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ArrayArray

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• Learn about arrays.• Explore how to declare and manipulate data into arrays.• Understand the meaning of “array index out of bounds.”• Become familiar with the restrictions on array processing. • Discover how to pass an array as a parameter to a method.• Discover how to manipulate data in a two- dimensional array.• Learn about multidimensional arrays.

Chapter Objectives

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Why do need array

• Let consider the following problem

How do we write Java program that read five numbers, find the sum, and prints the numbers in reverse order

• Normally, we need to store all the numbers in 5 variables before we can print it in reverse order• Let see the following code. (next slide).

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import java.util.*;public class ReverseOrder{

public static void main(String [] args){int item0, item1, item2, item3, item4;int sum;Scanner input = new Scanner(System.in);

System.out.println("Enter five integers one number per line");

item0 = input.nextInt();item1 = input.nextInt();

item2 = input.nextInt();item3 = input.nextInt();item4 = input.nextInt();

sum = item0 + item1 + item2 + item3 + item4;

System.out.println("The sum of the numbers = " + sum);System.out.println("The numbers in reverse order are: ");System.out.println(item4 + " " + item3 + " " + item2 + " " + item1 +

" " + item0);}

}

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• We need 5 variables to hold the data

• What happen if we want to read 100 (or more) numbers and print them in reverse order.

• So, we need 100 variables to hold all data. (item0, item1, item2, item3, item4, item5,…100)

• For large of data, this code is not desirable.

• We need an ARRAY.

continue

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What is Array

• A structured data type with a fixed number of components.

• Every component is of the same type.

• Components are accessed using their relative positions in the array.

• Types of array- One-Dimensional array- Two-Dimensional array- Multi Dimensional array

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One-Dimensional Arrays

• Syntax to declare an array:

<dataType>[] <arrayName> = new <dataType>[intExp]; Or

<dataType> <arrayName>[]= new <dataType>[intExp];

1. dataType : a type of data will be store in array or component type

2. arrayName : a reference variable for array 3. intExp : size of an array (> 0)

• Syntax to access an array component:

- arrayName[indexExp]

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Example

int[] num = new int[5]; or int num[] = new int[5];

• This statement declare and creates the array num of 5 components.

• Each component is int data type

• The components are num[0], num[1], num[2], num[3], num[4]

• The value in square bracket [ ] is call index and it start at 0

num

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continue

elementindex

In java, [ ] is call as array subscripting operator

Items in an array is called elements

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Continue

Array of five integers called test

Array of five characters

called grade

test[0] = 85;test[1] = 98;test[2] = 75;test[3] = 87;test[4] = 68;

grade[0] = ‘B’;grade[1] = ‘C’;grade[2] = ‘B’;grade[3] = ‘A’;grade[4] = ‘C’;

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Assign a value into array

int[] list = new int[10];

Assume the declaration as above.

Statement;

list[3] = 10;

list[6] = 35;

list[5] = list[3] + list[6];

will store 10, 45 and 35 into the array in list[3], list[5] and list[6] respectively. (see next figure)

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Specifying Array Size During Program Execution (dynamic array)

• Array that are created during program execution is called dynamic array• Enables user to specify the size of the array

int arraySize;System.out.print("Enter the size of the array: "); arraySize = input.nextInt();

int[] list = new int[arraySize];

• The system use the value of arraysize to instantiate the object list

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Array Initialization During Declaration

• We also can assign a value into the array during declaration

double[]sales = {12.25, 32.50, 16.90, 23.00, 45.68};

• The values, called initial values, are placed between braces and separated by commas • When declaring and initializing arrays, the size of the array is determined by the number of initial values within the braces.• If an array is declared and initialized simultaneously, we do not use the operator new to instantiate the array object.

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Arrays and the Instance Variable length

• A public instance variable length is associated with each array that has been instantiated. • The variable length contains the size of the array. • The variable length can be directly accessed in a program using the array name and the dot operator.

int[] list = {10, 20, 30, 40, 50, 60};

•This statement creates the array list of six components and initializes the components using the values given.• Here list.length is 6.

