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Chapter 7

Chapter 7. Arrays. Outline and Objective. Arrays in Visual Basic One- dimensional arrays Control arrays Two-dimensional arrays Searching Sorting. Array verses Simple Variable. Simple variable is used to store a single value.

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Chapter 7

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  1. Chapter 7 Arrays Chapter 7 - Visual Basic Schneider

  2. Outline and Objective • Arrays in Visual Basic • One- dimensional arrays • Control arrays • Two-dimensional arrays • Searching • Sorting Chapter 7 - Visual Basic Schneider

  3. Array verses Simple Variable • Simple variable is used to store a single value. • Array variable is used to represent many values of the same type with one variable name. Chapter 7 - Visual Basic Schneider

  4. Elements of an Array • Array Name: A valid variable name for the structure. • Subscript or Index : A value that refers to a particular array element. • Element: An individual data item within an array. Chapter 7 - Visual Basic Schneider

  5. Array Declaration • Syntax Dim arrayname ( 1 To n) As VarType Chapter 7 - Visual Basic Schneider

  6. Examples of arrays • Dim month ( 1 To 12) As String • Dim score (1 To 30) As Single • Dim students (1 To 30 ) As String Chapter 7 - Visual Basic Schneider

  7. The Dim Statement • Used when you need to define an array . • The Dim statement sets up storage for array. • A Dim statement must occur before the first reference to the array elements. Chapter 7 - Visual Basic Schneider

  8. Initializing an Array Private Sub cmdWhoWon_Click() Dim teamName( 1To 5) As String Dim n As Integer ' Fill array with World Series Winners teamName(1) = "Red Sox" teamName(2) = "Giants" teamName(3) = "White Sox" teamName(4) = "Cubs" teamName(5) = "Cubs" n = Val(txtNumber.Text) picWinner.Print "The "; teamName(n); " won World Series number"; n End Sub Chapter 7 - Visual Basic Schneider

  9. Array teamName() Array Name teamName( 1 To 5) As String Red Sox Giants White Sox Cubs Cubs teamName(1) Index Chapter 7 - Visual Basic Schneider

  10. Initializing an Array by Reading from a File Dim student ( 1 To 30) As String Open “STUDENTS.TXT” For Input As #1 For count = 1 To 30 Input #1, student ( count ) Next count Chapter 7 - Visual Basic Schneider

  11. Adding Rows in an Array Dim score( 1 To 30) As Single, student (1 To 30) As String Open “STUDENT.TXT” For Input As #1 For count = 1 To 30 Input #1, student (count), score (count) Next count sum = 0 For count = 1 To 30 sum = sum + score(count) Next count average = sum/30 Chapter 7 - Visual Basic Schneider

  12. Parallel Arrays • Two arrays are referred to as parallel if subscripted variables having the same subscript are related. Chapter 7 - Visual Basic Schneider

  13. Example of Parallel Arrays Dim nom ( 1 To 8) As String , score ( 1 To 8 ) As Integer Open “SCORE.TXT” For Input As #1 For student = 1 To 8 Input #1, nom(student), score(student) Next student Close #1 Chapter 7 - Visual Basic Schneider

  14. Ordered Array • An array is ordered if its values are in either ascending or descending order. • For string arrays, the ANSI table is used to evaluate the “less than or equal to” condition. Chapter 7 - Visual Basic Schneider

  15. Processing Arrays • Searching successive elements of an array is called Sequential Search. • Also called linear search or serial search. • A Sequential Search examines each element , from the first to the last, until the specified value is found or the end of the array is reached. Chapter 7 - Visual Basic Schneider

  16. Example of Sequential Search (finding the quiz grades greater than 8) Dim quiz ( 1 To 15) As Single For position =1 TO 15 If quiz(position) > 8 THEN picOutput.Print quiz (position) count = count + 1 End If Next Position Chapter 7 - Visual Basic Schneider

  17. Sequential Search • Useful for short lists. • Very inefficient for long lists ( example names in telephone book). • Use binary search if the list is sorted. Chapter 7 - Visual Basic Schneider

  18. Binary Search • In binary search, an ordered array is repeatedly divided in half. The half not containing the target value is ignored. • To use binary search, the data in the array must be arranged in ascending or descending order. Chapter 7 - Visual Basic Schneider

