VB.Net Introduction - 2

Counter

• Example: Keep track the number of times a user clicks a button

• Need to declare a variable:Dim Counter As Integer

• Need to increase the counter by 1Counter = Counter + 1

Question: Where to declare this variable?

Variable Scope• Block-level scope: declared within a block of code

terminated by an end, loop or next statement.– If city = “Rome” then

• Dim message as string = “the city is in Italy”

• MessageBox.Show(message)

– End if

• Procedural-level scope: declared in a procedure• Class-level, module-level scope: declared in a

class or module but outside any procedure with either Dim or Private keyword.

• Project-level scope: a module variable declared with the Public keyword.

Sum• Example: Create a form with one textbox to enter

number, and a button to add the number to a variable Sum to compute the total of all numbers, and button to show the Sum:

Dim Sum As Double

Sum = Sum + textbox1.text

• Interface issue: Should we clear the textbox after adding the number to Sum?

TextBox1.Clear()

• Data conversion issue: What if data entered is not numeric?

Error Handling with the Try Statement

Try Sum = Sum + TextBox1.Text TextBox1.Clear()Catch MessageBox.Show("Pls enter a number") End Try

Display Error Message with the Try Statement

Try Sum = Sum + TextBox1.Text TextBox1.Clear()Catch ex As Exception MessageBox.Show(ex.Message) End Try

Simple Calculator

• Create a form with two textboxes to enter numbers and buttons to do +, -, *, / and Mod.

Format

• C, F, N, P

• toString(“C”)

Multiple FormsTo open a form:

formName.Show, formName.ShowDialog

Two forms: Form1, Form2To Open Form2 from Form1:

Form2.ShowDialog

.

• Modeless form: Other forms can receive input focus while this form remains active.– FormName.Show()

• Modal form: No other form can receive focus while this form remains active.– FormName.ShowDialog()

Configure VB Project

• Project property page– Application– Compile– References

• Tools/Options– Environment

– Projects and Solutions

» VB defaults

VB Defaults

• Option Explicit: – On --- must declare variables before use

• Option Strict:– Off --- VB will convert the data

• Option Compare:– Binary --- case sensitive– Text --- case insensitive

• Option Infer– On --- When you set Option Infer to On, you can declare variables

without explicitly stating a data type. The compiler infers the data type of a variable from the type of its initialization expression.

Decision Structure

Decision: Action based on conditionExamples

• Simple condition:– If total sales exceeds $300 then applies 5%

discount; otherwise, no discount.

• More than one condition:• Taxable Income < =3000 no tax• 3000 < taxable income <= 10000 5% tax• Taxable income > 10000 15% tax

• Complex condition:– If an applicant’s GPA > 3.0 and SAT > 1200:

admitted

Relational Operators Test Conditions

• Usually a condition is formed using a relational operator

• A relational operator determines if a specific relationship exists between two values> Greater than< Less than= Equal to<> Not equal to>= Greater than or equal to<= Less than or equal to

The If … Then Statement• If condition Then

Statements

• End If

• If condition ThenStatements

ElseStatements

• End If

• Note: The If and the Then must be on the same line

If…Then Examples‘Bonus awarded if sales greater than 50000 If sales > 50000 Then

getsBonus = TrueEnd If

‘Bonus, 12% commission rate, and a day off‘awarded if sales greater than 50000If sales > 50000 Then

getsBonus = TruecommissionRate = 0.12daysOff = daysOff + 1

End If

Example: If total sales is larger than 1000, then give 5% discount

totalSales=textbox1.textdisCount=0If totalSales > 1000 Then discount=0.05End if

totalSales=textbox1.textIf totalSales > 1000 Then discount=0.05Else discount=0End if

Example: If with a block of statements

If totalSales > 1000 ThendiscountRate=0.05

NetPay=totalSales*(1-discountRate)MessageBox.Show(“Thank you very much”)

ElsediscountRate=0

NetPay=totalSalesMessageBox.Show(“Thank you”)

End if

More than one condition

Rules for bonus:

JobCode = 1 300JobCode = 2 500JobCode = 3 700JobCode = 4 1000

IF Statement with ElseIf

• IF condition THEN

statements

[ELSEIF condition-n THEN

[elseifstatements]

[ELSE

[elsestatements]]]

End If

Code Example

If JobCode = 1 Then Bonus = 300ElseIf JobCode = 2 Then Bonus = 500ElseIf JobCode = 3 Then Bonus = 700Else : Bonus = 1000End If

If JobCode = 1 Then Bonus = 300ElseIf JobCode = 2 Then Bonus = 500ElseIf JobCode = 3 Then Bonus = 700ElseIf JobCode = 4 Then Bonus = 1000End If

Example of ElseIf Usage

If sngAverage < 60 ThenlblGrade.Text = "F"

ElseIf sngAverage < 70 ThenlblGrade.Text = "D"

ElseIf sngAverage < 80 ThenlblGrade.Text = "C"

ElseIf sngAverage < 90 ThenlblGrade.Text = "B"

ElseIf sngAverage <= 100 ThenlblGrade.Text = "A"

End If

• Does the order of these conditions matter?

• What happens if we reverse the order?

