Lecture5

Introducing the Do While…Loop and Do Until…Loop Repetition Statements

Introducing the Do…Loop While and Do…Loop Until Repetition Statements

Do While…Loop Repetition Statement

• Loop-continuation condition– While loop-continuation condition remains

true, loop statement executes its body repeatedly

– When loop-continuation condition becomes false, loop terminates

• Code example:– Dim intProduct As Integer = 3

Do While intProduct <= 50 intProduct *= 3

Loop

Do While…Loop Repetition Statement

[intProduct <= 50]

[intProduct > 50]

triple product va lue

merge

dec ision

Corresponding Visua l Basic .NET statement:

intProduct *= 3

Figure 9.4 Do While Loop repetition statement UML activity diagram.

Do Until…Loop Repetition Statement

• Loop-continuation condition– While loop-continuation condition remains

false, loop statement executes its body repeatedly

– When loop-continuation condition becomes true, loop terminates

• Code example:– Dim intProduct As Integer = 3

Do Until intProduct > 50 intProduct *= 3

Loop

Do Until…Loop Repetition Statement

[intProduct <= 50]

[intProduct > 50]

triple product va lue

merge

decision

Corresponding Visual Basic .NET statement:

intProduct *= 3

Figure 9.5 Do Until Loop repetition statement UML activity diagram

Do…Loop While Repetition Statement

• Do…Loop While example: Dim intCouner As Integer = 1

Do 1stDisplay/Items.Add(intCounter) intCounter += 1 Loop While intCounter <= 3

• Set counter to 1– Excecute body of Do…Loop While statement while the counter is less than or equal to 3

Do…Loop While Repetition Statement

decision

action state

[intCounter <= 3]

[intCounter > 3]

Corresponding VB .NET Statements:lstDisplay.Items.Add(intCounter)intCounter += 1

loop-continuation

condition

Do…Loop While repetition statement UML activity diagram.

• Do…Loop While repetition statement– Body of the loop executes first

– Loop-continuation condition checked

• If False, loop exits

• If True, loop executes again

Do…Loop Until Repetition Statement

• Do…Loop Until example: Dim intCouner As Integer = 1

Do 1stDisplay/Items.Add(intCounter) intCounter += 1 Loop Until intCounter > =3

• Set counter to 1– Execute body of Do…Loop Until statement until the counter is greater than to 3

Do…Loop Until Repetition Statement

Decision

action state

[intCounter <= 3]

[intCounter > 3]

Corresponding VB .NET Statements:lstDisplay.Items.Add(intCounter)

intCounter += 1

Loop-termination c ondition

Do…Loop Until repetition statement UML activity diagram.

• Do…Loop Until repetition statement– Body of the loop executes

– Loop-continuation condition checked

• If False, loop executes again

• If True, loop exits

Introducing the Do…Loop While and Do…Loop Until Repetition Statements

Class Average Application

Test-Driving the Class Average Application

Application Requirements A teacher regularly issues quizzes to a class of ten students. The grades on these quizzes are integers in the range from 0 to 100 (0 and 100 are each valid grades). The teacher would like you to develop an application that computes the class average for a quiz.


Entering grades in the Class Average application.

• Entering quiz grades– Enter a grade into the Enter grade: TextBox

– Click Add Grade Button

– Grade added to ListBox– Focus given to TextBox


Figure 10.3 Class Average application after 10 grades have been input.

Ten quiz grades entered Disabled Add Grade Button

• Entering ten grades– Add Grade Button disabled

– Click Average Button to calculate average quiz grade


Figure 10.4 Displaying the class average.

Label displaying average

Click to calculate class average

• Calculating the average– Click the Average Button

– Average displayed in output Label– Add Grade Button enabled


Figure 10.5 Entering a new set of grades.

• Entering another set of grades– Old quiz grades removed from ListBox– New grade added to ListBox

Creating the Class Average Application

Event Control Event

Label all the application’s controls lblPrompt, lblGradeList, lblDescribeOutput

User enters grade txtInput

Retrieve grade entered by user in the Enter grade: TextBox

btnAdd Click

Display the grades lstGrades

Calculate the class average by reading ListBox values

btnAverage Click

Display the class average lblOutput

ACE table for the Class Average application.


