Class 11 | Arrays | Java Programming Notes

Class 11 | Arrays | Java Programming Notes
UNIT 5
Class 11 Java Programming

Arrays

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01

Introduction to Array

  • An array is a collection of similar type of data items in a single unit.
  • It is continuous memory locations that stores homogeneous data items.
  • Each item of an array is called its element.
  • Every element of an array is identified by index number. Index ranges from 0 to n − 1 where n is size of an array.
  • Name of array elements are same as that of array name but index ranges.
02

Declaration of Array in Java

  • It is the process of creating continuous memory space to store similar types of data items.
  • Array must be declared before using it in program.

Syntax

data_type [] arrayname;   // preferred way

OR

data_type arrayname[];   // not preferred way

Example

int [] num;
double [] names;
03

Creating Array

Array can be created using new keyword for memory allocation.

Syntax

array_name = new datatype[size];

Example

num = new int[10];

Array declaration and creation can be done in a single line as:

data_type [] arrayname = new datatype[size];

Example:
int [] num = new int[10];
Index: 01234 56789
num[0] num[1] num[8] num[9]
Array element last array element
Array num with index numbers 0 to 9
04

Array Initialization

Array elements can be initialized during declaration.

Syntax

type [] array = new type[] {val1, val2, val3};

OR

type [] array = {val1, val2, ... valN};

Example

int [] num = new int[] {2, 4, 6, 8, 10, 12, 14, 16, 18, 20};

OR

int [] num = {2, 4, 6, 8, 10, 12, 14, 16, 18, 20};

An array can be initialized using index number.

Example

int [] num = new int[5];   // array declaration
num[0] = 2;
num[1] = 4;
num[2] = 6;
num[3] = 8;
num[4] = 10;

Program Illustration Array Initialization Using System

class ArrayTest
{
    public static void main(String []args)
    {
        int [] num = new int[] {2, 4, 6, 8, 10};

        System.out.println("First array element:" + num[0]);
        System.out.println("Second array element:" + num[1]);
        System.out.println("Third array element:" + num[2]);
        System.out.println("Fourth array element:" + num[3]);
        System.out.println("Fifth array element:" + num[4]);
    }
}
05

Length Property

Length property is used to find length of array i.e. total number of array elements.

Syntax

array_name.length;

Program Illustrating Length Property

class ArrayTest
{
    public static void main(String []args)
    {
        int [] num = new int[5];   // array declaration
        int len;
        len = num.length;
        System.out.println("The length of an array is:" + len);
    }
}
06

Accessing Array Elements (Processing Array)

An array elements can be accessed using two ways:

  1. Using For Loop
  2. Using Foreach Loop

i) Using For Loop

  • Accessing array, reading data from elements of array and display to the output screen to the user.
  • It uses index number of each array element.

Program to Illustrate Accessing Array Elements

class AccessArray
{
    public static void main(String []args)
    {
        double [] nums = {1.5, 2.5, 3.5, 4.5};

        for (int i = 0; i < nums.length; i++)
        {
            System.out.println("Element:" + nums[i]);
        }
    }
}

Program to Input Age of 10 Students and Display Them Using Array

import java.util.*;

class AccessArr
{
    public static void main(String []args)
    {
        int [] age = new int[10];
        Scanner sc = new Scanner(System.in);

        System.out.println("Enter age of students:");
        for (int i = 0; i < age.length; i++)
        {
            age[i] = sc.nextInt();
        }

        System.out.println("Age of students are:");
        for (int i = 0; i < age.length; i++)
        {
            System.out.println(age[i]);
        }
    }
}

Java Program to Input 10 Numbers From User and Display Sum of Them

import java.util.*;

class CalculateSum
{
    public static void main(String []args)
    {
        int total = 0;
        int [] num = new int[10];   // array declaration
        Scanner sc = new Scanner(System.in);

        System.out.println("Enter numbers:");
        for (int i = 0; i < 10; i++)
        {
            num[i] = sc.nextInt();
        }

        for (int i = 0; i < 10; i++)
        {
            total = total + num[i];
        }

