1. Tony Gaddis Kip Irvine STARTING OUT WITH Visual Basic 2008
FOURTH EDITION
Tony Gaddis
Haywood Community College
Kip Irvine
Florida International University

2. Lists, Loops, Validation, and More
Chapter
Lists, Loops,
Validation, and More 5
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley

3. Introduction This chapter covers the Visual Basic looping statements
Do … While
Do … Until
For … Next
It also discusses the use of
List Boxes
Combo Boxes
As well as presenting some properties and events used for user input validation

4. Input Boxes
5.1
Input Boxes Provide a Simple Way to Gather Input Without Placing a Text Box on a Form
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley

5. Format of the InputBox Function
InputBox(Prompt [,Title] [,Default] [,Xpos] [,Ypos])
Prompt - message to the user (required)
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
Square brackets around Title and following arguments indicate these are optional

6. Sample InputBox Usage
strUserInput = InputBox("Enter the distance.", _ "Provide a Value", "150")
If the users clicks OK without entering a value, 150 will be assigned to strUserInput due to the default value

7. Xpos & Ypos
Xpos specifies the distance from the left of the screen to the left side of the box
Ypos specified the distance from the top of the screen to the top of the box
Both are specified in pixels

8. List Boxes
5.2
List Boxes Display a List of Items and Allow the User to Select an Item From the List
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley

9. The ListBox Control
A ListBox control displays a list of items and allows the user to select one or more
Drag from Toolbox to create this control on a form

10. ListBox Items Property
The Items property holds an entire list of values from which the user may choose
The list of values may be established at run time or as part of the form design
To set list values in the form design:
Select the list box in the Design window
View properties & click the Items ellipsis button
This property is a collection, a list of values
Type each value on a separate line

11. ListBox Items.Count Property
This property returns an integer with the number of entries stored in the Items property
Example of use:
The number of entries in the list can be assigned to an integer variable
If lstEmployees.Items.Count = 0 Then
MessageBox.Show("The list has no items!")
End If
numEmployees = lstEmployees.Items.Count

12. Item Indexing
The Items property values can be accessed from your VB code
Each item value is given a sequential index
The first item has an index of 0
The second item has an index of 1, etc.
Example:
name = lstCustomers.Items(2)
' Access the 3rd item value

13. Index Out of Range Error
The index of the last item is always list.Items.Count-1
Reference to an index greater than Count-1 or less than zero throws an exception
An exception handler can trap this error
The variable ex captures the exception thrown
Try
strInput = lstMonths.Items(n).ToString()
Catch ex as Exception
MessageBox.show(ex.Message)
End Try

14. ListBox SelectIndex Property
The SelectIndex property returns an integer with the index of the item selected by the user
If no item is selected, the value is set to -1 (an invalid index value)
Can use SelectIndex to determine if an item has been selected by comparing to -1
Example:
If lstLocations.SelectedIndex <> -1 Then
location = lstLocations.Items(lstLocations.SelectedIndex)
End If

15. ListBox SelectedItem Property
Instead of using the SelectedIndex property as follows:
The SelectedItem property can be used to retrieve the value of a selected item as follows:
If lstMonths.SelectedIndex <> -1 Then
month = lstMonths.Items(lstMonths.SelectedIndex)
End If
If lstMonths.SelectedIndex <> -1 Then
month = lstMonths.SelectedItem.ToString)
End If

16. ListBox Sorted Property
Sorted is a boolean property
When set to true, values in the Items property are displayed in alphabetical order
When set to false, values in the Items property are displayed in the order they were added

17. ListBox Items.Add Method
Items can be added to the end of a ListBox list in your VB code using the Add method
Format is
ListBox.Items.Add(Item)
ListBox is the name of the control
Item is a string value to add to the Items property
Example:
lstStudents.Items.Add("Sharon")

18. ListBox Items.Insert Method
Items can be added at a specific position of a ListBox in VB code using the Insert method
ListBox.Items.Insert(Index, Item)
Index specifies position where Item is placed
Index is zero based similar to SelectedIndex property
Items that follow are “pushed” down
Example inserting "Jean“ as the 3rd item
lstStudents.Items.Insert(2, "Jean")

19. ListBox Methods to Remove Items
ListBox.Items.RemoveAt(Index)
Removes item at the specified index
ListBox.Items.Remove(Item)
Removes item with value specified by Item
ListBox.Items.Clear()
Removes all items in the Items property
Examples:
lstStudents.Items.RemoveAt(2) ‘remove 3rd item
lstStudents.Items.Remove(“Jean”) ‘remove item Jean
lstStudents.Items.Clear() ‘remove all items