Class Average application’s Form in design view.

• Template application– Contains:

• ListBox

• Buttons• Output Label

18


Clearing the ListBox and output Label after a calculation.

Clearing the grade list and class average

• Clearing ListBox and output Label– If output Label contains text

• Clear output Label of previous result

• Clear ListBox of previous quiz grades

19


Adding the grade input to the ListBox and clearing the Enter grade: TextBox.

• Displaying grades– Add grade to ListBox– Clear TextBox txtInput

Adding a numeric grade to the ListBox and

clearing the user input from the TextBox

20


Transferring the focus to the TextBox control.

Transferring the focusof the application to

the TextBox

• Transferring focus to a control– Makes application easier to user

– Use method Focus of that control

21


• Accepting ten grades– Use method lstGrades.Items.Count to determine if ten grades

entered

– If ten grades entered:• Disable Add Grade Button

• Give focus to Average Button

22


Application accepts only 10 grades.

Disabling the Add

grade Button and transferringthe focus to the

Average Button

23


Do…Loop Until summing grades.

Using the Do…LoopUntil repetition

statement to sumgrades in the ListBox.

24


• Adding quiz grades– Use a Do…Loop Until statement to retrieve ten grades

• Use lstGrades.Items.Item() to retrieve grade from ListBox

• Add grade to intTotal

• Increment counter

25


• Displaying average– Calculate average of ten quiz grades

– Format and display average– Enable Add Grade Button– Transfer the focus to txtInput TextBox

26


Displaying the result of the average calculation.

Calculating the class average,

enabling the Add

Grade Button, and transferring the

focus to the Enter

Grade: TextBox.

27

ClassAverage.vb(1 of 3)

1 Public Class FrmClassAverage2 Inherits System.Windows.Forms.Form

3 4 ' Windows Form Designer generated code

5 6 ' handles Add Grade Button’s Click event

7 Private Sub btnAdd_Click(ByVal sender As System.Object, _8 ByVal e As System.EventArgs) Handles btnAdd.Click

9 10 ' clear previous grades and calculation result

11 If lblOutput.Text <> ““ Then12 lblOutput.Text = ““

13 lstGrades.Items.Clear()14 End If

15 16 ' display grade in ListBox

17 lstGrades.Items.Add(Val(txtInput.Text))18 txtInput.Clear() ' clear grade from TextBox19 txtInput.Focus() ' transfer focus to TextBox

20 21 ' prohibit users from entering more than 10 grades

22 If lstGrades.Items.Count >= 10 Then23 btnAdd.Enabled = False ' disable Add Grade Button

24 btnAverage.Focus() ' transfer focus to Average

Button25 End If

Disabling the Add Grade Button and transferring the focus to the Average Button

28


26 27 End Sub ' btnAdd_Click

28 29 ' handles Average Button’s Click event

30 Private Sub btnAverage_Click(ByVal sender As System.Object, _31 ByVal e As System.EventArgs) Handles btnAverage.Click

32 33 ' initialization phase

34 Dim intTotal As Integer = 035 Dim intGradeCounter As Integer = 0

36 Dim intGrade As Integer = 037 Dim dblAverage As Double = 0

38 39 ' sum grades in ListBox

40 Do41

42 ' read grade from ListBox43 intGrade = lstGrades.Items.Item(intGradeCounter)

44 intTotal += intGrade ' add grade to total45 intGradeCounter += 1 ' increment counter

46 Loop Until intGradeCounter >= 1047

Using a Do…Loop Until statement to calculate the class average

29


48 dblAverage = intTotal / 10 ' calculate average49 lblOutput.Text = String.Format(“{0:F}”, dblAverage)

50 btnAdd.Enabled = True ' enable Add Grade Button 51 txtInput.Focus() ' reset focus to Enter grade: TextBox

52 End Sub ' btnAverage_Click53

54 End Class ' FrmClassAverage

Enabling the Add

Grade Button and transferring the focus to the Enter grade: TextBox

Engineering

Lecture5