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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

  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.

  4. Parts of an Array • Array Name: A valid variable name for the structure. • Subscript or Index : A value that refers to a particular array element. • Which must be defined as an integer • Element: An individual data item within an array.

  5. Array Declaration • Syntax Dim arrayname ( 1 To n) As VarType

  6. Examples of arrays • Dim strMonth ( 1 To 12) As String • Dim intScore (1 To 30) As Integer • Dim strStudents (1 To 30 ) As String • Dim strStudents (30) as String • Will the last two arrays hold the same number of elements?

  7. The Dim Statement • Used when you need to define an array . • The Dim statement sets up storage area in memory for the array. • ******A Dim statement mustoccur before the first reference to the array elements.*******

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

  9. Array strTeamName() Array Name strTeamName( 1 To 5) As String Red Sox Giants White Sox Cubs Cubs strTeamName(1) Index

  10. Initializing an Array by Reading from a File Dim strCereal ( 1 To 30) As String Dim strMfr (1 to 30) as string Dim intCount as Integer Open “Cereal.txt” For Input As #1 For intCount = 1 To 70 Input #1, strCereal ( intCount ), strMfr(intcount) Next intCount

  11. Parallel Arrays, Use the same index Dim intScore( 1 To 30) As Single, strStudent (1 To 30) As String Dim sngAverage as Single , intSum as Integer Open “StudentFile.txt” For Input As #1 intSum = 0 For intCount = 1 To 30 Input #1, strStudent (intCount), intScore (intCount) intSum = intSum + intScore(intCount) Next intCount sngAverage = sum/30

  12. Parallel Arrays • Arrays are parallel if subscripted variables have the same subscript and are related.

  13. Example of Parallel Arrays Dim strName ( 1 To 8) As String Dim intScore ( 1 To 8 ) As Integer Dim intStudent as Integer Open “Score.txt” For Input As #1 For intStudent = 1 To 8 Input #1, strName(intStudent), intScore(intStudent) Next intStudent Close #1 intStudent is the ?

  14. RateOfReturn 1 2 3 4 5 Summing the Elements of an Array in a For-Next Loop For K = 1 To 5 Sum = Sum + RateOfReturn(K) Next K When K = 1,the statement is Sum = Sum + RateOfReturn(1)

  15. Ordered Array • An array is ordered if its values are in either ascending or descending order. • For string arrays, the ASCI table is used to evaluate the “less than or equal to” condition. • Is “Ace” > “Nine”

  16. Processing Arrays • Searching successive elements of an array is called a 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.

  17. Example of Sequential Search (finding the sngQuiz grades greater than 8) Dim sngQuiz ( 1 To 15) As Single Dim intPosition as Integer For intPosition =1 TO 15 If sngQuiz(intPosition) > 8 THEN picOutput.Print sngQuiz (intPosition) intCount = intCount + 1 End If Next intPosition picOutput.Print “Number of Grades found =“ intCount

  18. Sequential Search • Useful for short lists. 1 to 500 elements • Very inefficient for long lists ( example names in telephone book). • Use binary search if the list is sorted.

  19. 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. (Sorted)

  20. SampleArray Binary search with a target value of 76 Lower bound 1 2 3 4 5 6 7 8 9 10 11 Begin search at midpoint of array (element 6) mid = (lowerbound + upperbound) \ 2 If value in element 6 is = 76, then we are done. If value in element 6 is < 76, then set lower bound to mid + 1. Otherwise, set upper bound to mid - 1. Upper bound

  21. 1 2 3 4 5 6 7 8 9 10 11 Lower bound SampleArray Binary search with a target value of 76 Now recompute the midpoint mid = (lowerbound + upperbound) \ 2 If value in element 9 is = 76, then we are done. If value in element 9 is < 76, then set lower bound to mid + 1. Otherwise, set upper bound to mid - 1. Upper bound

  22. SampleArray Binary search with a target value of 76 1 2 3 4 5 6 7 8 9 10 11 Now recompute the midpoint mid = (lowerbound + upperbound) \ 2 Recall that the arithmetic operator (\) means integer division (15 \ 2 = 7) Lower bound Upper bound We have now found the value we were searching for. If we had not found the value, we would have adjusted the lower or upper bound as appropriate and then computed a new midpoint.

  23. Dim strFirm(1 to 100) as String Private Sub BinarySearch(strCorp As String, strResult As String) Dim first as Integer, middle as Integer, last as Integer Dim Found as Integer, intFirms as Integer, strFirm as String Found = 0 '1 indicates strCorp has been found first = 1; last = intFirmsDo While (first <= last) And (foundFlag = 0) middle = Int((first + last) / 2) Select Case UCase(strFirm(middle)) Case strCorp Found = 1 Case Is > strCorp last = middle - 1 Case Is < strCorp first = middle + 1 End Select Loop End Sub

  24. Passing an Array • An array can be passed to another procedure by Reference.

  25. Example of Passing an array: Private Sub cmddisplay_Click() ' Pass array to Sub Procedure and function Dim intScore(1 To 5) As Integer Call FillArray(intScore( ) ) picAverage.Cls picAverage.Print ”Average is"; Sum(intScore( ) ) / 5 End Sub Passing array intScore User Function

  26. Array intScore is passed to a Subprogram Private Sub FillArray(s( ) As Integer) ' Fill array with intScores 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 intScore

  27. Array intScore is passed to a Function Private Function Sum(s ( ) As Integer) As Integer Dim intTotal As Integer, intIndex As Integer ' Add up intScores intTotal = 0 For intIndex = 1 To 5 intTotal = intTotal + s(intIndex) Next intIndex Sum = intTotal End Function

  28. 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.

  29. 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

  30. 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.

  31. 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 intIndex property for the elements of the control array must be passed. Example: Private Sub txtBox_GotFocus (intIndex As Integer)

  32. Creating Control Array at Run Time: • You can create elements of a control array by using the Load statement at run time.

  33. 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.

  34. The location and size of a control Set the index property to zero!

  35. 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) LoadtxtInfo(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

  36. 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.

  37. Types of Sorting • Bubble Sort • Shell Sort

  38. 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.

  39. Bubble Sort ForpassNum = 1 To 4 'Number of passes is 1 less than number of items For intIndex = 1 To 5 - passNum If name(intIndex) > name(intIndex + 1) Then Call SwapData( name( ), intIndex) End If Next intIndex Next passNum

  40. Swapping two variables Private Sub SwapData ( A ( ) As String, intIndex As Integer) Dim temp As String temp = A ( intIndex) A(intIndex ) = A (intIndex + 1) A ( intIndex + 1) = temp End Sub

  41. 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.

  42. Shell Sort • Is similar to the bubble sort, but instead of comparing and swapping adjacent elements A(intCount) and A(intCount+1), it compares and swaps nonadjacent elements A(intCount) and A(intCount + Gap) , where Gap starts considerably greater than 1.

  43. 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.

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

  45. 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

  46. 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.

  47. Declaration of Two-Dimensional Array • Syntax: • Dim arrayname (r1 To n1, c2 To n2) As vartype • Example: • Dim rm (1 To 4, 1 To 4) As Single column row

  48. 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)

  49. 0, 2054, 802, 738 2054, 0, 2786, 2706 802, 2786, 0, 100 738, 2706, 100, 0 Example of Input File

  50. 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

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