DECLARING ARRAYS AND ACCESSING ARRAY

COMPONENTS

Chapter 7.1:

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).

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);}

}

Why do need array

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.

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 arrayTwo-Dimensional arrayMulti Dimensional array

One-Dimensional Arrays Syntax to declare an array:

Syntax to access an array component: arrayName[indexExp]

<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)

Example

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

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

num

Continue

In java, [ ] is call as array subscripting operator

Items in an array is called elements

elementindex

ContinueArray 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’;

Assign a value into array

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)

int[] list = new int[10];

Specifying Array Size During ProgramExecution (dynamic array) Array that are created during program execution is called

dynamic array Enables user to specify the size of the array

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

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

int[] list = new int[arraySize];

Array Initialization During Declaration We also can assign a value into the array during

declaration

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.

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

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.

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

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;

2. Input data into an arraydouble[] sales = new double[10];int index;

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

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

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

3. Printing an array

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;

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];

Suppose the array sales is as figure 9.5

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

example Consider the following declaration:

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)

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

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]