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Chapter 7

Chapter 7. Arrays. Outline and Objective. In this chapter we will Learn about arrays One-dimensional arrays Two-dimensional arrays Learn about searching Learn about sorting. Array verses Simple Variable. Simple variable is used to store a single value. Ex: Dim X as Integer. X(2).

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Chapter 7

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  1. Chapter 7 Arrays

  2. Outline and Objective In this chapter we will Learn about arrays One-dimensional arrays Two-dimensional arrays Learn about searching Learn about sorting

  3. Array verses Simple Variable Simple variable is used to store a single value. Ex: Dim X as Integer X(2) X(4) X(1) X(3) 0 0 0 0 X X 0 • Array variable is used to represent many values of the same type with one variable name. • Ex: Dim X(1 to 4) as Integer

  4. Elements of an Array Array Name: A valid variable name for the structure. (X) Subscript or Index : A value that refers to a particular array element. X(1) Element or Subscripted variable: An individual data item within an array. Ex: X(1), X(2) X(2) X(4) X(1) X(3) 0 0 0 0 X

  5. Array Grade() Dim Grade( 1 To 4) As Integer Array Name 35 10 5 25 Grade(1) Index 5

  6. Array Declaration SyntaxDimarrayName(mTon) AsvarTypewhere m and n are integers Declaring arrays - specify: name type of array number of elements

  7. Array Declaration Examples Dim month(1 To 5) As String Dim stdID(1 To 3) As Integer Month(2) Month(3) Month(4) Month(5) Month(1) This statement creates array of 3 integer elements stdID(2) stdID(3) stdID(1) 0 0 0 7

  8. The Dim Statement Used to declare an array A Dimstatement must occur before the first reference to the array elements.

  9. Initializing an Array Private Sub cmdExample_Click() Dim Grade(1 To 5) As Integer Grade (1) = 5 Grade (2) = 30 Grade (3) = 25 Grade (4) = 15 Print Grade(1) + Grade(3) Print Grade(4) Print Grade(1+2) End Sub Grade(1) 5 0 Grade(2) 0 30 0 25 Grade(3) 0 15 Grade(4) 0 Grade(5) 30 15 25

  10. The statements Dim arrayName(0 to n) as VarType can be placed by the statement Dim arrayName(n) as VarType • Ex. Dim X(0 To 3) As Integer • Ex. Dim X(3) As Integer x(1) x(2) x(3) x(0) 0 0 0 0 x(1) x(2) x(3) x(0) 0 0 0 0

  11. Example • Dim Y(-1 to 2) As Integer Y(0) Y(1) Y(2) Y(-1) 0 0 0 0 • Dim Z(-2 to 0) As Single Z(-1) Z(0) Z(-2) 0 0 0

  12. Example • Dim Y(-1 to -1) As Integer Y(-1) 0 • Dim Z(2 to 2) As Single Z(2) 0

  13. Example • Dim Y(2) As Integer Y(0) Y(1) Y(2) 0 0 0 • Dim Z(2 to 2) As Single Z(2) 0

  14. Example • Dim Y(10 to 1) As Integer • Compile ERROR • Dim Z(2 to -2) As Single • Compile ERROR

  15. Example i = 5 Dim arr(i) As Integer • Invalid statements

  16. Example 5 0 0

  17. Example

  18. Example m, n must be a constant

  19. 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 STUDENTS.txt Ali Ahmed .. .. .. Huda 1 2 30 Student Ali Ahmed Huda 19 Chapter 7 - Visual Basic Schneider

  20. 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 Students.txt Ali 22 Ahmed 19 .. .. .. Huda 25 1 2 30 Student Ali Ahmed Huda 1 2 30 Score 19 22 25 Chapter 7 - Visual Basic Schneider 20

  21. Dim XY (1 To 5) As Integer Dim count As Integer count = 1 Open “DATA.TXT” For Input As #1 Do While NOT EOF(1) Input #1, XY(count) count= count + 1 Loop Example

