1. Chapter 7 - Visual Basic Schneider1 Chapter 7 Arrays

2. Chapter 7 - Visual Basic Schneider2 Outline and Objective Arrays in Visual Basic One-dimensional arrays Control arrays Two-dimensional arrays Searching Sorting

3. Chapter 7 - Visual Basic Schneider3 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. Chapter 7 - Visual Basic Schneider4 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.

5. Chapter 7 - Visual Basic Schneider5 Array Declaration Syntax Dim arrayName(m To n) As varType where 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

6. Chapter 7 - Visual Basic Schneider6 The Dim Statement Used to declare an array A Dim statement must occur before the first reference to the array elements.

7. Chapter 7 - Visual Basic Schneider7 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

8. Chapter 7 - Visual Basic Schneider8 Array teamName() Dim teamName( 1 To 4) As String Red Sox Giants White Sox Cubs team N ame(1 ) Array Name Index

9. Chapter 7 - Visual Basic Schneider9 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

10. Chapter 7 - Visual Basic Schneider10 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

11. Chapter 7 - Visual Basic Schneider11 Parallel Arrays Two arrays are said to be parallel if subscripted variables having the same subscript are related.

12. Chapter 7 - Visual Basic Schneider12 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

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 Schneider13

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 Schneider14

15. Chapter 7 - Visual Basic Schneider15 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.

16. Chapter 7 - Visual Basic Schneider16 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.

17. Chapter 7 - Visual Basic Schneider17 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

18. Chapter 7 - Visual Basic Schneider18 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

19. Chapter 7 - Visual Basic Schneider19 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

20. Chapter 7 - Visual Basic Schneider20 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

21. Chapter 7 - Visual Basic Schneider21 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.

22. Chapter 7 - Visual Basic Schneider22 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

23. Chapter 7 - Visual Basic Schneider23 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

24. Chapter 7 - Visual Basic Schneider24 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.

25. Chapter 7 - Visual Basic Schneider25 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

26. Chapter 7 - Visual Basic Schneider26 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.

27. Chapter 7 - Visual Basic Schneider27 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

28. Chapter 7 - Visual Basic Schneider28 Efficiency of Bubble and Shell sort (average number of comparisons) Array Elements Bubble Sort Shell Sort 51015 15105115 25300302 30435364 1004,9502,638 500124,75022,517

29. Chapter 7 - Visual Basic Schneider29 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.

30. Chapter 7 - Visual Basic Schneider30 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

31. Chapter 7 - Visual Basic Schneider31 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

32. Chapter 7 - Visual Basic Schneider32 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.

33. Chapter 7 - Visual Basic Schneider33 Binary Search In a binary search, an ordered array is repeatedly divided in half. The half not containing the target value is ignored.

34. Chapter 7 - Visual Basic Schneider34 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

35. Chapter 7 - Visual Basic Schneider35 Binary Search (continued) If foundFlag = True Then result = "found” Else result = "not found End If End Sub

36. Chapter 7 - Visual Basic Schneider36 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.

37. Chapter 7 - Visual Basic Schneider37 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

38. Chapter 7 - Visual Basic Schneider38 Example of a two-dimensional array: rm(1 To 4, 1 To 4) Chicago Los Angeles New York Philadelphia ChicagoLos AngelesNew YorkPhiladelphia rm (2,2) rm (3,4)

39. Chapter 7 - Visual Basic Schneider39 Example of Input File 0, 2054, 802, 738 2054, 0, 2786, 2706 802, 2786, 0, 100 738, 2706, 100, 0

40. Chapter 7 - Visual Basic Schneider40 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 For col = 1 To 4 Input #1, rm(row, col) Next col Next row

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 Schneider41