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Chapter 5, Slide 1 Starting Out with Visual Basic 3rd Edition
Chapter 5
Lists, Loops,
Validation, and More
Chapter 5, Slide 2 Starting Out with Visual Basic 3rd Edition
Introduction
Chapter 5, Slide 3 Starting Out with Visual Basic 3rd Edition
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
Chapter 5, Slide 4 Starting Out with Visual Basic 3rd Edition
Section 5.1Input Boxes
Input Boxes Provide a Simple Way to Gather Input Without Placing a Text Box on a Form
Chapter 5, Slide 5 Starting Out with Visual Basic 3rd Edition
Format of the InputBox Function
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
InputBox(Prompt [,Title] [,Default] [,Xpos] [,Ypos])
Chapter 5, Slide 6 Starting Out with Visual Basic 3rd Edition
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
Chapter 5, Slide 7 Starting Out with Visual Basic 3rd Edition
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
Chapter 5, Slide 8 Starting Out with Visual Basic 3rd Edition
Section 5.2List Boxes
List Boxes Display a List of Items and Allow the User to Select an Item From the List
Chapter 5, Slide 9 Starting Out with Visual Basic 3rd Edition
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
Chapter 5, Slide 10 Starting Out with Visual Basic 3rd Edition
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 ThenMessageBox.Show("The list has no items!")
End If
numEmployees = lstEmployees.Items.Count
Chapter 5, Slide 11 Starting Out with Visual Basic 3rd Edition
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
Chapter 5, Slide 12 Starting Out with Visual Basic 3rd Edition
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
TrystrInput = lstMonths.Items(n).ToString()
Catch ex as ExceptionMessageBox.show(ex.Message)
End Try
Chapter 5, Slide 13 Starting Out with Visual Basic 3rd Edition
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 Thenlocation = lstLocations.Items(lstLocations.SelectedIndex)
End If
Chapter 5, Slide 14 Starting Out with Visual Basic 3rd Edition
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 Thenmonth = lstMonths.Items(lstMonths.SelectedIndex)
End If
If lstMonths.SelectedIndex <> -1 Thenmonth = lstMonths.SelectedItem.Value)
End If
Chapter 5, Slide 15 Starting Out with Visual Basic 3rd Edition
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
Chapter 5, Slide 16 Starting Out with Visual Basic 3rd Edition
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")
Chapter 5, Slide 17 Starting Out with Visual Basic 3rd Edition
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")
Chapter 5, Slide 18 Starting Out with Visual Basic 3rd Edition
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 itemlstStudents.Items.Remove(“Jean”) ‘remove item JeanlstStudents.Items.Clear() ‘remove all items
Chapter 5, Slide 19 Starting Out with Visual Basic 3rd Edition
Other ListBox Methods 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”)
Chapter 5, Slide 20 Starting Out with Visual Basic 3rd Edition
Section 5.3The Do While Loop
A Loop Is Part of a ProgramThat Repeats
Chapter 5, Slide 21 Starting Out with Visual Basic 3rd Edition
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
Chapter 5, Slide 22 Starting Out with Visual Basic 3rd Edition
Do While Flowchart
The Do While loop If the expression is
true, the statement(s)are executed
Expression is thenevaluated again
As long as the expression remains true, the statement(s) will continue to be executed
Expression statement(s)
False
True
Chapter 5, Slide 23 Starting Out with Visual Basic 3rd Edition
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 expressionstatement(s)
Loop
Chapter 5, Slide 24 Starting Out with Visual Basic 3rd Edition
Do While Example
Private Sub btnRunDemo_Click(ByVal sender As System.Object, _ByVal e As System.EventArgs) Handles btnRunDemo.Click' Demonstrate the Do While loopDim intCount As Integer = 0
Do While intCount < 10lstOutput.Items.Add("Hello")intCount += 1
LoopEnd Sub
Note that programming styledictates the body of theloop be indented for clarity
Chapter 5, Slide 25 Starting Out with Visual Basic 3rd Edition
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
Chapter 5, Slide 26 Starting Out with Visual Basic 3rd Edition
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 beginsDim intCount As Integer = 0
The counter is then modified in the body of the loopintCount += 1
The test expression ends the loop when the counter compares to some value
Chapter 5, Slide 27 Starting Out with Visual Basic 3rd Edition
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
Chapter 5, Slide 28 Starting Out with Visual Basic 3rd Edition
Posttest Loop Syntax and Flowchart
The statement(s) mustbe executed at leastonce, irrespective of the expression used
Dostatement(s)
Loop While expression
Expression
statement(s)
False
True
Chapter 5, Slide 29 Starting Out with Visual Basic 3rd Edition
A Posttest Running Total LoopintCount = 1 ' Initialize the counterdecTotal = 0 ' Initialize totalDo
strInput = InputBox("Enter the sales for day " & _intCount.ToString, "Sales Amount Needed")
If strInput <> "" ThendecSales = CDec(strInput) decTotal += decSales ' Add sales to totalintCount += 1 ' Increment the counter
End IfLoop 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
Chapter 5, Slide 30 Starting Out with Visual Basic 3rd Edition
Section 5.4The Do Until andFor 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
Chapter 5, Slide 31 Starting Out with Visual Basic 3rd Edition
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
Chapter 5, Slide 32 Starting Out with Visual Basic 3rd Edition
Do Until: Pretest & Posttest Forms
Pretest:
Posttest:
Tutorial 5-6 provides a hands-on example of a pretest Do Until loop
Do Until expressionstatement(s)
Loop
Dostatement(s)
Loop Until expression
Chapter 5, Slide 33 Starting Out with Visual Basic 3rd Edition
Do Until Loop – Test Score AveragestrInput = InputBox("How many test scores do you " _
& “want to average?", "Enter a Value")intNumScores = CInt(strInput)
‘ Store the starting values in total and countsngTotal = 0intCount = 1
‘ Get the test scoresDo 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 thensngAverage = sngTotal / intNumScores
ElsesngAverage = 0.0
End If
Chapter 5, Slide 34 Starting Out with Visual Basic 3rd Edition
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]
Chapter 5, Slide 35 Starting Out with Visual Basic 3rd Edition
For…Next Flowchart
Counter =EndValue? statement(s)
False
True
setcounter
to StartValue
incrementcounter
Chapter 5, Slide 36 Starting Out with Visual Basic 3rd Edition
For…Next Example
For intCount = 1 To 10intSquare = CInt(intCount ^ 2)strTemp = "The square of " & intCount.ToString _
& “ is “ & intSquare.ToStringlstOutput.Items.Add(strTemp)
Next intCount
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
Chapter 5, Slide 37 Starting Out with Visual Basic 3rd Edition
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 10MessageBox.Show("x is now " & x.ToString)
Next x
For x = 10 To 1 Step -1MessageBox.Show("x is now " & x.ToString)
Next x
Chapter 5, Slide 38 Starting Out with Visual Basic 3rd Edition
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
Chapter 5, Slide 39 Starting Out with Visual Basic 3rd Edition
Example: Exit a Loop Prematurely
maxNumbers = CInt(InputBox("How many numbers do " & _"you wish to sum?"))