  22. Parallel Arrays Two arrays are said to be parallel if subscripted variables having the same subscript are related. Students.txt Ali 22 Ahmed 19 .. .. .. Huda 25 1 2 30 Student Ali Ahmed Huda 1 2 30 Score 19 22 25

  23. Example of Parallel Arrays Dim nom(1 To 3) As String, score(1 To 3) As Integer Dim student As Integer Open “SCORE.TXT” For Input As #1 For student = 1 To 3 Input #1, nom(student), score(student) Next student Close #1

  24. Form_Load() Chapter 7 - Visual Basic Schneider 24 Use it to Initialize Form Level variables Use it to initialize Form Level Arrays Use it to Dim Dynamic Arrays

  25. 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. x(1) x(2) x(3) x(0) 10 15 30 40

  26. Passing an Array An array can be passed to another procedure (Only) by reference. the name of the array, followed by an empty set of parenthesis, must appear as an argument in calling statement, and an array variable name of the same type must appear as corresponding parameters in the procedure definition of the procedure that is to receiver the array Parenthesis are optional with the arguments

  27. Example 27

  28. Example

  29. Example

  30. Sorting A common practice involving arrays is to sort the elements of the array in either ascending or descending order. A sort is an algorithm for ordering an array Sorting techniques Bubble Sort Shell Sort Ascending : lowest to highest descending: highest to lowest

  31. Bubble Sort The bubble sort involves comparing adjacent elements and swapping the values of those elements when they are out of order.

  32. 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

  33. 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

  34. Searching Arrays The process of finding the position of a value in an array is called searching For example Note: search(x) means search about x Search(-10) = 3 Search (3) = 1 Search (5) = Not Found stdID(2) stdID(3) stdID(1) 3 6 -10

  35. Searching techniques A sequential search examines each element, beginning with the first, until the specified value is found or the end of the array is reached For example Search about number 4  Search(4) = 5 stdID(2) stdID(3) stdID(5) stdID(6) stdID(1) stdID(4) 3 7 -10 1 4 10 Is 4 = 3: False Is 4 = 7: False Is 4 = -10: False Is 4 = 1: False Is 4 = 4: True The number of comparisons to find the value 4 = 5 35

  36. 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.

  37. Binary Search In a binary search, an ordered array is repeatedly divided in half. The half not containing the target value is ignored.

  38. 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.

  39. 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

  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

  41. Examples: How many elements? 97 98 99 0 0 0 0 0 0 0 1 0 0 0 2 0 0 0 3  12 elements 41 Dim rm(0 To 3, 97 To 99) As Integer

  42. What is the output of: Dim (1 to 3 , 1 to 3) as integer For i = 1 To 3 For j = 1 To 3 a(i, j) = i * j Print a(i, j); Next j Print Next i The output is 1 2 3 2 4 6 3 6 9

  43. Examples: How many elements? rm(2) rm(3) rm(1) rm(4) 0 0 0 0  4 elements Dim rm(1 To 4) As Single

  44. Examples: How many elements? 1 2 3 4 0 0 0 0 0 0 0 0 0 1 0 0 0 0 2 0 0 0 0 3 4 0 0 0 0  20 elements 44 Dim arr(4, 1 To 4) As Single

  45. Examples: How many elements? 5 -5 -4 -3  3 elements 45 Dim arr(-5 To -3, 5 To 5) As String

  46. Examples: How many elements?  ERROR 46 Dim xy(-3 To -5) As Integer

  47. Examples 97 98 99 0 0 0 0 0 20 0 1 0 0 0 2 0 0 15 3 47 • Dim arr(0 To 3, 97 To 99) As Integer • arr(1,98)= 20 • arr(3,99)=15 • arr(0,2)  Error: index out of range • arr(-1,99)  Error: index out of range

  48. Examples 97 98 99 0 0 0 0 10 10 10 1 0 0 0 2 0 0 0 3 48 Dim arr(0 To 3, 97 To 99) As Integer For i = 97 To 99 arr(1,i) = 10 Next i

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