20. Other ListBox Methods Returns true if Item is found in the collection
ListBox.Items.Contains(Item)
Returns true if Item is found in the collection
ListBox.Items.IndexOf(Item)
Returns an integer with the index position of the first occurrence of Item in the collection
Examples:
Tutorial 5-1 provides more examples of ListBox controls, methods and properties
blnFound = lstMonths.Items.Contains(“March”)
intIndex = lstMonths.Items.IndexOf(“March”)

21. A Loop Is Part of a Program That Repeats
5.3
The Do While Loop
A Loop Is Part of a Program That Repeats
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley

22. Repetition Structure (or Loop)
Visual Basic has three structures that allow a statement or group of statements to repeat
Do While
Do Until
For...Next

23. Do While Flowchart The Do While loop
If the expression is true, the statement(s) are executed
Expression is then evaluated again
As long as the expression remains true, the statement(s) continue to be repeated
Expression
statement(s)
False
True

24. Do While Syntax Do, While, and Loop are new keywords
The Do While statement marks the beginning of the loop
The Loop statement marks the end
The statements to repeat are found between these and called the body of the loop
Do While expression
statement(s)
Loop

25. Do While Example
Private Sub btnRunDemo_Click(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles btnRunDemo.Click
' Demonstrate the Do While loop
Dim intCount As Integer = 0
Do While intCount < 10
lstOutput.Items.Add("Hello")
intCount += 1
Loop
End Sub
Note that programming style dictates the body of the loop be indented for clarity

26. Infinite Loops A loop must have some way to end itself
Something within the body of the loop must eventually force the test expression to false
In the previous example
The loop continues to repeat
intCount increases by one for each repetition
Finally intCount is not <10 and the loop ends
If the test expression can never be false, the loop will continue to repeat forever
This is called an infinite loop

27. Counters
Variables called counters are frequently used to control Do While loops
intCount in previous example is a counter
Counters generally initialized before loop begins
Dim intCount As Integer = 0
Counter must be modified in body of loop
intCount += 1
The test expression ends the loop when the counter compares to some value

28. Pretest vs. Posttest Loops
Previous Do While loops are in pretest form
Expression is tested before the body of the loop is executed
The body may not be executed at all
Do While loops also have a posttest form
The body of the loop is executed first
Then the expression is evaluated
Body repeats as long as expression is true
A posttest loop always executes the body of the loop at least once

29. Posttest Loop Syntax and FlowchartDo
statement(s)
Loop While expression
statement(s)
The statement(s) must be executed at least once, irrespective of the expression used
Expression
True
False

30. A Posttest Running Total Loop
intCount = 1 ' Initialize the counter
decTotal = 0 ' Initialize total Do
strInput = InputBox("Enter the sales for day " & _
intCount.ToString, "Sales Amount Needed")
If strInput <> "" Then
decSales = CDec(strInput)
decTotal += decSales ' Add sales to total
intCount += 1 ' Increment the counter
End If
Loop While intCount <= 5
Tutorial 5-4 uses the code above in pretest form as part of a more complete example
Tutorial 5-5 demonstrates how to structure a loop such that the user can specify the iterations

31. The Do Until and For Next Loops
5.4
The Do Until and For Next Loops
A Do Until Loop Iterates Until Its Test Expression Is True
The For...Next Loop Is Designed to Use a Counter Variable and Iterates a Specific Number of Times
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley

32. Do Until vs. Do While A Do While loop
Repeats as long as its test expression is true
Ends when its test expression becomes false
A Do Until loop
Repeats as long as its test expression is false
Ends when its test expression becomes true
The Do Until loop has a pretest and posttest form just as a Do While loop

33. Do Until: Pretest & Posttest Forms
Tutorial 5-6 provides a hands-on example of a pretest Do Until loop
Do Until expression
statement(s)
Loop Do
statement(s)
Loop Until expression

34. Do Until Loop – Test Score Average
strInput = InputBox("How many test scores do you " _
& “want to average?", "Enter a Value")
intNumScores = CInt(strInput)
‘ Store starting values
sngTotal = 0
intCount = 1
‘ Get the test scores
Do Until intCount > intNumScores
strInput = InputBox("Enter the value for test score " _
& intCount.ToString, "Test Score Needed")
sngTotal = sngTotal + CSng(strInput)
intCount = intCount + 1
Loop
‘ Calculate the average
If intNumScores > 0 then
sngAverage = sngTotal / intNumScores
Else
sngAverage = 0.0
End If