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Loops and Arrays

• Loops can be used to process array in several ways:

1. Initialing an array to a specific value 2. Input data into an array 3. Printing an array 4. Find the sum and average of an array 5. Determine the largest element in the array

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1. Initializing an array to a specific value

eg.

to initialize every component of the array sale with a value of 10.00

double[] sales = new double[10];int index;

for (index = 0; index < sales.length;index++) sales[index] = 10.00;

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2. Input data into an array

double[] sales = new double[10];int index;

for (index = 0; index < sales.length;index++) sales[index] = input.nextDouble();

3. Printing an array

double[] sales = new double[10];int index;

for(index = 0; index < sales.length;index++)System.out.print(sales[index] + " ");

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4. Find the sum and average of an array

double[] sales = new double[10];int index, sum;double average;

sum = 0;for(index = 0; index < sales.length;index++)

sum = sum + sales[index];

if (sales.length != 0) average = sum / sales.length;else average = 0.0;

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5. Determining the largest element in the array

double[] sales = new double[10];int index, maxIndex;double largestSale;

maxIndex = 0;

for(index = 1; index<sales.length;index++) if (sales[maxIndex] < sales[index]) maxIndex = index;largestSale = sales[maxIndex];

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continue

Suppose the array sales is as figure 9.5

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Array Index Out of Bounds

• An array is in bounds if: 0 <= index <= arraySize – 1

• An array is in out bounds if:

index < 0 or index > arraySize

If an array is out of bounds;

i. ArrayIndexOutOfBoundsException exception is thrown. ii. The program will terminates with an appropriate error message

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example

Consider the following declaration:

double[] num = double[10];int i;

• The component num[i] is valid if i = 0, 1, 2….9

• When i < 0 or i >= 10, the component num[i] is

invalid (the index is out of bounds)

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Consider the following loopsfor (i = 0; i <= 10; i++) list[i] = 5;

• When i = 10; list[i] = list[10] = 5;• The program tries to access list[10] but does not exist

• We say the index is out of bound

5

5

5

5

5

5

5

5

5

5

list[0]

list[1]

list[2] list[3]

list[4]

list[5]

list[6]

list[7]

list[8]

list[9]

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Manipulate data into arrays

• Searching a value

• Calculation

• Reverse element

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Searching a value

• Eg.- A method to search an array of integer

• The Search method return the location of

the first array element equal to the search value

int Search (int[ ] num, int search value){ int location; for (i=0; i =num.length; i++) if(num[i] = = search Value) location = i; return location;}

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• Assume the num values as below: int[] num = {10,20,30,40,50,60,70,80,90,100}

• If searchValue is 60, the

method will return 5 to Search

method.

40

10

20

30

50

60

70

80

90

100

num[0]

num[1]

num[2]

num[3]

num[4]

num[5]

num[6]

num[7]

num[8]

num[9]

Location, i

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Calculation in array

• Eg:- add a number from Array1 and Array2,

and store the total in Array3

• Assume Array1, Array2 and Array3

declarations as below:

int[ ] Array1 = {10,20,30,40,50,60,70,80,90,100};

int[ ] Array2 = {11,22,33,44,55,66,77,88,99,110};

int[ ] Array3 = new int[10];

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+

10 21

42

63

84

:

:

:

:

:

:

11

20

30

40

50

60

70

80

90

100

22

33

44

55

66

77

88

99

110

Array[0]

Array[1]

Array[2]

Array[3]

Array[4]

Array[5]

Array[6]

Array[7]

Array[8]

Array[9]

Array[0]

Array[1]

Array[2]

Array[3]

Array[4]

Array[5]

Array[6]

Array[7]

Array[8]

Array[9]

Array[0]

Array[1]

Array[2]

Array[3]

Array[4]

Array[5]

Array[6]

Array[7]

Array[8]

Array[9]

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public static void ArraySum(){ int[] Array1 = {10,20,30,40,50,60,70,80,90,100}; int[] Array2 = {11,22,33,44,55,66,77,88,99,110}; int[] Array3 = new int[10]; int i;

for (i=0; i < 10; i++) {

Array3[i] = Array1[i] + Array2[i];

System.out.println("Array3["+i+"]=“ +Array3[i]); }}

Output

Array3[0] = 21

Array3[1] = 42

Array3[2] = 63

Array3[3] = 84

Array3[4] = 105

Array3[5] = 126

Array3[6] = 147

Array3[7] = 168

Array3[8] = 189

Array3[9] = 210

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for (i=0; i < 10; i++) Array3[i] = Array1[i] + Array2[i];

Values of Array3 during for loop iterations

i Array1[i] Array2[i] Array3[i]

