1. Lists, Loops, Validation, and More
Chapter 5
Lists, Loops,
Validation, and More

2. Introduction

3. Chapter 5 Topics
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. Section 5.1 Input Boxes
Input Boxes Provide a Simple Way to Gather Input Without Placing a Text Box on a Form

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, 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 1/440th inch

8. Section 5.2 List Boxes
List Boxes Display a List of Items and Allow the User to Select an Item From the List

9. ListBox Items Property
This property holds the entire list of items from which the user may choose
The list 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
Click on ellipses button for the Items property
This property is a collection, a list of values
Type each value on a separate line

10. ListBox Items.Count Property
This property holds the number of items that are stored in the Items property
Example of use:
The number of items 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

11. 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

12. 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

13. ListBox SelectIndex Property
Use the SelectIndex property to retrieve the index of an 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

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

15. ListBox Sorted Property
Sorted is a boolean property
If true, causes the values in the Items property to be displayed in alphabetical order
If false, values in the Items property are displayed in the order they were added

16. 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 the value to add to the Items property
Example:
lstStudents.Items.Add("Sharon")

17. 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")

18. 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

19. Other ListBox Methods Examples:
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”)

20. Section 5.3 The Do While Loop
A Loop Is Part of a Program That Repeats

21. 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

22. 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) will continue to be executed
Expression
statement(s)
True
False

23. 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

24. 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

25. 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

26. Counters
Variables called counters are frequently used to control Do While loops
See intCount from the previous example
Counters are typically initialized before the loop begins
Dim intCount As Integer = 0
The counter is then modified in the body of the loop
intCount += 1
The test expression ends the loop when the counter compares to some value

27. 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
The body of a posttest loop must be executed at least once

28. 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

29. 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

30. Section 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

31. Do Until vs. Do While A Do While loop A Do Until 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

32. 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

33. Do Until Loop – Test Score Average
strInput = InputBox("How many test scores do you " _
& “want to average?", "Enter a Value")
intNumScores = CInt(strInput)
‘ Store the starting values in total and count
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

34. 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 a counter increment other than 1 for each iteration of the loop
For CounterVariable = StartValue To EndValue [Step]
statement
Next [CounterVariable]

35. For…Next Flowchart False statement(s) True set counter to StartValue
EndValue?
statement(s)
increment
counter
False
True

36. 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

37. 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

38. 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

39. 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)

40. 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

41. 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

42. 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

43. 5.6 Nested Loops
Nested Loops Are Necessary When a Task Performs a Repetitive Operation and That Task Itself Must Be Repeated

44. 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

45. Nested Loop Example A clock is an example of a nested loop
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

46. 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 requires:
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

47. Section 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

48. 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

49. 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 - the user clicks an item once to select it, again to check it
True - the user clicks an item only once to both select it and check it

50. 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.”)

51. 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

52. 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

53. Combo Box Styles Simple Combo Box Drop-down Combo Box
List is always shown
Drop-down Combo Box
List appears when user clicks down arrow

54. 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

55. Section 5.7 Input Validation
As Long As the User of an Application Enters Bad Input, the Application Will Produce Bad Output
Applications Should Be Written to Filter Out Bad Input

56. 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

57. 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

58. 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

59. 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

60. Section 5.8 Tool Tips
Tool Tips Are a Standard and Convenient Way of Providing Help to the Users of an Application
Visual Basic Provides the ToolTip Control, Which Allows You to Assign Tool Tips to the Other Controls on a Form

61. 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

62. 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 have a ToolTip property
This new property holds the text string that will be displayed for that control

63. 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

64. Section 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.