Chapter 7 Arrays Chapter 7 Visual Basic Schneider

Chapter 7 Arrays Chapter 7 - Visual Basic Schneider 1

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

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 3

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 4

Array Declaration • Syntax Dim array. Name(m To n) As var. Type 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 Chapter 7 - Visual Basic Schneider 5

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 6

Initializing an Array Private Sub cmd. Who. Won_Click() Dim team. Name(1 To 4) As String Dim n As Integer team. Name(1) = "Red Sox" team. Name(2) = "Giants" team. Name(3) = "White Sox" team. Name(4) = "Cubs" n = Val(txt. Number. Text) pic. Winner. Print "The "; team. Name(n); " won World Series #"; n End Sub Chapter 7 - Visual Basic Schneider 7

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

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 9

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 10

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

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 12

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 13

Dynamic Arrays • Re. Dim can only be used inside a procedure • Defines or dimensions a Dynamic array – Re. Dim array. Name (m To n) as var. Type – m, n can be a variables or expressions • Use Dim in General Declaration as – Dim array. Name() as var. Type – Cannot be used till you Re. Dim it in a procedure 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

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 16

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 Fill. Array(score()) pic. Average. Print ”Average is"; Sum(score()) / 5 End Sub Chapter 7 - Visual Basic Schneider 17

Array Score is passed to a Sub procedure Private Sub Fill. Array(s() As Integer) ' Fill array with scores s(1) = 85 s(2) = 92 This array is pointing to the same s(3) = 75 location as array score s(4) = 68 s(5) = 84 End Sub Chapter 7 - Visual Basic Schneider 18

Array score is passed to a Function Private Function Sum(s() 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 19

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 20

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 21

Bubble Sort For pass. Num = 1 To 4 For index = 1 To 5 - pass. Num If name(index) > name(index + 1) Then Call Swap. Data(name(), index) End If Next index Next pass. Num Chapter 7 - Visual Basic Schneider 22

Swapping two variables Private Sub Swap. Data (A() As String, index As Integer) Dim temp As String temp = A(index) = A(index + 1) = temp End Sub Chapter 7 - Visual Basic Schneider 23

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 24

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 25

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 26

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

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

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 29

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 30

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

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 32

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 33

Binary Search Private Sub Binary. Search(corp As String, result As String) Dim found. Flag As Boolean, first as Integer, last as Integer first = 1 last = num. Firms Do While (first <= last) And (found. Flag = False) middle = Int((first + last) / 2) Select Case UCase(firm(middle)) Case corp found. Flag = 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 34

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

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 36

Declaration of Two-Dimensional Array • Syntax: – Dim array. Name(m 1 To m 2, n 1 To n 2) As var. Type • Example: – Dim rm(1 To 4, 1 To 4) As Single column row Chapter 7 - Visual Basic Schneider 37

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

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

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 Chapter 7 - Visual Basic Schneider 40

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(0 To 3, 97 To 100) Dim rm (-5 To -3, 5 To 5) Dim (-3 To -5) Chapter 7 - Visual Basic Schneider 41