35. For…Next Loop Ideal for loops that require a counter
For, To, and Next are keywords
CounterVariable tracks number of iterations
StartValue is initial value of counter
EndValue is counter number of final iteration
Optional Step allows the counter to increment at a value other than 1 at each iteration of the loop
For CounterVariable = StartValue To EndValue [Step]
statement
Next [CounterVariable]

36. For…Next Flowchart False statement(s) True Counter = EndValue? set
to StartValue
increment

37. For…Next Example
The following code from Tutorial 5-7 uses a For…Next loop to place the squares of the numbers 1 through 10 in a ListBox
Tutorial 5-8 uses a For…Next loop to move a PictureBox control around a window
For intCount = 1 To 10
intSquare = CInt(intCount ^ 2)
strTemp = "The square of " & intCount.ToString _
& “ is “ & intSquare.ToString
lstOutput.Items.Add(strTemp)
Next intCount

38. More on the StepValue
It’s optional and if not specified, defaults to 1
The following loop iterates 11 times with counter values 0, 10, 20, …, 80, 90, 100
StepValue may be negative, causing the loop to count downward
For x = 0 To 100 Step 10
MessageBox.Show("x is now " & x.ToString)
Next x
For x = 10 To 1 Step -1
MessageBox.Show("x is now " & x.ToString)
Next x

39. Exiting a Loop Prematurely
In some cases it is convenient to end a loop before the test condition would end it
The following statements accomplish this
Exit Do (used in Do While or Until loops)
Exit For (used in For Next loops)
Use this capability with caution
It bypasses normal loop termination
Makes code more difficult to debug

40. Example: Exit a Loop Prematurely
maxNumbers = CInt(InputBox("How many numbers do " & _
"you wish to sum?"))
total = 0
For x = 1 to maxNumbers
input = InputBox("Enter a number.")
If input = "" Then
Exit For
Else
num = CDbl(input)
total += num
End If
Next x
MessageBox.Show(“Sum of the numbers is " & total.ToString)

41. When to Use the Do While Loop
Use Do While when the loop should repeat as long as the test expression is true
Can be written as a pretest or posttest loop
A pretest Do While is ideal when the body should not be perfomed for a test expression that is initially false
Posttest loops are ideal when you always want the loop to iterate at least once

42. When to Use the Do Until Loop
Use Do Until when the loop should repeat as long as the test expression is false
Can be written as a pretest or posttest loop
A pretest Do Until is ideal when the body should not be perfomed for a test expression that is initially true
Posttest loops are ideal when you always want the loop to iterate at least once

43. When to Use the For Next Loop
The For...Next loop is a pretest loop ideal when a counter is needed
It automatically increments the counter variable at the end of each iteration
The loop repeats as long as the counter variable is not greater than an end value
Used primarily when the number of required iterations is known

44. A Loop that is Inside Another Loop is Called a Nested Loop
5.5
Nested Loops
A Loop that is Inside Another Loop is Called a Nested Loop
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley

45. Nested Loops
The body of a loop can contain any type of VB statements including another loop
When a loop is found within the body of another loop, it’s called a nested loop

46. Nested Loop Example A clock is an example of a nested loop
Minute hand repeats 60 times for each hour
Second hand repeats 60 times for each minute
For hours = 0 To 24
lblHours.Text = hours.ToString
For minutes = 0 To 59
lblMinutes.Text = minutes.ToString
For seconds = 0 To 59
lblSeconds.Text = seconds.ToString
Next seconds
Next minutes
Next hours

47. Nested Loop Example Analysis
The innermost loop will iterate 60 times for each iteration of the middle loop
The middle loop will iterate 60 times for each iteration of the outermost loop
24 iterations of the outermost loop require:
1,440 iterations of the middle loop
86,400 iterations of the innermost loop
An inner loop goes through all its iterations for each iteration of the outer loop
Multiply iterations of all loops to get the total iterations of the innermost loop

48. Multicolumn List Boxes, Checked List Boxes and Combo Boxes
5.6
Multicolumn List Boxes, Checked List Boxes and Combo Boxes
A Multicolumn List Box Displays Items in Columns A Checked List Box Displays a Check Box Next to Each Item in the List
A Combo Box Is Like a List Box Combined With a Text Box
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley

49. List Box Multicolumn Property
The ListBox has a Multicolumn property
Boolean property with default value of false
If set to true, entries can appear side by side
Below, ColumnWidth is set to 30
Note the appearance of a horizontal scroll bar in this case

50. Checked List Box
A form of ListBox with the list box properties and methods already discussed
One item at a time may be selected but many items in a Checked List Box can be checked
The CheckOnClick property determines how items may be checked
False - user clicks item once to select it, again to check it
True - user clicks item only once to both select it and check it

51. Finding the Status of Checked Items
The GetItemChecked method returns true if the item at Index has been checked
CheckedListBox.GetItemChecked(Index)
Dim i as Integer
Dim intCheckedCities as Integer = 0
For i = 0 to clbCities.Items.Count – 1
If clbCities.GetItemChecked(i) = True Then
intCheckedCities += 1
End If
Next i
MessageBox.Show(“You checked “ & _
intCheckedCities.Tostring() & “ cities.”)