0 10 11 21

1 20 22 42

2 30 33 63

3 40 44 84

4 50 55 105

5 60 66 126

6 70 77 147

7 80 88 168

8 90 99 189

9 100 110 210

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Reverse element

Eg- Read 10 integer numbers, and print the numbers in reverse order

public static void ReverseOrder(){ int item[] = new int[10]; int i; //Read integers number and store in item[i] System.out.println("Enter ten integers number:");

for(i = 0; i < 10; i++) item[i] = input.nextInt();

//Print the output in reverse order are:"); System.out.println("The numbers in reverse order are:");

for(i = 9; i >= 0; i--) System.out.println(item[i]);

}

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Output Enter ten integers number:56656743647639774784The numbers in reverse order are:84477739766443676556

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Arrays as Formal Parameters to Methods

Arrays can be passed as parameter to methods

Eg.

public static void arrayAsFormalParameter(int[] listA,double[] listB, int num)

{//…

}

Formal parameter

The above method have 3 formal parameters – listA, listB and num

Statement to call the method

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arrayAsFormalParameter(intList, doubleNumList, number);

Actual parameter

int[] intList = new int[10];double[] doubleNumList = new double[15];int number;

Suppose we have the following statement

example 1

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public class PassingParameter {public static void main(String[] args){

int num[] = {10,20,30,40,50,60,70};System.out.println(“ The number of elements: "

+ num.length);printArray(num);

}

public static void printArray(int[] number){for (int index = 0; index < number.length; index++)

System.out.println(number[index] + "");}

}

Passing parameter

OUTPUT:

The number of elements: 7

10

20

30

40

50

60

70

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public static void main(String[] args){ int[] listA = {11,22,36,42,15,46,27,48,19,10} int[] listB = new int[10]; int Total, Largest; // call sumArray method and return a value to Total

Total = sumArray (listA, listA.length); System.out.println(“\n The sum of ListA is :” + Total);

// call indexLargestElement and return the indux value to Largest

indLargest = indexLargestElement (listA, list.length); System.out.println(“\n The largest element is :” + listA[Largest]);

example 2

continue

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public static int sumArray(int[] list, int noOfElements){ int index; int sum = 0; for (index = 0; index < noOfElement; index++)

sum = sum + list[index]; return sum;}

public static int indexLargestElement(int[] list, int noOfElement){ int index; int maxIndex = 0; for (index = 1; index < noOfElement; index++) if(list[maxIndex] < list[index]) maxIndex = index; return maxIndex;

}

Array of String Objects

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String[] nameList = new String[5]

nameList[0] = “Amanda Green”;nameList[1] = “Vijay Arora”;nameList[2] = “Sheila Mann”;nameList[3] = “Rohit Sharma”;nameList[4] = “Mandy Johnson”;

Array of Object

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• Can use arrays to manipulate objects.• Example: Create an array named array1 with N object of type T:

T[] array1 = new T[N]

• Can instantiate array1 as follows:

for(int j=0; j < array1.length; j++) array1[j] = new T();

• Eg: a) clock – hour, minute, second b) student – name, matric, age

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import java.util.*;

public class ArrayOfObj {

int N = 3;

StudentInfo[] student = new StudentInfo[N];

public static void main (String[] args)

{

int N = 3;

int i;

ArrayOfObj arr = new ArrayOfObj();

StudentInfo[] Std = new StudentInfo[N];

Std = arr.InputData();

arr.PrintInfo(Std);

}

exampleInput students information's (name,matric, age) into array and print out the output

class StudentInfo{

String name;

String matric;

int age;

}

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public StudentInfo[] InputData() int i;

StudentInfo[] student = new StudentInfo[N];

System.out.println("\nEnter Students Information ");

System.out.println("___________________________ \n");

for (i = 0; i< N; i++)

{

student[i] = new StudentInfo();

System.out.print("Name : ");

student[i].name = input.readLine();

System.out.print("Matric No : ");

student[i].matric = input.nextLine();

System.out.print("Age : ");

student[i].age = input.nextInt();

System.out.println();

}

return student;

}

public void PrintInfo(StudentInfo[] Std)

{

int i;

System.out.println("List of students :\n");

for (i=0;i<N;i++)

{

System.out.println((i+1) + ". " + Std[i].matric + " " +

Std[i].name + " " + " " + Std[i].age);

}

}

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Enter Students Information ___________________________

Name : BAHARUDIN OSMAN

Matric No : S11111

Age : 30

Name : BADRUL HAZMI

Matric No : S23212

Age : 28

Name : NUR BADRINA

Matric No : S34213

Age : 27

List of students :

