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Chapter 8

Chapter 8. Using Repetition with Loops and Lists. Class 8: Loops and Lists. Write Do loops to execute statements repeatedly Write For loops to execute statements repeatedly Use generic collections of objects to locate items in a list and to add, change, and delete list items

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Chapter 8

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  1. Chapter 8 Using Repetition with Loops and Lists

  2. Class 8: Loops and Lists • Write Do loops to execute statements repeatedly • Write For loops to execute statements repeatedly • Use generic collections of objects to locate items in a list and to add, change, and delete list items • Use controls that operate on lists, including the ListBox and ComboBox controls • Use the DataGridView control to work withtabular data

  3. Introduction to Loops • Chapter 7 discussed the decision-making structure • This chapter discusses the repetition structure • After completing this chapter, you will know about the three primary control structures • The sequence structure • The decision-making structure • The repetition structure • These structures are part of nearly every program written

  4. Executing Statements Repeatedly • Situations where repetition is used • Calculating payroll – a loop is used to calculate the payroll for each employee • Reading multiple records in a file – a loop is used to process each record in a file • Graphical animations – a loop is used to move graphical objects about the screen • Accounting problems – depreciation and loan balances are calculated repeatedly for multiple periods

  5. DoWhile and DoUntil Loops (Syntax) • Do While and DoUntil loops execute statements repeatedly Do [While | Until] condition [statements] [Exit Do] [statements] Loop statements

  6. DoWhile and DoUntil Loops (Syntax, continued) • DoWhile and DoUntil loops test the condition before executing the statements in a loop • The condition evaluates to a Boolean value • The DoWhile form executes statements while the condition is True • The DoUntil form executes statements until the condition becomes True • The Exit Do statement causes the Do loop to exit immediately • statements following the loop execute

  7. Figure 8-1: Execution Flow of a Do Until Loop

  8. Figure 8-2: Execution Flow of a Do While Loop

  9. Loops and Counters • A counter is a variable whose value increases by a constant value each time through a loop • The constant value used to increment the counter is typically 1 • A counter takes the following general form: • Counter = Counter + 1 • Counter += 1

  10. Loops and Counters (Example) • Print the counting numbers 1 through 10 using a Do Until loop Dim Counter As Integer = 1 Do Until Counter > 10 Debug.WriteLine(Counter) Counter += 1 Loop

  11. Loops and Counters (Example, continued) • Print the counting numbers 1 through 10 using a Do While loop Dim Counter As Integer = 1 Do While Counter <= 10 Debug.WriteLine(Counter) Counter += 1 Loop

  12. Types of Loops • Pre-test loops test a condition before executing the statements in a loop • Post-test loops execute the loop's statements first, and then test the loop's condition

  13. Post-Test Do Loops (Syntax) • In a post-test loop, the condition is tested after the loop statements have executed Do [statements] [Exit Do] [statements] Loop [While | Until] condition statements

  14. Post-Test Do Loops (Syntax, continued) • Note the statement(s) in the loop are first executed, and then the condition is tested • Thus, the loop's statements will always execute at least once • The DoLoopWhile variation executes the loop's statements while a condition is True • The DoLoopUntil variation executes the loop's statements until a condition is True

  15. Post-Test Do Loops (Example) • Print the counting numbers 1 through 10 Dim Counter As Integer = 1 Do Debug.WriteLine(Counter) Counter += 1 Loop While Counter < 10

  16. Figure 8-3:Execution Flow of a Post-test Do Loop

  17. The Role of an Accumulator • An accumulator is similar to a counter • An accumulator is updated each time the statements in a loop execute • An accumulator is used to calculate (accumulate) a total • An accumulator takes the following general form: • Accumulator = Accumulator + value • Accumulator += value

  18. Accumulator (Example) • Store the sum of the counting numbers 1 through 10 Dim Counter As Integer = 1 Dim Accumulator As Integer = 0 Do While Counter <= 10 Debug.Write (Counter) Debug.WriteLine(“ “ & Accumulator) Accumulator += Counter Counter += 1 Loop

  19. Accumulator (Example) (continued)

  20. Infinite Loops • A condition must ultimately occur causing a loop to exit • Loops can be mistakenly written so that statements execute indefinitely • These loops are called infiniteloops • Infinite loop example (Counter is always equal to 1): Dim Counter As Integer = 1 Do While Counter < 10 Debug.WriteLine(Counter) Loop

  21. Nested Do Loops and Decision-making • Loops can be nested, just as decision-making statements can be nested • Loops can contain decision-making statements • Decision-making statements can contain loops • Both can contain nested sequence structures

  22. Combining Looping and Decision-making Statements (Example) • Determine whether a number is prime Private Shared Function IsPrime( _ ByVal arg As Integer) As Boolean Dim Count As Integer = 2 Do While Count < arg If (arg Mod Count) = 0 Then Return False End If Count += 1 Loop Return True End Function

  23. For Loops (Introduction) • For loops execute statements repeatedly • Use a For loop in place of a Do loop when the iteration count (the number of times the loop will execute) is known in advance • For loops run more quickly than comparable Do loops • For loops are more readable than equivalent Do loops

  24. For Loops (Syntax) For counter = start To end [Step increment] [statements] [Exit For] [statements] Next [counter] statements

  25. For Loops (Syntax, continued) • The For and Next statements mark the beginning and end of a For loop • counter is first initialized to start • The value of counter is incremented automatically • Do not explicitly set the value of counter • By default, counter is incremented by one each time through the loop • This value can be changed by including the Stepincrement clause