52. Combo Boxes Similar to List Boxes
Both display a list of items to the user
Both have Items, Items.Count, SelectedIndex, SelectedItem, and Sorted properties
Both have Items.Add, Items.Clear, Items.Remove, and Items.RemoveAt methods
These properties and methods work the same with combo boxes and list boxes

53. Additional Combo Box Features
A combo box also functions like a text box
The user may enter text into a combo box
Or the user may select the text from a series of list box type choices

54. Combo Box Styles Simple Combo Box List is always shown
Drop-down Combo Box
List appears when user clicks down arrow
User can type text or select

55. Combo Box Styles Drop-down List Combo Box
Behaves like a Drop-Down Combo Box, but the user may not enter text directly
Tutorial 5-9 demonstrates each style of combo box

56. Choosing a Combo Box Style
If restricting the user to select items listed
If empty space – use ListBox
If limited space – use drop-down list ComboBox
If allowing user to select an item listed or enter an entirely new item
If empty space – use simple ComboBox
If limited space – use drop-down ComboBox

57. 5.7
Input Validation
As Long As a User Enters Bad Input, the Application Will Produce Bad Output Applications Should Be Written to Filter Out Bad Input
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley

58. Examples of Input Validation
Numbers are checked to ensure they are within a range of possible values
For example, there are 168 hours in a week
A person can’t work more than 168 hours a week
Values are checked for their “reasonableness”
A person might possibly work 168 hours in a week
However, this is highly improbable
Items selected from a menu or a set of choices are checked to ensure these options are available
Variables are checked for values that might cause problems, such as division by zero

59. CausesValidation/Validating Event
A control’s Validating event is triggered when focus is shifting from that control to a control whose CausesValidation property is true
The Validating event of the control losing the focus fires before focus is lost
This allows your code to validate an entry just before focus shifts
If user shifts focus, input must be complete
Tutorial 5-10 demonstrates this capability

60. The Validated Event A control’s Validated event is triggered
After the Validating event
After focus has been lost
Allows operations on input that should occur only after the user moves away from the field

61. Select Text with SelectAll Method
SelectAll makes correcting invalid input easier
If you know which TextBox is in error you can:
Use the Focus method to position the cursor
Use the SelectAll method to select the text
Then the user need not erase incorrect text
Simply start typing to enter the corrected text
If txtName is in error:
txtName.Focus()
txtName.SelectAll()

62. Using the With…End Statement
A With statement establishes a default object in effect until an End With is encountered
Instead of repeating txtNum1 in this code
With allows you to reference the txtNum1 object without specifying it repeatedly
txtNum1.SelectionStart = 0
txtNum1.SelectionLength = txtNum1.Text.Length
With txtNum1
.SelectionStart = 0
.SelectionLength = .Text.Length
End With

63. 5.8
Tool Tips
Tool Tips Are a Standard, Convenient Way of Providing Help to Users of an Application
The ToolTip Control Allows You to Assign Pop-up Hints to the Other Controls on a Form
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley

64. What is a Tool Tip?
A Tool Tip is the short text message you see when holding the mouse over a control
These are easy to set up and use in Visual Basic forms
The ToolTip control allows you to create ToolTips for other controls on a form

65. Setting Up ToolTips Display the form design window
Double-click the ToolTip tool in the Toolbox
The ToolTip control is invisible at runtime so
It appears in the component tray, not the form
Component tray shows at the bottom of the design window
ToolTips are now enabled for this form
Form controls now have a ToolTip property
This new property holds the text string that will be displayed for that control

66. Controlling the Tool Tips
Select the ToolTip control from the tray
View Properties window to see the following
An InitialDelay property that regulates the delay before a tip appears
An AutoPopDelay that determines how long a tip is displayed
ReshowDelay determines the time between the display of different tips as the user moves the mouse from control to control

67. Building the Vehicle Loan Calculator Application
5.9
Building the Vehicle Loan Calculator Application
This application utilizes loops, input validation, and tool tips.
It also makes use of some Visual Basic intrinsic financial functions.
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley

68. Vehicle Loan Calculator

69. Calculate Button Flowchart

70. Radio Button CheckChanged Events

71. Cost Textbox Validating Event