        System.out.println("Total/sum of entered is:" + total);
    }
}

Program to Input n Numbers, Store in Array and Display in Ascending Order

Source transcription note: The scanned program visibly contains repeated declarations for n and num. They are retained rather than silently corrected. 
import java.util.*;

class SortNum
{
    public static void main(String []args)
    {
        int n, i, j;
        int [] num = new int[100];

        Scanner sc = new Scanner(System.in);
        System.out.println("How many numbers:");
        int n = sc.nextInt();
        int [] num = new int[n];

        System.out.println("Enter numbers:");
        for (int i = 0; i < n; i++)
            num[i] = sc.nextInt();

        int temp;
        for (int i = 0; i < n; i++)
        {
            for (int j = i + 1; j < n; j++)
            {
                if (num[i] >= num[j])
                {
                    temp = num[i];
                    num[i] = num[j];
                    num[j] = temp;
                }
            }
        }

        System.out.println("The numbers in ascending order:");
        for (int i = 0; i < n; i++)
            System.out.println(num[i] + " ");
    }
}

Processing (Accessing) Array Using Foreach Loop

  • Elements of an array can be accessed using foreach loop.
  • Foreach loop reads each element of an array and assigns in the counter variable until there is no more elements in array.

Syntax

for (type var : array)
{
    statements using var;
}

Program to Input Age of ‘n’ Students and Display Using Foreach Loop

import java.util.*;

class Program
{
    public static void main(String args[])
    {
        Scanner sc = new Scanner(System.in);

        System.out.println("How many students:");
        int n = sc.nextInt();

        int [] age = new int[n];

        System.out.println("Age of students are");
        for (int i = 0; i < n; i++)
            age[i] = sc.nextInt();

        System.out.println("The Age of students are");
        for (int i : age)
        {
            System.out.println(" " + i);
        }
    }
}
07

Types of Arrays

Three types of array can be declared in Java.

1. One Dimensional Array

One-dimensional array is strings of data stored in a single line. One-dimensional array only contains one continuous row of data. The elements of one-dimensional arrays can be added or printed in a single line using loops.

2. Two Dimensional Array

Two-dimensional arrays are the more frequently used type of array in Java. They form a matrix of rows and columns and find applications in a lot of fields outside development such as simulations, robotics, and machine learning.

3. Multi-Dimensional Array

Arrays can each have more than two dimensions. While arrays with multiple dimensions are not easy to visualize, their applications are only increasing by the day. They can also hold large amounts of data which is a useful feature when it comes to data analysis.

08

Two Dimensional Array

  • An array having two dimensions that stores data in rows and columns i.e. in matrix form.
  • It has two subscripts. Value of first subscript represents no. of rows and second subscript represents no. of columns.

Declaration of 2-D Array

Syntax

type [][] arrayName = new type[row][column];
typeis valid data type
arrayNameis name of array i.e. valid identifier
[row]specifies no. of rows
[column]specifies no. of columns
row × columntotal no. of element in that array
row indexranges from 0 to row − 1
column indexranges from 0 to column − 1

Name of array elements are same as that of array name but index of row and column varies.

Example

int [][] num = new int[2][3];
Two-dimensional array num with 2 rows and 3 columns
09

2-D Array Initialization in Java

2-D array can be initialized during declaration time. Value are initialized row wise.

Syntax

type [][] arrayName = new type[][] {
    {row0 values},
    {row1 values},
    ...,
    {rowN values}
};

OR

type [][] arrayName = {
    {row0 values},
    {row1 values},
    ...,
    {rowN values}
};

Example

int [][] num = new int[][] {{2, 4, 6}, {8, 10, 12}};

int [][] num = {{2, 4, 6}, {8, 10, 12}};

Program to Demonstrate of 2-D Array in Java

public class program
{
    public static void main(String []args)
    {
        int [][] num = new int[2][3];   // 2D array declaration

        num[0][0] = 2;
        num[0][1] = 4;
        num[0][2] = 6;
        num[1][0] = 8;
        num[1][1] = 10;
        num[1][2] = 12;

        System.out.println("num[0][0]:" + num[0][0]);
        System.out.println("num[0][1]:" + num[0][1]);
        System.out.println("num[0][2]:" + num[0][2]);
        System.out.println("num[1][0]:" + num[1][0]);
        System.out.println("num[1][1]:" + num[1][1]);
        System.out.println("num[1][2]:" + num[1][2]);
    }
}

