Week 6 Lists, Loops, Validation, and More Slide 2 2 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 Slide 3 Input Boxes Input Boxes Provide a Simple
Way to Gather Input Without Placing a Text Box on a Form Slide 4 4
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]) Slide 5 5
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 Slide 6 6 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 Slide 7 List Boxes List
Boxes Display a List of Items and Allow the User to Select an Item
From the List Slide 8 8 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 9 9
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 Slide 10 10 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 Slide 11 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 Slide 12 12
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
Slide 13 13 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 Slide 14 14 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 Slide 15 15 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") Slide 16 16 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 3 rd item lstStudents.Items.Insert(2, "Jean") Slide 17 17
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 3 rd item
lstStudents.Items.Remove(Jean)remove item Jean
lstStudents.Items.Clear()remove all items Slide 18 18 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: blnFound = lstMonths.Items.Contains(March)
intIndex = lstMonths.Items.IndexOf(March) Slide 19 The Do While
Loop A Loop Is Part of a Program That Repeats Slide 20 20
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 Slide 21 21 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 Slide 22 22 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 Slide 23 23 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 = intCount + 1 Loop End Sub
Note that programming style dictates the body of the loop be
indented for clarity Slide 24 24 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 32 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 Slide 33 33
ForNext 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] Slide 34 34 ForNext Flowchart Counter = EndValue?
statement(s) False True set counter to StartValue increment counter
Slide 35 35 The following code uses a ForNext loop to place the
squares of the numbers 1 through 10 in a ListBox ForNext Example
For intCount = 1 To 10 intSquare = CInt(intCount ^ 2) strTemp =
"The square of " & intCount.ToString _ & is &
intSquare.ToString lstOutput.Items.Add(strTemp) Next intCount Slide
36 36 More on the StepValue Its 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
Slide 37 37 Example: User Specify End Value Dim intCount,
intMaxNumbers As Integer Dim dblTotal As Double = 0 Dim dblNum As
Double Dim strInput As String intMaxNumbers = CInt(InputBox("How
many numbers do " & _ "you wish to sum?")) For intCount = 1 to
maxNumbers strInput = InputBox("Enter a number.") dblNum =
CDbl(strInput) dblTotal = dblTotal + dblNum Next intCount
MessageBox.Show(Sum of the numbers is " & total.ToString) Slide
38 38 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 Slide 39 39 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 Slide 40 40 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 Slide 41
Nested Loops A Loop that is Inside Another Loop is Called a Nested
Loop Slide 42 42 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, its called a nested loop Slide 43
43 Nested Loop Example For intHours = 0 To 24 lblHours.Text =
intHours.ToString For intMinutes = 0 To 59 lblMinutes.Text =
intMinutes.ToString For intSeconds = 0 To 59 lblSeconds.Text =
intSeconds.ToString Next intSeconds Next intMinutes Next intHours 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 Slide 44
44 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 Slide 45 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
Slide 46 46 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 Slide 47 47 Example For intNumber = 0 To 100
lstNumbers.Items.Add(intNumber) Next Slide 48 48 Checked List Box
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 Slide 49 49 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.) Slide 50 50 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 Slide 51 51 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 Slide 52 52 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 Slide 53
53 Combo Box Styles Drop-down List Combo Box Behaves like a
Drop-Down Combo Box, but the user may not enter text directly Slide
54 54 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 Slide 55 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 Slide
56 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 cant 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 Slide 57 57
CausesValidation/Validating Event A controls 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 Slide 58 58 The Validated
Event A controls 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 Slide 59
59 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() Slide 60 60 Using the
WithEnd 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
Slide 61 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
Slide 62 62 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 Slide 63
63 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 Slide 64 64 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
