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

Array verses Simple Variable Simple variable is used to store a single value. Ex:Dim X as Integer 0 X 0000 X(1) X(2) X(3) X(4) X 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) 0000 X(1) X(2) X(3) X(4) X

5 Array Grade() Dim Grade( 1 To 4) As Integer Grade(1 ) Array Name Index

6 Array Declaration Syntax Dim arrayName(m To n) As varType where 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(1) Month(2)Month(3)Month(4)Month(5) 000 stdID(1) stdID(2)stdID(3) This statement creates array of 3 integer elements

8 The Dim Statement Used to declare an array A Dim statement 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) Grade(2) Grade(3) Grade(4) Grade(5)

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 0000 x(0) x(1)x(2)x(3) 0000 x(0) x(1)x(2)x(3)

Example Dim Y(-1 to 2) As Integer 0000 Y(-1) Y(0)Y(1)Y(2) Dim Z(-2 to 0) As Single 000 Z(-2) Z(-1)Z(0)

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

Example Dim Y(2) As Integer Dim Z(2 to 2) As Single 0 Z(2) 000 Y(0)Y(1)Y(2)

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

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

Example

m, n must be a constant

Chapter 7 - Visual Basic Schneider 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 Ali Ahmed.. Huda STUDENTS.txt AliAhmed Huda Student 12 30

Chapter 7 - Visual Basic Schneider20 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 AliAhmed Huda Student Score 1230 Ali 22 Ahmed 19.. Huda 25 Students.txt

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. AliAhmed Huda Student Score 1230 Ali 22 Ahmed 19.. Huda 25 Students.txt

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

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 Schneider24

Dynamic Arrays Defines or dimensions a Dynamic array ReDim arrayName (m To n) as varType m, n can be a variables or expressions ReDim can only be used inside a procedure Use Dim in General Declaration as Dim arrayName() as varType Cannot be used till you ReDim it in a procedure 25

About ReDim: Dim A(5) as integer ReDim A(7)  Invalid Redim A(5) Dim A(7)  Invalid ReDim A(5) ReDim A(7) as integer  Invalid

About ReDim: Dim A() as integer Redim A(7) as integer  Valid X=1 ReDim A(x) as integer  Valid Dim A() as integer A(1)= 5  Invalid

28 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(0) x(1)x(2)x(3)

Chapter 7 - Visual Basic Schneider29 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.

BasemWalid Student1 1 2 AhmedMajid Student2 12 Student3 12 Osama 34 Nabeel Ali 34 Salim Merging two ordered arrays SalimAhmedAliBasemMajidNabeelOsamaWalid

15 0 Arr1(1) Arr1(2) Arr1(3) Arr2(1) Arr2(2) Arr2(3) Is 5 <= 10  T: Copy the number 5 from Arr1 to Arr3, cross the number 5 off the original array (Arr1) Arr Is 13 <=10  F: Copy the number 10 from Arr2 to the Arr3, cross it off the original array Repeat the step with the new number in the array until you reach the end of either array. Copy the numbers from the remaining array onto the third array. Merging two ordered arrays 026

32 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

33 Example

36 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

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

38 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

39 Efficiency of Bubble and Shell sort (average number of comparisons) Array Elements Bubble Sort Shell Sort ,9502, ,75022,517

40 Searching Arrays The process of finding the position of a value in an array is called searching For example Search(-10) = 3 Search (3) = 1 Search (5) = Not Found stdID(1) stdID(2)stdID(3)

41 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(4) = stdID(1) stdID(2)stdID(3) 1014 stdID(4) stdID(5)stdID(6) 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

42 The Sequential Search Dim nom(1 To 6) As Integer Dim index As Integer, foundFlag As Boolean Do While (index <= 6) And (Not foundFlag) If nom(index)= 4 Then foundFlag = True Else index = index + 1 End If Loop picOutput.Print “The Number 4 is located at position“; index

43 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.

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

8 3 7 stdID(1) stdID(2) stdID(3) stdID(4) stdID(5) stdID(6) stdID(7) stdID(8) stdID(9) stdID(10) stdID(11) stdID(12) Search(78) = First = 1 Last = 12 Middle = Int((first + last) / 2) Middle = Int(1+12)/2=6 Middle=6 Is 78 = 77  false Is 78< 77  false Is 78>77  First =7 First = 7 Middle = Int(7+12)/2=9 Middle= 9 Is 78 = 81  false Is 78 < 81  Last = 8 Last = 8 Middle = Int(7+8)/2=7 Middle = 7 Is 78 = 78  true STOP Search(78) =7

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(m1 To m2, n1 To n2) As varType Example: Dim rm(1 To 4, 1 To 4) As Single column row

48 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

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

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

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

Examples: How many elements? Dim arr(4, 1 To 4) As Single  20 elements

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

Examples: How many elements? Dim xy(-3 To -5) As Integer 54  ERROR

Examples 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

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