F" ElseIf average < 70 Then lblGrade.Text = "D" ElseIf average < 80 Then lblGrade.Text = "C" ElseIf average < 90 Then lblGrade.Text = "B" ElseIf average <= 100 Then lblGrade.Text = "A" End If

Contrast the Preceding Code To This Code Without the Elses If average < 60 Then lblGrade.Text = "F" End If If average < 70 Then lblGrade.Text = "D" End If If average < 80 Then lblGrade.Text = "C" End If If average < 90 Then lblGrade.Text = "B" End If If 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 Then lblGrade.Text = "F" ElseIf average < 70 Then lblGrade.Text = "D" ElseIf average < 80 Then lblGrade.Text = "C" ElseIf average < 90 Then lblGrade.Text = "B" ElseIf average <= 100 Then lblGrade.Text = "A" Else lblGrade.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) > Then If Val(txtYearsOnJob.Text) > 2 Then lblMessage.Text = "The applicant qualifies." Else lblMessage.Text = "The applicant does not qualify." End If Else If Val(txtYearsOnJob.Text) > 5 Then lblMessage.Text = "The applicant qualifies." Else lblMessage.Text = "The applicant does not qualify." End If Note how the convention of indentations emphasizes the structure of nested Ifs.

Flowchart Version Val(txtSalary.Text) > Val(txtSalary.Text) > Val(txtSalary.Text) > lblMessage.Text = "The applicant qualifies." lblMessage.Text = "The applicantdoes not qualify." lblMessage.Text = "The applicant qualifies." lblMessage.Text = "The applicantdoes not qualify." False True

Logical Operators Logical Operators Connect Two or More Relational Expressions Into One, or Reverse the Logic of an Expression

Visual Basic.NET Logical Operators OperatorEffect AndBoth operands must be true for the overall expression to be true, otherwise it is false Or One or both operands must be true for the overall expression to be true, otherwise it is false Not Reverses the logical value of an expression

The 'And' Operator Truth Table for 'And' Operator Expression 1 Expression 2 Expression 1 And Expression 2 TrueFalseFalse FalseTrueFalse FalseFalseFalse TrueTrueTrue If temperature 12 Then lblMessage.Text = "The temperature is in the danger zone." End If

The 'Or' Operator Truth Table for 'Or' Operator Expression 1 Expression 2 Expression 1 Or Expression 2 TrueFalseTrue FalseTrueTrue FalseFalseFalse TrueTrueTrue If temperature 100 Then lblMessage.Text = "The temperature is in the danger zone." End If

The 'Not' Operator Truth Table for 'Not' Operator Expression 1 Not Expression 1 TrueFalse FalseTrue If Not temperature > 100 Then lblMessage.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 Then lblMessage.Text = "The value is in the acceptable range." End If If x 40 Then lblMessage.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 - Description MessageBoxButtons.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, I Select Case Val(txtInput.Text) Case 1 MessageBox.Show("Day 1 is Monday.") Case 2 MessageBox.Show("Day 2 is Tuesday.") Case 3 MessageBox.Show("Day 3 is Wednesday.") Case 4 MessageBox.Show("Day 4 is Thursday.") Case 5 MessageBox.Show("Day 5 is Friday.") Case 6 MessageBox.Show("Day 6 is Saturday.") Case 7 MessageBox.Show("Day 7 is Sunday.") Case Else MessageBox.Show("That value is invalid.") End Select

Select Case Statement Example, II Select Case animal Case "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 Then MessageBox.Show("Please enter positive numbers for " & _ " sales and/or advance pay.") EndIf

4.12 Radio 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 Then MessageBox.Show("You selected Choice 1") ElseIf radChoice2.Checked = True Then MessageBox.Show("You selected Choice 2") ElseIf radChoice3.Checked = True Then MessageBox.Show("You selected Choice 3") End If

Checking Check Boxes in Code If chkChoice4.Checked = True Then message = 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