  19. Binary Search Private Sub BinarySearch(corp As String, result As String foundFlag = 0 '1 indicates corp found first = 1 last = numFirmsDo While (first <= last) And (foundFlag = 0) middle = Int((first + last) / 2) Select Case UCase(firm(middle)) Case corp foundFlag = 1 Case Is > corp last = middle - 1 Case Is < corp first = middle + 1 End Select Loop End Sub Chapter 7 - Visual Basic Schneider

  20. Binary Search If foundFlag = 1 Then result = "found” Else result = "not found End If Chapter 7 - Visual Basic Schneider

  21. Merging • A common practice involving arrays is merging two sorted arrays. Chapter 7 - Visual Basic Schneider

  22. The method for merging two sorted arrays : 1.Compare the two first elements of the first and second arrays. A. If one name alphabetically precedes the other, copy it onto the third list and cross it off the original array. B. If the names are the same, copy the name onto the third list and cross out the name from both arrays. 2. Repeat Step 1 with the new name in the array until you reach the end of either array. 3. Copy the names from the remaining array onto the third array. Chapter 7 - Visual Basic Schneider

  23. Passing an Array • An array can be passed to another procedure by reference. Chapter 7 - Visual Basic Schneider

  24. Example of Passing an array: Private Sub cmddisplay_Click() ' Pass array to subprogram and function Dim score(1 To 5) As Integer Call FillArray(score( ) ) picAverage.Cls picAverage.Print ”Average is"; Sum(score( ) ) / 5 End Sub Passing array score Chapter 7 - Visual Basic Schneider

  25. Array Score is passed to a Subprogram Private Sub FillArray(s( ) As Integer) ' Fill array with scores s(1) = 85 s(2) = 92 s(3) = 75 s(4) = 68 s(5) = 84 End Sub This array is pointing to the same location as array score Chapter 7 - Visual Basic Schneider

  26. Array Score is passed to a Function Private Function Sum(s ( ) As Integer) As Integer Dim total As Integer, index As Integer ' Add up scores total = 0 For index = 1 To 5 total = total + s(index) Next index Sum = total End Function Chapter 7 - Visual Basic Schneider

  27. Control Array • A means of constructing arrays of text boxes, labels, and command buttons. • At least one element of a control array must be created when the form is designed. • The remaining elements can be created either during form design, or with the Load statement. Chapter 7 - Visual Basic Schneider

  28. Creating Control Array during Form Design • Add one instance of the desired control to the form • Set the Index property to a number • Set any other properties that will be common to all other elements • Click on the control and then press Ctrl+C • Press Ctrl + V, to create the next element Chapter 7 - Visual Basic Schneider

  29. Control Array • All the properties of the first element are passed to other elements of the control array including the Top and Left properties. • The only property that differs from first element is the Visible property. • The Load statement sets the Visible property to False. Chapter 7 - Visual Basic Schneider

  30. Control Array Event Procedures: • Even though we may have many elements in the txtBox( ) control array, we will have just one txtBox_GotFocus event procedure to deal with. • The value of Index property for the elements of the control array must be passed. Example: Private Sub txtBox_GotFocus (Index As Integer) Chapter 7 - Visual Basic Schneider

  31. Creating Control Array at Run Time: • You can create elements of a control array via Load statement at run time. Chapter 7 - Visual Basic Schneider

  32. Creating Control Array at Run Time: • The standard unit of measurement is called Twip. • To place a new element of a control array, adjust the Top and Left properties during run time. • Set the Visible property to True. Chapter 7 - Visual Basic Schneider

  33. The location and size of a control Chapter 7 - Visual Basic Schneider

  34. Example ( Creating control array during run time) Private Sub Form_Load() Dim i As Integer, monthNames As String monthNames = "FebMarAprMayJunJulAugSepOctNovDec" For i = 1 To 11 Load lblMonth(i) Load txtInfo(i) lblMonth(i).Top = lblMonth(i - 1).Top + txtInfo(0).Height txtInfo(i).Top = txtInfo(i - 1).Top + txtInfo(0).Height lblMonth(i).Caption = Mid(monthNames, 3 * i - 2, 3) lblMonth(i).Visible = True txtInfo(i).Visible = True Next i End Sub