1. S11111 BAHARUDIN OSMAN 30

2. S23212 BADRUL HAZMI 28

3. S34213 NUR BADRINA 27

output

Statement below create an array of arrivalTimeEmp

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Clock[] arrivalTimeEmp = new Clock[100];

Instantiating of Array Objects

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for (int j = 0; j < arrivalTimeEmp.length; j++) arrivalTimeEmp[j] = new Clock();

Continue

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• Setting a time for index 49

arrivalTimeEmp[49].setTime(8, 5, 10);

Delete Object

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• Step

i. Identify the element to deleteii. Point the object to delete nulliii. Move up all elements (after deleted object)iv. Point the last element to null

Example Step 1 : Identify the element to delete Step 2 : Point the object to delete to null

- if the sixth element to delete

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for (i=0; i < student.length; i++) if(i==5) then

student[i] = null

null

student

NameMatricIC

NameMatricIC

NameMatricIC

array 49

element A

element B

element C

element D

element E

element

element G

element H

element I

element J

[0]

[1]

[2]

[3]

[4]

[5]

[6]

[7]

[8]

[9]

element A

element B

element C

element D

element E

element G

element H

element I

element J

[0]

[1]

[2]

[3]

[4]

[5]

[6]

[7]

[8]

[9]

iii. Move up all elements (after deleted object)iv. Point the last element to null

for (i = 0; i < student.length; i++) if (i= =5) student[i] = student[student.length -1)if (i= = (student.length – 1)) student[i] = null

Set the last element to null

nullbefore

studentstudent

after

Two-Dimension Array

A collection of a fixed number of components arranged in rows and columns.

All components are in same type. Data is sometimes in table form (difficult to represent

using a one-dimensional array).

10 11 21 45

20 22 42 34

30 33 66 21

40 44 84 32

50 55 105 13

60 66 126 21

70 77 147 33

80 88 168 22

90 99 189 123

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continue

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• To declare/instantiate a two-dimensional array:

dataType[ ][ ] arrayName = new data Type[intRow][intCol];

intRow => number of rowsintCol => number of columns

intRow and intCol > 0

• Eg.

double[ ][ ] sales = new double[10][15];

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double[ ][ ] sales = new double[10][5];

Accessing Array Components

To access a component of a two-dimensional array:arrayName[indexExp1][indexExp2];

indexExp1 = row positionindexEXp2 = column position

Eg.

The above statement stores 25.75 into row number 5 and column number 3; (the 6th row and the 4th column)

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sales [5][3] = 25.75;

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Sales [5][3] = 25.75;

Array Initialization During Declaration

2-Dimensional array can be initialized during declaration Eg.

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int[ ][ ] board = { {2,3,1}, {15,25,13},

{20,4,7}, {11,18,14}};

Processing 2-Dimensional Array

eg.• Initialization• Print• Input data/store data into 2-Dimensional array• Sum the data• Find the largest element

Suppose the declaration as below:

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int row;

int column;

int matix = new int[7][6];

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Initialization

for (row = 0; row < matrix.length; row++) for (col = 0; col < matrix[row].length; col++)

matrix[row][col] = 10;

matrix

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Print

for (row = 0; row < matrix.lenth; row++){ for ( col = 0; col < matrix[row].length; col++)

System.out.println(matrix[row][col]); System.out.println();

}

Read Data

for (row = 0; row < matrix.length; row++) for (col = 0; col < matrix[row].length; col++) matrix[row][col] = Integer.parseInt(keyboard.readLine())

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Largest Element in Each Row

for (row = 0; row < matrix.length; row++){ largest = matrix[row][0]; for (col = 1; col < matrix[row].length; col++)

if (largest < matrix[row][col]) largest = matrix[row][col];

System.out.println(“The largest element of row” + (row+1) + “=“ + largest);}

Can define three-dimensional arrays or n-dimensional arrays (n can be any number).

Syntax to declare and instantiate array:

dataType[][]…[] arrayName = new dataType[intExp1][intExp2]…[intExpn];

Syntax to access component:arrayName[indexExp1][indexExp2]…[indexExpn]

○ intExp1, intExp2, ..., intExpn = positive integers○ indexExp1,indexExp2, ..., indexExpn = non-

negative integers

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Multidimensional Arrays

double[][][] carDealers = new double[10][5][7];

for (i = 0; i < 10; i++) for (j = 0; j < 5; j++) for (k = 0; k < 7; k++) carDealers[i][j][k] = 10.00;

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Loops to Process Multidimensional Arrays