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STARTING OUT WITH
Visual Basic 2008FOURTH EDITION
Tony GaddisHaywood Community College
Kip IrvineFlorida International University
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Chapter
Lists, Loops,
Validation, and More5
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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
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Input Boxes5.1
Input Boxes Provide a Simple Way to Gather Input Without Placing a
Text Box on a Form
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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])
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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
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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
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List Boxes5.2
List Boxes Display a List of Items and Allow the User to Select an
Item From the List
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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
Slide 5- 9
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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
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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 ThenMessageBox.Show("The list has no items!")
End If
numEmployees = lstEmployees.Items.Count
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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
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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
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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 Thenlocation = lstLocations.Items(lstLocations.SelectedIndex)
End If
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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 Thenmonth = lstMonths.Items(lstMonths.SelectedIndex)
End If
If lstMonths.SelectedIndex <> -1 Thenmonth = lstMonths.SelectedItem.ToString)
End If
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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
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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")
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ListBox Items.Insert Method
Items can be added at a specific position of a ListBox in VB code using the Insert methodListBox.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")
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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
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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”)
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The Do While Loop5.3
A Loop Is Part of a ProgramThat Repeats
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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
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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) continue to be repeated
Expression statement(s)
False
True
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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
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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
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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
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Counters
Variables called counters are frequently used to control Do While loops intCount in previous example is a counter
Counters generally initialized before loop beginsDim intCount As Integer = 0
Counter must be modified in body of loopintCount += 1
The test expression ends the loop when the counter compares to some value
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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
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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
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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
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The Do Until andFor Next Loops5.4
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
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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
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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
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Do Until Loop – Test Score AveragestrInput = InputBox("How many test scores do you " _
& “want to average?", "Enter a Value")intNumScores = CInt(strInput)
‘ Store starting valuessngTotal = 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
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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]
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For…Next Flowchart
Counter =EndValue? statement(s)
False
True
setcounter
to StartValue
incrementcounter
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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…Next Example
For intCount = 1 To 10intSquare = CInt(intCount ^ 2)strTemp = "The square of " & intCount.ToString _
& “ is “ & intSquare.ToStringlstOutput.Items.Add(strTemp)
Next intCount
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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
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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
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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)
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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
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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
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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
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Nested Loops5.5
A Loop that is Inside Another Loop is Called a Nested Loop
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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
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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 Minute hand repeats 60 times for each hour Second hand repeats 60 times for each minute
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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
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Multicolumn List Boxes,Checked List Boxesand Combo Boxes
5.6
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
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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
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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
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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.”)
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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
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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
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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
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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
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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
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Input Validation5.7
As Long As a User Enters Bad Input, the Application Will Produce Bad Output
Applications Should Be Written to Filter Out Bad Input
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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
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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
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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
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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()
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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
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Tool Tips5.8
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
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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
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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
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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
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Building the Vehicle Loan Calculator Application5.9
This application utilizes loops, input validation, and tool tips.
It also makes use of some Visual Basic intrinsic financial functions.
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Vehicle Loan Calculator
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Calculate Button Flowchart
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Radio Button CheckChanged Events
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Cost Textbox Validating Event
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