Program to Input a 2×3 Matrix and Display Its Elements

Source transcription note: The display loops use i and j without visible declarations in that portion of the scanned program. This is retained as shown. 
import java.util.*;

class Program
{
    public static void main(String []args)
    {
        Scanner sc = new Scanner(System.in);
        int [][] mat = new int[2][3];

        System.out.println("Enter element of matrix:");
        for (int i = 0; i < 2; i++)
        {
            for (int j = 0; j < 3; j++)
            {
                mat[i][j] = sc.nextInt();
            }
        }

        System.out.println("Elements of matrix are:");
        for (i = 0; i < 2; i++)
        {
            for (j = 0; j < 3; j++)
            {
                System.out.println(mat[i][j] + " ");
            }
            System.out.println("\n");
        }
    }
}
10

Arrays Class

  • The Arrays class is built-in class included within java.util package.
  • This class provides static methods to create and manipulate arrays dynamically.
  • Methods of an Arrays class are used to manipulate array in unique ways such as sort, search, fill, etc.

Some methods of an Arrays class includes:

1) sort() Method

Method that sorts an array into ascending numerical order or alphabetical order for strings.

Syntax

Arrays.sort(arr);
Arrays is built-in class sort() is method of Arrays class arr is argument passed to method

Program to Sort an Integer Array Using Arrays Class

import java.util.*;

class Integer
{
    public static void main(String args[])
    {
        int [] arr = {8, 6, 9};

        Arrays.sort(arr);

        for (int i : arr)
        {
            System.out.println(i + " ");
        }
    }
}

2) Fill()

  • The Arrays.fill() method is used to fill an entire array with a single value.
  • It can be useful when we want to reset all values in an array and initialize them to a specific value.

Syntax

Arrays.fill(arr, value);

It takes two arguments, name of array and value by which array element are filled.

Example

import java.util.*;

class program
{
    public static void main(String []args)
    {
        int [] arr = new int[5];

        Arrays.fill(arr, 2);

        for (int i : arr)
        {
            System.out.println(i + " ");
        }
    }
}

3) toString()

  • Used to convert an entire array into a string format.
  • It takes an array as input and returns string representation of the array.
  • Useful for printing or logging purpose.

Syntax

Arrays.toString(arr);

Example

import java.util.*;

class program
{
    public static void main(String []args)
    {
        int [] arr = {1, 2, 3};

        System.out.println(Arrays.toString(arr));
    }
}
O/P 
[1,2,3]

4) copyOf()

  • Used to create a new array that is copy of an existing array.
  • It takes two arguments: original array and length of new array.
  • Useful when we want to manipulate an array without affecting the original data.

Syntax

Arrays.copyOf(arr, length);

Example

int [] original = {1, 2, 3};
int [] copy = Arrays.copyOf(original, original.length);

System.out.println(Arrays.toString(copy));
O/P 
[1,2,3]

5) Arrays.equals()

  • equals() method is used to checks if two array are equal, meaning their length, order and element are the same.
  • It returns logical value either true or false.

Syntax

Arrays.equals(arr1, arr2);

Example

Source transcription note: The scanned example prints the literal text "isEqual", rather than visibly printing the boolean variable itself. It is preserved as written. 
import java.util.*;

class ArrayTest
{
    public static void main(String []args)
    {
        int [] arr1 = {1, 2, 3};
        int [] arr2 = {1, 2, 3};

        boolean isEqual = Arrays.equals(arr1, arr2);

        System.out.println("isEqual");
    }
}

6) Arrays.binarySearch()

  • This method is used to search specific element in array.
  • It uses the binary search algorithm which is more efficient than a linear search but required the array to be sorted first.
  • It takes two arguments: name of array and element to be search.

Syntax

Arrays.binarySearch(arrays, element);

Example

import java.util.*;

class ArrayTest
{
    public static void main(String []args)
    {
        int [] array = {1, 2, 4, 6, 8, 14};

        int index = Arrays.binarySearch(array, 4);

        System.out.println(index);
    }
}
O/P 
1
Source note: The scanned page shows output 1 for this example. That source output is retained without correction.
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