---
title: "Write a program to find the missing number in an array of integers."  
description: "Write a program to find the missing number in an array of integers."  
author: "Revati S Misra"  
published: 2023-04-19  
updated: 2023-04-24  
canonical: https://www.mindstick.com/forum/157932/write-a-program-to-find-the-missing-number-in-an-array-of-integers  
category: "java"  
tags: ["java", "programs"]  
reading_time: 2 minutes  

---

# Write a program to find the missing number in an array of integers.

Write a [program](https://www.mindstick.com/blog/12337/scaling-up-your-mentorship-program) to find the [missing](https://answers.mindstick.com/qa/52138/international-missing-children-s-day-2019-was-observed-on) number in an [array of integers](https://www.mindstick.com/forum/157951/write-a-program-to-find-the-second-smallest-element-in-a-given-array-of-integers).

## Replies

### Reply by Aryan Kumar

```java
public class MissingNumber {
   public static void main(String[] args) {
       int[] arr = {1, 2, 3, 4, 6, 7, 8, 9, 10}; // array with missing number
       int n = arr.length + 1; // length of original array

       int sum = n * (n + 1) / 2; // sum of integers from 1 to n

       for (int i = 0; i < arr.length; i++) {
           sum -= arr[i]; // subtract each element in the array from the sum
       }

       System.out.println("The missing number is: " + sum); // output the missing number
   }
}
```

This program calculates the sum of integers from 1 to n (where n is the length of the original array plus one), and then subtracts each element in the array from the sum. The result is the missing number in the array. In the example array above, the missing number is 5.

### Reply by Krishnapriya Rajeev

The code given below shows how to find the missing number in an [array](https://www.mindstick.com/articles/335/jagged-array-in-c-sharp-dot-net) of integers:

```plaintext
class missingNum
{
    public static void main(String[] args)
    {
        int[] nums = { 1, 2, 3, 5, 4, 8, 6};	// Declare and initialize and an integer array.
        int n = nums.length;	// Store length of the integer array
        int sum = ((n + 1) * (n + 2)) / 2;	// Sum of n numbers
        for (int i = 0; i < n; i++)
            sum -= nums[i];		// Subtracts each element from sum
         System.out.println("The missing number is " + sum + ".");
    }
}
OUTPUT:
The missing number is 7.
```

We can obtain the missing element by subtracting the sum of all elements from the sum of the first n natural numbers, where n is the largest number in the array.


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Original Source: https://www.mindstick.com/forum/157932/write-a-program-to-find-the-missing-number-in-an-array-of-integers

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