Slide 4- 24

The Same Rules Without ElseIfIf sngAverage < 60 Then

lblGrade.Text = "F"End IfIf sngAverage < 70 Then

lblGrade.Text = "D"End IfIf sngAverage < 80 Then

lblGrade.Text = "C"End IfIf sngAverage < 90 Then

lblGrade.Text = "B"End IfIf sngAverage <= 100 Then

lblGrade.Text = "A"End If

• Does this code function correctly? What is assigned to lblGrade for a 65 average? 75?

Bonus Example

If JobCode = 1 Then Bonus = 300End IfIf JobCode = 2 Then Bonus = 500End IfIf JobCode = 3 Then Bonus = 700End IfIf JobCode = 4 Then Bonus = 1000End If

If JobCode = 1 Then Bonus = 300ElseIf JobCode = 2 Then Bonus = 500ElseIf JobCode = 3 Then Bonus = 700ElseIf JobCode = 4 Then Bonus = 1000End If

Use of a Trailing Else

If sngAverage < 60 ThenlblGrade.Text = "F"

ElseIf sngAverage < 70 ThenlblGrade.Text = "D"

ElseIf sngAverage < 80 ThenlblGrade.Text = "C"

ElseIf sngAverage < 90 ThenlblGrade.Text = "B"

ElseIf sngAverage <= 100 ThenlblGrade.Text = "A"

ElselblGrade.Text = "Invalid"

End If

• If average is greater than 100, lblGrade is assigned the text “Invalid”

Nested IF

• State University calculates students tuition based on the following rules:– State residents:

• Total units taken <=12, tuition = 1200

• Total units taken > 12, tuition = 1200 + 200 per additional unit.

– Non residents:• Total units taken <= 9, tuition = 3000

• Total units taken > 9, tuition = 3000 + 500 per additional unit.

Decision Tree

Resident or Not

Units <= 12 or Not

Units <= 9 or Not

Nested If Example

If sngSalary > 30000 ThenIf intYearsOnJob > 2 Then

lblMessage.Text = “Applicant qualifies."Else

lblMessage.Text = “Applicant does not qualify."End If

ElseIf intYearsOnJob > 5 Then

lblMessage.Text = “Applicant qualifies."Else

lblMessage.Text = “Applicant does not qualify."End If

End If Note how the convention of indentationsemphasizes the structure of nested Ifs.

• A bank customer qualifies for a special loan if:– Earns over 30000 & on the job more than 2 years– Or been on the job more than 5 years

Select Case Structure

• SELECT CASE testexpression

[CASE expressionlist-n

[Statements]

[CASE ELSE

[elsestatements]

END SELECT

Select Case Example• SELECT CASE temperature

CASE <40Text1.text=“cold”

CASE < 60Text1.text=“cool”

CASE 60 to 80Text1.text=“warm”

CASE ELSEText1.text=“Hot”

End Select

The Expression list can contain multiple expressions, separated by commas.

Select Case number

Case 1, 3, 5, 7, 9

textBox1.text=“Odd number”

Case 2, 4, 6, 8, 10

textBox1.text=“Even number”

Case Else

End Select

Complex Condition

• Examples:– A theater charges admission fee based on

customer’s age:• 12 <= Age <= 65: Fee = $5

• Otherwise: Fee = $3

– X University admission rules:• If GPA > 3.5 or SAT > 1500: Admitted

– Y University admission rules:• If GPA > 3.0 and SAT > 1200: Admitted

Logical Operators: AND, OR, NOT

• AND• Cond1 Cond2 Cond1 AND Cond2

T TT FF TF F

• OR• Cond1 Cond2 Cond1 OR Cond2

T TT FF TF F

• NOT• Cond NOT Cond

TF

Examples

• Write a complex condition for: 12 <= Age <= 65• Use a complex condition to describe age not

between 12 and 65.• X <= 15 is equivalent to: X<15 AND X =15? (T/F)• This complex condition is always false:

– X < 5 AND X > 10

• This complex condition is always true:– X >= 5 OR X <= 10

Example• Electric Company charges customers based on KiloWatt-Hour

used. The rules are:– First 100 KH, 20 cents per KH

– Each of the next 200 KH • (up to 300 KH), 15 cents per KH

– All KH over 300, 10 cents per KH

Complex Condition

• University admission rules: Applicants will be admitted if meet one of the following rules:– 1. Income >= 100,000– 2. GPA > 2.5 AND SAT > 900

• An applicant’s Income is 150,000, GPA is 2.9 and SAT is 800. Admitted?– Income >= 100,000 OR GPA > 2.5 AND SAT >900

• How to evaluate this complex condition?

• Scholarship: Business students with GPA at least 3.2 and major in Accounting or CIS qualified to apply:– 1. GPA >= 3.2– 2. Major in Accounting OR CIS

• Is a CIS student with GPA = 2.0 qualified?– GPA >= 3.2 AND Major = “Acct” OR Major = “CIS”

• Is this complex condition correct?

NOTSet 1: Young: Age < 30

Set 2: Rich: Income >= 100,000

Young Rich

Condition with Not

• University admission rules: Applicants will be admitted if meet all the rules:– 1. SAT > 900 OR Income >= 50,000– 2. Not GPA < 2.5

• Condition:– SAT > 900 OR Income >= 50,000 AND Not GPA < 2.5

– Correct?

Order of Evaluation

• 1. ()

• 2. Not

• 3. AND

• 4. OR