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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 or Subscripted variable: An individual data item within an array. Chapter 7 - Visual Basic Schneider

  5. Array Declaration • SyntaxDim arrayName(m To n) As varTypewhere m and n are integers • Examples • Dim month(1 To 12) As String • Dim population(1987 To 2003) As Single • Dim students(1 To 30) As String Chapter 7 - Visual Basic Schneider

  6. The Dim Statement • Used to declare an array • A Dim statement must occur before the first reference to the array elements. Chapter 7 - Visual Basic Schneider

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

  8. Array teamName() Array Name Dim teamName( 1 To 4) As String Red Sox Giants White Sox Cubs teamName(1) Index Chapter 7 - Visual Basic Schneider

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

  10. Adding Up Elements in an Array Dim score(1 To 30) As Single, student(1 To 30) As String Dim count As Integer, average as Integer, sum As Integer 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

  11. Parallel Arrays • Two arrays are said to be parallel if subscripted variables having the same subscript are related. Chapter 7 - Visual Basic Schneider

  12. Example of Parallel Arrays Dim nom(1 To 8) As String, score(1 To 8) As Integer Dim student 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

  13. Form_Load() • Use it to Initialize Form Level variables • Use it to initialize Form Level Arrays • Use it to Dim Dynamic Arrays Chapter 7 - Visual Basic Schneider

  14. Dynamic Arrays • ReDim can only be used inside a procedure • Defines or dimensions a Dynamic array • ReDim arrayName (m To n) as varType • m, n can be a variables or expressions • Use Dim in General Declaration as • Dim arrayName() as varType • Cannot be used till you ReDim it in a procedure Chapter 7 - Visual Basic Schneider

  15. 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

  16. Merging two ordered 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

  17. Passing an Array • An array can be passed to another procedure by reference. • Example: Private Sub cmddisplay_Click() 'Pass array to subprogram and function Dim score(1 To 5) As Integer Call FillArray(score()) picAverage.Print ”Average is"; Sum(score()) / 5 End Sub Chapter 7 - Visual Basic Schneider

  18. Array Score is passed to a Sub procedure 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

  19. 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

  20. Sorting • A common practice involving arrays is to sort the elements of the array in either ascending or descending order. • Sorting techniques • Bubble Sort • Shell Sort Chapter 7 - Visual Basic Schneider

  21. 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 an array is called a pass. Chapter 7 - Visual Basic Schneider

  22. Bubble Sort For passNum = 1 To 4 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

  23. 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

  24. Bubble Sort • The number of passes used to sort the elements in an array is equal to the number of elements in the array less 1. • Bubble sort works well for sorting small arrays, but is often too slow for very large ones. Chapter 7 - Visual Basic Schneider

  25. Shell Sort • Similar to the bubble sort • Instead of comparing and swapping adjacent elements A(count) and A(count+1), Shell sort compares and swaps non-adjacent elements A(count) and A(count + Gap), where Gap starts at roughly half the size of the array Chapter 7 - Visual Basic Schneider

  26. Shell Sort • At the end of each pass, if no elements have been swapped in the current pass, then Gap is halved for the next pass. • Eventually, Gap becomes one, and adjacent elements are compared and swapped as necessary. Chapter 7 - Visual Basic Schneider

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

  28. Efficiency of Bubble and Shell sort (average number of comparisons) Array ElementsBubble SortShell Sort 5 10 15 15 105 115 25 300 302 30 435 364 100 4,950 2,638 500 124,750 22,517 Chapter 7 - Visual Basic Schneider

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

  30. Searching Arrays • The process of finding the position of a value in an array is called searching • A sequential search examines each element, beginning with the first, until the specified value is found or the end of the array is reached Chapter 7 - Visual Basic Schneider

  31. Example of a Sequential Search Dim nom(1 To 15) As String, index As Integer, foundFlag As Boolean Do While (index <= 15) And (Not foundFlag) If nom(index) = “Fred” Then foundFlag = True Else index = index + 1 End If Loop picOutput.Print “Fred is located at position“; index Chapter 7 - Visual Basic Schneider

  32. Sequential Search • Useful for small arrays. • Very inefficient for large arrays (for example, names in a telephone book). • For any size array, if the array is ordered, the more efficient binary search can be used. Chapter 7 - Visual Basic Schneider

  33. Binary Search • In a binary search, an ordered array is repeatedly divided in half. The half not containing the target value is ignored. Chapter 7 - Visual Basic Schneider

  34. Binary Search Private Sub BinarySearch(corp As String, result As String) Dim foundFlag As Boolean, first as Integer, last as Integer first = 1 last = numFirms Do While (first <= last) And (foundFlag = False) middle = Int((first + last) / 2) Select Case UCase(firm(middle)) Case corp foundFlag = True Case Is > corp last = middle - 1 Case Is < corp first = middle + 1 End Select Loop ‘continued on next slide Chapter 7 - Visual Basic Schneider

  35. Binary Search (continued) If foundFlag = True Then result = "found” Else result = "not found End If End Sub Chapter 7 - Visual Basic Schneider

  36. 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

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

  38. 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

  39. 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

  40. 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

  41. Examples: How many elements? • Dim rm(1 To 4) As Single • Dim rm(4, 1 To 4) As Single • Dim rm(1 To 4, 1 To 4) As Single • Dim rm(4, 1 To 4) As Single • Dim rm(0 To 3, 97 To 100) • Dim rm (-5 To -3, 5 To 5) • Dim (-3 To -5) Chapter 7 - Visual Basic Schneider

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