Chapter 6 – Repetition

1. Chapter 6 – Repetition 6.1 Do Loops 6.2 For...Next Loops 6.3 List Boxes and Loops

2. 6.3 List Boxes and Loops • Some Properties, Methods, and Events of List Boxes • List Boxes Populated with Strings • List Boxes Populated with Numbers • Searching • Unordered Lists • Ordered Lists

3. # of Items in a Listbox • The total number of items in a list box is: listBox.Items.Count • Each item in listBox is identified by an index number from: 0 to listBox.Items.Count – 1

4. Accessing ListBox Items • listBox.Items(i) is used to access the value of an item in a list box (where i is the index number) • The index identifies the relative position of items in the list. • The first item’s index number is 0 • The second item’s index number is 1 • The last item’s index number is listbox.items.count - 1 • The first element of listBox.Items can be displayed in a text box as follows: txtBox.Text = CStr(listBox.Items(0)) • The last element of listBox.Items can be displayed in a text box as follows: n = listbox.items.Count - 1txtBox.Text = CStr(listBox.Items(n))

5. Anatomy of a Listbox What does following statement do? class = cstr(listbox1.items(2)) What does following statement do? n = listbox1.items.count What does following statement do? Textbox.text = listbox1.items (listbox1.items.count – 1)

6. Object Data Type • The Objectdata type is the parent of all data types. • The data type of the elements in the listBox.Items(i) array are Object. • You may have noticed we used the convert each item to string in the list box. txtBox.Text = CStr(lstBox.Items(0))

7. Stepping Through EachItem in a ListBox The following code copies the items from one list box to a second list box. For i As Integer = 0 To ListBox1.Items.Count – 1 ListBox2.Items.Add(CStr(ListBox1.Items(i))) Next

8. The Sorted Property • Items can be placed into the list at design time or run time • The Sortedproperty causes items in the list to be sorted automatically as strings • Caution: Numbers in a sorted list box will not necessarily be in increasing numerical order

9. Copy and Sort This code is the sameas the previous code,but now Listbox2’s sorted property is True. For i As Integer = 0 To ListBox1.Items.Count – 1 ListBox2.Items.Add(CStr(ListBox1.Items(i))) Next

10. SelectedIndex The index numberof the currently highlighted itemis given by: ListBox2.Items.Add(ListBox1.SelectedIndex)

11. Currently Highlighted Item in a List Boxes The value of the currently highlighted item as a string can be obtained with thefollowing: ListBox2.Items.Add(ListBox1.Text)

12. States Join the Union: Form In this program we will list the last 10 states to join the union. lstStates Copy from thetext file included on drive P: lstStates is filled at design time with the names of the U.S. states in the order they joined the union.

13. States Join the Union: Code Private Sub btnDisplay_Click(...) Handles _ btnDisplay.Click 'Display last ten states to join the union Dim n As Integer = lstStates.Items.Count For i As Integer = (n - 1) To (n - 10) Step -1 lstLastTen.Items.Add(lstStates.Items(i)) Next End Sub n-1 is the index of the last item in the list

14. States Join the Union: Output

15. Average Age of Presidents: Form lstAges txtAvgAge lstAges is filled at design time with the ages of the U.S. presidents when taking office.

16. Average Age of Presidents: Code Private Sub btnCalculate_Click(...) Handles _ btnCalculate.Click 'Calculate average age of presidents when assuming office Dim n As Integer = lstAges.Items.Count Dim sum As Double = 0 For i As Integer = 0 To n – 1 sum += CDbl(lstAges.Items(i)) Next txtAvgAge.Text = FormatNumber(sum / n, 2) End Sub

17. Average Age of Presidents: Output

18. Flags • A flag is a variable that keeps track of whether a certain situation has occurred. • The data type most suited to flags is Boolean.

19. More About Flags When flagVaris a variable of Boolean type, the statements IfflagVar = True Then and IfflagVar = False Then can be replaced by IfflagVarThen and If NotflagVarThen

20. More About Flags (continued) The statements Do While flagVar = True and Do While flagVar = False can be replaced by Do While flagVar and Do While Not flagVar

21. Searching an Unsorted List • A flag is used to indicate whether or not the sought-after item has been found. • The flag variable is initially set to False and then set to True if and when the item is found.

22. Searching an Unordered List Private Sub Button1_Click(…) Handles Button1.Click Dim key As String, found As Boolean = False Dim i As Integer = 0 key = TextBox1.Text Do While Not found And i <= ListBox1.Items.Count - 1 If key = CStr(ListBox1.Items(i)) Then found = True End If i = i + 1 Loop If found Then ListBox2.Items.Add(“Found at index # " & (i - 1)) Else ListBox2.Items.Add("The item is not in the list.") End If End Sub Initialize It may be easier to think of this as: Do … Loop Until found or i > # items Once the value is located set found to true

23. Searching a List of Strings • A searchoften can be terminated early if the list is sorted. • This is particularly the case when the sought-after item is not in the list. The search can be terminated when an item is reached that follows the sought-after item alphabetically.

24. Search With Early Exit Private Sub Button1_Click(…) Handles Button1.Click Dim key As String Dim found As Boolean = False, doneEarly As Boolean = False Dim i As Integer = 0 key = TextBox1.Text Do While Not found And i <= ListBox1.Items.Count - 1 AndNot doneEarly If key = CStr(ListBox1.Items(i)) Then found = True End If Ifkey < CStr(ListBox1.Items(i)) Then doneEarly = True End If i = i + 1 Loop

25. Search With Early Exit (Cont) If found Then ListBox2.Items.Add("The item you search for is at index # " & (i - 1)) Else If doneEarly Then ListBox2.Items.Add("Not in the List - left early") Else ListBox2.Items.Add("Not in the List") End If End If End Sub