How to find missing number in array

Welcome back, visitors. ‘Find missing number questions’ are intermediate problems commonly asked in interviews. If you haven’t solved the ‘locate missing number’ program, your journey ends here. This question is also available on platforms like LeetCode and GeeksforGeeks.

Understand Logic Behind How To Search The Missing Number In A Series

It is important to first analyze the question, create a blueprint in your mind, and then start writing code. So, let’s explore ‘how to find missing numbers from an array’. The interviewer may phrase this as ‘search for missing numbers in a sequence’ or ‘locate missing numbers in a worksheet’. In both cases, your answer would be the same

First, read the question. It requires us to identify a missing number from an array or a sorted array. In other words, the interviewer will provide us with a sequence of numbers, such as an array or a sorted array and you have to get missing number from it. I believe you now have a clear understanding of the problem we will address in this article.

Logic Part:

Let’s assume we have an array or a sorted array named ‘questionArray’ and a variable to store the absent number named ‘output’. These are basic requirements to start.

To locate the missing number, we will use a loop to find the greatest number from the sequence and use a variable to store the sum of all numbers named ‘totalOfArrayElements’. Now that we have the maximum number, we will implement another loop to find the total sum named ‘mainTotal’ from 0 to the maximum number. Then, we subtract ‘totalOfArrayElements’ from ‘mainTotal’. The resulting amount is our answer, so we store that number in ‘output’

Program To Find Missing Number From Array

Let’s implement the logic part in code; it’s easy to execute. If you’re a regular visitor, you’ve already started implementing our first part. If not, don’t worry; I’ll explain it step by step.

Step 1

The first step is always to create a blueprint in your mind, and then, according to the illustration, create the required variables. In our case, I’m going to create a method with an int array parameter. In this method, I’ll initialize the ‘output’ variable to store our answer and return it. Our method’s return type must be int.

static void Main(string[] args)
{
    int[] questionArray = new int[] { 1,10,4,3,5,7,8,9,2 };

    Console.WriteLine("Missing number is : "+GetMissingNumberFromArray(questionArray));
    
}

private static int GetMissingNumberFromArray(int[] questionArray)
{
    int output = 0;

    return output;
}

Step 2

Now, for the second step, as discussed in the logic part section, we require a variable to store the total sum of the given array. Therefore, I’m going to create a variable named ‘totalOfArrayElements’. In addition to that, we need to find the maximum number from the given array or a sorted array. To do that, I have declared a variable named ‘maxNumber’.

Now, I’m going to implement a loop to find those values. By doing that, we’ll have half of the logic to discover the missing number from the series.

 private static int GetMissingNumberFromSortedArray(int[] questionArray)
 {
     int output = 0;

     int maxNumber = 0;
     int totalOfArrayElements = 0;
     
     for (int i = 0; i < questionArray.Length; i++)
     {
         if (maxNumber < questionArray[i])
         {
             maxNumber = questionArray[i];
         }

         totalOfArrayElements += questionArray[i];
     }
    
     return output;
 }

Step 3

This is our last step. I believe we are on the same page. If not, or if you find this difficult to understand, please check our Beginner Level Programs, where I have posted an in-depth tutorial to solve beginner-level programs.

Our final step is to create a loop using ‘maxNumber’, and in each iteration, increase ‘mainTotal’ by the current element of the iteration (in our case, denoted by ‘i’). By doing this, we’ll have two values: ‘totalOfArrayElements’ and ‘mainTotal’. To find the missing number, subtract ‘mainTotal’ from ‘totalOfArrayElements’ and store the result in our ‘output’ variable.

private static int GetMissingNumberFromSortedArray(int[] questionArray)
{
    int output = 0;

    int maxNumber = 0;
    int totalOfArrayElements = 0;
    
    for (int i = 0; i < questionArray.Length; i++)
    {
        if (maxNumber < questionArray[i])
        {
            maxNumber = questionArray[i];
        }

        totalOfArrayElements += questionArray[i];
    }
    
    int mainTotal = 0;
    
    for (int i = 0;i <= maxNumber;i++)
    {
        mainTotal += i;
    }

    output = mainTotal - totalOfArrayElements;

    return output;
}

Summarizing How to Find The Missing Number With Answer

In summary, to find a missing number from an array, first, define a method with an integer array parameter and initialize the ‘output’ variable to store the result. Next, create variables to store the total sum of the array elements (‘totalOfArrayElements’) and the maximum number in the array (‘maxNumber’). Then, implement a loop to calculate the total sum and find the maximum number. Proceed to iterate from 0 to ‘maxNumber’, adding each value to ‘mainTotal’. Finally, subtract ‘totalOfArrayElements’ from ‘mainTotal’ to find the missing number and store it in ‘output’. This systematic approach ensures a clear understanding of the problem and an efficient solution.

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