  35. Sorting • A common practice involving arrays is sorting the elements of the array in either ascending or descending order. • You can sort an array in alphabetic order or numeric order. • There are various methods to sort data items. Chapter 7 - Visual Basic Schneider

  36. Types of Sorting • Bubble Sort • Shell Sort Chapter 7 - Visual Basic Schneider

  37. Bubble Sort • The bubble sort involves comparing adjacent elements and swapping the values of those elements when they are out of order. • One complete time through the array is called a pass. Chapter 7 - Visual Basic Schneider

  38. Bubble Sort For passNum = 1 To 4 'Number of passes is 1 less than number of items For index = 1 To 5 - passNum If name(index) > name(index + 1) Then Call SwapData( name( ), index) End If Next index Next passNum Chapter 7 - Visual Basic Schneider

  39. Swapping two variables Private Sub SwapData ( A ( ) As String, index As Integer) Dim temp As String temp = A ( index) A(index ) = A (index + 1) A ( index + 1) = temp End Sub Chapter 7 - Visual Basic Schneider

  40. Bubble Sort • The maximum number of passes necessary to sort the elements in an array is equal to the number of elements in the array less 1. • The minimum number of passes to sort an array may be one. • It works well for a small number of data items. • It is too slow for really long lists. Chapter 7 - Visual Basic Schneider

  41. Shell Sort • Is similar to the bubble sort, but instead of comparing and swapping adjacent elements A(count) and A(count+1), it compares and swaps nonadjacent elements A(count) and A(count + Gap) , where Gap starts considerably greater than 1. Chapter 7 - Visual Basic Schneider

  42. Shell Sort • Gap is set to one-half the length of the array. • After each pass if flag is set to 1, then Gap is halved again for the next pass. • At the end Gap becomes one, and adjacent elements are compared and swapped. Chapter 7 - Visual Basic Schneider

  43. Shell Sort gap = Int(numParts / 2) Do While gap >= 1 Do doneFlag = 1 For index = 1 To numParts - gap If part(index) > part(index + gap) Then Call Swap( part(index), part(index + gap) doneFlag = 0 End If Next index Loop Until doneFlag = 1 gap = Int(gap / 2) 'Halve the length of the gap Loop Chapter 7 - Visual Basic Schneider

  44. Efficiency of Bubble and Shell sort Array ElementsBubble SortShell Sort 5 10 15 15 105 115 25 300 302 30 435 364 100 4950 2638 500 124,750 22,517 Chapter 7 - Visual Basic Schneider

  45. Two-Dimensional Arrays • Store values as a table, grouped into rows and columns. • The first subscript of a two-dimensional array refers to the row and the second subscript to the column. Chapter 7 - Visual Basic Schneider

  46. Declaration of Two-Dimensional Array • Syntax: • Dim arrayname (m1 To n1, m2 To n2) As vartype • Example: • Dim rm (1 To 4, 1 To 4) As Single column row Chapter 7 - Visual Basic Schneider

  47. Example of a two-dimensional array: rm(1 To 4, 1 To 4) Chicago Los Angeles New York Philadelphia Chicago Los Angeles New York Philadelphia rm (2,2) rm (3,4) Chapter 7 - Visual Basic Schneider

  48. 0, 2054, 802, 738 2054, 0, 2786, 2706 802, 2786, 0, 100 738, 2706, 100, 0 Example of Input File Chapter 7 - Visual Basic Schneider

  49. Manipulating a Two-Dimensional Array • Use nested For ….. Next loops to assign or access elements of a two-dimensional array. • Example: For row = 1 To 4 Forcol = 1 To 4 Input #1, rm(row, col) Next col Next row Chapter 7 - Visual Basic Schneider

  50. Example of two-dimensional array Dim a(1 To 4, 1 To 5 ) As Integer Private Sub cmdDisplay_Click() For j = 1 To 4 For k = 1 To 5 a (j, k) = (j - k) * j picOutput.Print a(j,k); Next k picOutput.Print Next j End Sub Chapter 7 - Visual Basic Schneider

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