total = 0For x = 1 to maxNumbers
input = InputBox("Enter a number.")If input = "" Then
Exit ForElse
num = CDbl(input)total += num
End IfNext xMessageBox.Show(“Sum of the numbers is " & total.ToString)
Chapter 5, Slide 40 Starting Out with Visual Basic 3rd Edition
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
Chapter 5, Slide 41 Starting Out with Visual Basic 3rd Edition
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
Chapter 5, Slide 42 Starting Out with Visual Basic 3rd Edition
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
Chapter 5, Slide 43 Starting Out with Visual Basic 3rd Edition
5.6Nested Loops
Nested Loops Are Necessary When a Task Performs a Repetitive Operation and That Task Itself Must Be
Repeated
Chapter 5, Slide 44 Starting Out with Visual Basic 3rd Edition
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
Chapter 5, Slide 45 Starting Out with Visual Basic 3rd Edition
Nested Loop Example
For hours = 0 To 24lblHours.Text = hours.ToStringFor minutes = 0 To 59
lblMinutes.Text = minutes.ToStringFor seconds = 0 To 59
lblSeconds.Text = seconds.ToStringNext seconds
Next minutesNext hours
A clock is an example of a nested loop
Chapter 5, Slide 46 Starting Out with Visual Basic 3rd Edition
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
Chapter 5, Slide 47 Starting Out with Visual Basic 3rd Edition
Section 5.6Multicolumn List Boxes,
Checked List Boxesand Combo Boxes
A Multicolumn List Box Displays Items in ColumnsA 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
Chapter 5, Slide 48 Starting Out with Visual Basic 3rd Edition
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
Chapter 5, Slide 49 Starting Out with Visual Basic 3rd Edition
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
Chapter 5, Slide 50 Starting Out with Visual Basic 3rd Edition
Finding the Status of Checked Items
The GetItemChecked method returns true if the item at Index has been checkedCheckedListBox.GetItemChecked(Index)
Dim i as IntegerDim intCheckedCities as Integer = 0
For i = 0 to clbCities.Items.Count – 1If clbCities.GetItemChecked(i) = True Then
intCheckedCities += 1End If
Next i
MessageBox.Show(“You checked “ & _intCheckedCities.Tostring() & “ cities.”)
Chapter 5, Slide 51 Starting Out with Visual Basic 3rd Edition
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
Chapter 5, Slide 52 Starting Out with Visual Basic 3rd Edition
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
Chapter 5, Slide 53 Starting Out with Visual Basic 3rd Edition
Combo Box Styles Simple Combo Box List is always shown
Drop-down Combo Box List appears when user
clicks down arrow
Chapter 5, Slide 54 Starting Out with Visual Basic 3rd Edition
Combo Box Styles Drop-down List Combo Box
Behaves like a Drop-DownCombo Box, but the usermay not enter text directly
Tutorial 5-9 demonstrates each style of combo box
Chapter 5, Slide 55 Starting Out with Visual Basic 3rd Edition
Section 5.7Input 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
Chapter 5, Slide 56 Starting Out with Visual Basic 3rd Edition
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
Chapter 5, Slide 57 Starting Out with Visual Basic 3rd Edition
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
Chapter 5, Slide 58 Starting Out with Visual Basic 3rd Edition
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
Chapter 5, Slide 59 Starting Out with Visual Basic 3rd Edition
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 = 0txtNum1.SelectionLength = txtNum1.Text.Length
With txtNum1.SelectionStart = 0.SelectionLength = .Text.Length
End With
Chapter 5, Slide 60 Starting Out with Visual Basic 3rd Edition
Section 5.8Tool 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
Chapter 5, Slide 61 Starting Out with Visual Basic 3rd Edition
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
Chapter 5, Slide 62 Starting Out with Visual Basic 3rd Edition
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
Chapter 5, Slide 63 Starting Out with Visual Basic 3rd Edition
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
Chapter 5, Slide 64 Starting Out with Visual Basic 3rd Edition
Section 5.9Building 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.
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