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Unit 8 List Boxes and the Do While Looping Structure Chapter 5 Lists, Loops, Validation, and More. Chapter 5 Topics. This chapter covers the Visual Basic .NET looping statements Do … While Do … Until For … Next It also discusses the use of List Boxes Combo Boxes. - PowerPoint PPT Presentation
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© 2006 ITT Educational Services Inc. Introduction to Computer Programming: Unit 8: Chapter 5: Slide 1
Unit 8
List Boxes and the Do While Looping Structure
Chapter 5
Lists, Loops, Validation, and More
© 2006 ITT Educational Services Inc. Introduction to Computer Programming: Unit 8: Chapter 5: Slide 2
Chapter 5 Topics
• This chapter covers the Visual Basic .NET looping statements Do … While Do … Until For … Next
• It also discusses the use of List Boxes Combo Boxes
© 2006 ITT Educational Services Inc. Introduction to Computer Programming: Unit 8: Chapter 5: Slide 3
Input Boxes
Input Boxes Provide a Simple Way to Gather Input Without Placing a Text
Box on a Form
© 2006 ITT Educational Services Inc. Introduction to Computer Programming: Unit 8: Chapter 5: Slide 4
Format of the InputBox Function
• Prompt - message to the user• 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• Title and beyond are optional arguments
InputBox(Prompt [,Title] [,Default] [,Xpos] [,Ypos])
© 2006 ITT Educational Services Inc. Introduction to Computer Programming: Unit 8: Chapter 5: Slide 5
Sample InputBox Usage
• userInput = InputBox("Enter the distance.", "Provide a Value", "150")
© 2006 ITT Educational Services Inc. Introduction to Computer Programming: Unit 8: Chapter 5: Slide 6
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 1/440 inch
© 2006 ITT Educational Services Inc. Introduction to Computer Programming: Unit 8: Chapter 5: Slide 7
List Boxes
List Boxes Display a List of Items and Allow the User to Select an Item
From the List
© 2006 ITT Educational Services Inc. Introduction to Computer Programming: Unit 8: Chapter 5: Slide 8
ListBox Items Property
• This property holds the list of items from which the user may choose
• The value may be established at design time and/or run time
© 2006 ITT Educational Services Inc. Introduction to Computer Programming: Unit 8: Chapter 5: Slide 9
ListBox Items.Count Property
• This property holds the number of items that are stored in the Items property
• Example of use:
If lstEmployees.Items.Count = 0 ThenMessageBox.Show("There are no items in the list!")
End If
© 2006 ITT Educational Services Inc. Introduction to Computer Programming: Unit 8: Chapter 5: Slide 10
Item Indexing
• The Item property values can be accessed under program control
• 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
© 2006 ITT Educational Services Inc. Introduction to Computer Programming: Unit 8: Chapter 5: Slide 11
ListBox SelectIndex Property
• The index of the user selected item is available as the value of the SelectIndex property
• If the user did not select any item, the value is set to -1 (an invalid index value)
• Example:
If lstLocations.SelectedIndex <> -1 Thenlocation = lstLocations.Items(lstLocations.SelectedIndex)
End If
© 2006 ITT Educational Services Inc. Introduction to Computer Programming: Unit 8: Chapter 5: Slide 12
ListBox SelectedItem Property
• When an item has been selected, this property, SelectedItem, contains the selected item itself
© 2006 ITT Educational Services Inc. Introduction to Computer Programming: Unit 8: Chapter 5: Slide 13
ListBox Sorted Property
• The value of this property, if true, causes the items in the Items property to be displayed in alphabetical order
© 2006 ITT Educational Services Inc. Introduction to Computer Programming: Unit 8: Chapter 5: Slide 14
ListBox Items.Add Method
• To add items to the end of a ListBox list at run time, use the Add method
• ListBox.Items.Add(Item)• Example
lstStudents.Items.Add("Sharon")
© 2006 ITT Educational Services Inc. Introduction to Computer Programming: Unit 8: Chapter 5: Slide 15
ListBox Items.Insert Method
• To add items at a specific position in a ListBox list at run time, use the Insert method
• ListBox.Items.Insert(Index, Item)• Example making the 3rd item "Jean"
lstStudents.Items.Insert(2, "Jean")
© 2006 ITT Educational Services Inc. Introduction to Computer Programming: Unit 8: Chapter 5: Slide 16
ListBox Items.Remove, Items.Clear,and Items.RemoveAt Methods
• ListBox.Items.Remove(Item) Removes the item named by value
• ListBox.Items.RemoveAt(Index) Removes the item at the specified index
• ListBox.Items.Clear() Removes all items in the Items property
© 2006 ITT Educational Services Inc. Introduction to Computer Programming: Unit 8: Chapter 5: Slide 17
The Do While Loop
A Loop Is Part of a ProgramThat Repeats
© 2006 ITT Educational Services Inc. Introduction to Computer Programming: Unit 8: Chapter 5: Slide 18
Repetition Structure (or Loop)
• Visual Basic .NET has three structures for repeating a statement or group of statements Do While Do Until For Next
© 2006 ITT Educational Services Inc. Introduction to Computer Programming: Unit 8: Chapter 5: Slide 19
Do While Flowchart
• The Do While loop• If/While the
expression is true,the statement(s)are executed Expression statement(s)
False
True
© 2006 ITT Educational Services Inc. Introduction to Computer Programming: Unit 8: Chapter 5: Slide 20
Do While Syntax
• "Do", "While", and "Loop" are new keywords• The statement, or statements are known as
the body of the loop
Do While expressionstatement(s)
Loop
© 2006 ITT Educational Services Inc. Introduction to Computer Programming: Unit 8: Chapter 5: Slide 21
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 count As Integer = 0Do While count < 10
lstOutput.Items.Add("Hello")count += 1
LoopEnd Sub
© 2006 ITT Educational Services Inc. Introduction to Computer Programming: Unit 8: Chapter 5: Slide 22
Infinite Loops
• Generally, if the expression is true and, hence the starts executing:
• Something with the body of the loop must eventually make the test expression false
• Otherwise, the Do While loop will continuously loop forever - called an infinite loop
© 2006 ITT Educational Services Inc. Introduction to Computer Programming: Unit 8: Chapter 5: Slide 23
Counters
• Variables called counters are frequently used to control Do While loops (see count in the previous example
• Counters are invariably initialized before the loop begins (above: Dim count As Integer = 0)
• They are also usually modified within the body of the loop (above: count += 1)
© 2006 ITT Educational Services Inc. Introduction to Computer Programming: Unit 8: Chapter 5: Slide 24
Pretest vs. Posttest Loops
• The preceding Do While loops were written in their pretest syntax
• The expression is always tested before the body of the loop is executed
• Do While loops also have a posttest form• In these, the body of the loop is always
executed first, then the expression is evaluated to check to see if additional iterations are needed
© 2006 ITT Educational Services Inc. Introduction to Computer Programming: Unit 8: Chapter 5: Slide 25
Posttest Do While Syntax and Flowchart
• The statement(s) willalways be done once,irrespective of theexpression used
Dostatement(s)
Loop While expression
Expression
statement(s)
False
True
© 2006 ITT Educational Services Inc. Introduction to Computer Programming: Unit 8: Chapter 5: Slide 26
Example: Keeping a Running Total
count = 1 ' Initialize the countertotal = 0 ' Initialize totalDo
input = InputBox("Enter the sales for day " & _count.ToString, "Sales Amount Needed")
If input <> "" Thensales = CDec(input) total += sales ' Add sales to totalcount += 1 ' Increment the counter
End IfLoop While count <= 5