Java Program to Find a Missing Number in an Array

Java Program to Find a Missing Number in an Array

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In the previous article, we have seen Java Program to Find the Common Strings in Two String Arrays

In this article we will see how to find a missing number in an array using Java Language.

Java Program to Find a Missing Number in an Array

Array is a data structure which stores a fixed size sequential collection of values of single type. Where with every array elements/values memory location is associated. Each array elements have it’s own index where array index starts from 0.

In Array set of variables referenced by a single variable name and it’s array index position. It is also called as a container object which contains elements of similar type.

Declaration of an array:

dataType[] arrayName; (or)                              //Declaring an array
dataType []arrayName; (or)
dataType arr[];

Instantiation of an Array:

arrayName = new datatype[size];                    //Allocating memory to array

Combining both Statements in One:

dataType[] arrayName = new dataType[size] //Declaring and Instantiating array

Initialization of an Array:

arrayName[index-0]= arrayElement1             //Initializing the array

...

arrayName[index-s]= arrayElementS

Combining all Statements in One:

dataType arrayName[ ]={e1,e2,e3};               //declaration, instantiation and initialization

Let’s see different ways to find a missing number in an array.

Method-1: Java Program to Find a Missing Number in an Array By Using summation formula (Static Input)

Approach:

  • Static array taken.
  • Calculate the sum of first n natural numbers as arraySum= n*(n+1)/2
  • Traverse the array from start to end.
  • Update the value of sum as arraySum -= array[i]
  • Return the variable arraySum.

Program:

public class MissingNum 
{
    //Driver method
    public static void main(String[] args) 
    {
        //An array declared and initialized
        int[] arr = {1, 2, 4, 5, 6, 7};
        //findMissing() method called
        System.out.println("The missing number is : "+ findMissing(arr));
    }
    
    //findMissing() user defined method
    //it willfind the missing number
    public static int findMissing(int arr[]) 
    {
        // calculating the sum upto n integers
        int arraySum = (arr[arr.length-1] * (arr[arr.length-1] + 1)) / 2;
        for (int i : arr) 
        {
            arraySum -= i;
        }
        return arraySum;
    }
}

Output:

The missing number is : 3

Method-2: Java Program to Find a Missing Number in an Array By Using summation formula (Dynamic Input)

Approach:

  • Dynamic array taken.
  • Calculate the sum of first n natural numbers as arraySum= n*(n+1)/2
  • Traverse the array from start to end.
  • Update the value of sum as arraySum -= array[i]
  • Return the variable arraySum.

Program:

import java.util.*;

public class MissingNum 
{
    //Driver method
    public static void main(String[] args) 
    {
        //Scanner class object created
        Scanner sc = new Scanner(System.in);
        System.out.println("Enter number of elements in the array : ");
        int n = sc.nextInt();
        int[] arr = new int[n];
        
        // initializing the array elements
        System.out.println("Enter Array elements : ");
        for (int i = 0; i < n; i++) 
        {
            arr[i] = sc.nextInt();
        }
        
        //calling the findMissing() user defined method
        System.out.println("Missing number is : "+findMissing(arr));

    }
    
    //findMissing() user defined method
    //it willfind the missing number
    public static int findMissing(int arr[]) 
    {
        // calculating the sum upto n integers
        int arraySum = (arr[arr.length-1] * (arr[arr.length-1] + 1)) / 2;
        for (int i : arr) 
        {
            arraySum -= i;
        }
        return arraySum;
    }
}

Output:

Enter number of elements in the array : 5
Enter Array elements : 1 2 4 5 6
Missing number is : 3

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