  26. For Loops (Example) • Print the counting numbers 1 to 10 Dim Counter As Integer For Counter = 1 To 10 Debug.WriteLine(Counter) Next Counter

  27. Figure 8-5: Execution of a For Loop

  28. Nested For Loop (Example) • Print a multiplication table Dim Factor1, Factor2, Result As Integer For Factor1 = 1 To 10 For Factor2 = 1 To 10 Result = Factor1 * Factor2 Debug.Write(Result.ToString() & " ") Next Debug.WriteLine("") Next

  29. Using the Step Keyword • Use the Step keyword to modify a counter's value by a value other than 1 • Example to decrement a counter by 5 Dim CurrentYear As Integer For CurrentYear = 2000 To 1900 Step –5 Debug.WriteLine(CurrentYear.ToString) Next CurrentYear

  30. Introduction to Collections • Collections store repeating items as a list • All collections store references to objects having the same data type or different data types • Some collections are predefined • The Controls collection, for example • It is possible to create custom collections

  31. Members of Collections • The Add method adds an item to the end of a collection • The Count property returns the number of items in a collection • The Item method references an item in a collection • The method is 0-based • The RemoveAt method removes a collection item • The method is also 0-based

  32. Categories of Collections • One category of collection can store items with different data types • A second category of collection requires the data type of all items be the same • This type of collection is called a generic list • A third category of collection stores items always having the same data type

  33. Creating a List • The List class is used to create collections that store items with the same data type • This type of a collection is called a generic list • The List class is new to Visual Studio 2005 • Example to declare a list of type Employee: Private EmployeeList As New _ List(Of Employee)

  34. Adding Items to a List • An instance of the List class is empty when first created • There are no items in the list • Calling the Add method adds an item to the end of the list • The Add method accepts one argument – the item to add • The item added must have the same data type as the data type expected by the list

  35. Adding Items to a List (Example) • Add two items to a list Dim EmpCurrent As New Employee EmpCurrent.EmployeeID = 18223 EmpCurrent.EmployeeName = "Joe Smith" EmployeeList.Add(EmpCurrent) EmpCurrent = New Employee EmpCurrent.EmployeeID = 24428 EmpCurrent.EmployeeName = "Mary Deems" EmployeeList.Add(EmpCurrent)

  36. Referencing a List Item (Example) • The Item method is used to reference a list item • Reference the first Employee in the list named EmployeeList Dim EmpCurrent As Employee EmpCurrent = EmployeeList.Item(0) Debug.WriteLine(EmpCurrent.EmployeeID.ToString) Debug.WriteLine(EmpCurrent.EmployeeName)

  37. Deleting a Collection Item • Call the RemoveAt method with one argument – the 0-based index of the item to remove • An exception will be thrown if the index value is invalid • Example to remove the first collection item: Try EmployeeList.RemoveAt(0) Catch ex As ArgumentOutOfRangeException MessageBox.Show("Employee not found.") End Try

  38. Determining the Size and Capacity of a Collection • The Count property stores the number of items in a collection • The Capacity property stores the number of items the collection can store • The initial value for the Capacity property is 4 • The value doubles, as necessary, as Count reaches Capacity • This is done for performance reasons

  39. Enumerating a Collection with a For loop • A Do loop or For loop can be used to enumerate the items in a collection • A For loop is preferable • Enumerate from 0 to Count - 1 • A ForEach loop can also be used to enumerate the items in a collection

  40. Enumerating a Collection with a For Loop (Example) • Enumerate the EmployeeList collection Dim Index As Integer Dim EmpCurrent As Employee For Index = 0 To EmployeeList.Count – 1 EmpCurrent = EmployeeList(Index) Debug.WriteLine( _ EmpCurrent.EmployeeID.ToString) Debug.WriteLine(EmpCurrent.EmployeeName) Next

  41. Introduction to the ForEach Loop • The ForEach loop is used to enumerate collections • Conceptually, it works the same way as a For loop • Each time through the loop, an object is returned from the collection

  42. The ForEachLoop (Syntax) For Each object In group [statements] [Exit For] [statements] Next statements

  43. The For Each Loop (Syntax, continued) • object contains an object reference • group contains the collection • Each time through the ForEach loop an object is returned

  44. The ForEach Loop (Example) • Enumerate the EmployeeList Dim EmpCurrent As Employee For Each EmpCurrent In EmployeeList Debug.WriteLine( _ EmpCurrent.EmployeeID.ToString) Debug.WriteLine( _ EmpCurrent.EmployeeName.ToString) Next

  45. Introduction to the ComboBox Control • The ComboBox control displays a list of items from which the end user selects one item • The DropDownStyle property defines the visual appearance of the ComboBox • DropDown • Simple • DropDownList

  46. Introduction to the ListBox Control • The ListBox control is similar to the ComboBox control • The visible region does not drop down • The SelectedItems property returns a collection of the selected items • The SelectionMode property defines whether the user can select one item or many items • The ClearSelected method clears the selected items

  47. Working with the ComboBoxand ListBox Controls • Common operations • Items must be added to the list • Conditional actions must be performed as the end user selects list items • Items must be selected programmatically

  48. Adding Items to a ComboBoxor ListBox • Items can be added at design time using the String Collection Editor • Items can be added at run time by calling the Add method as follows: For Count = 0 To 6 lstDemo.Items.Add("Item " & _ Count.ToString) Next

  49. Figure 8-13: The String Collection Editor Dialog Box

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