Python Program to find missing number in array

In this tutorial, you will learn writing a program using Python to find the missing number in integer array of 1 to 100. 

This Program is not generic, It has some limitations like 

• It can find only one missing number in an array.
• Here we are just adding numbers from 0 to n, where n is number of elements array can have.
•  User can give input only between 0 to n.
• To find missing number, subtract sum of numbers given by user from total sum of numbers from 0 to n.

Also Read This:  Java Program to find missing number in array. 

Python Program to find missing number in Array

Program 1: Using the Formula for the Sum

The sum of the first 𝑛n natural numbers is given by the formula 𝑛×(𝑛+1)/2​. If one number is missing, the sum of the array will be less than this total by exactly the missing number.

# Initialize the array
arr = [1, 2, 3, 5]

# Calculate the total number of elements expected (including the missing one)
n = len(arr) + 1

# Calculate the sum of the first n natural numbers
total_sum = (n * (n + 1)) // 2  # Use integer division for safety

# Calculate the sum of the elements in the array
sumArr = sum(arr)

# Calculate the missing number
missing_number = total_sum - sumArr

# Print the missing number
print("Missing number =", missing_number)

Output

Missing number = 4

Explanation

This method calculates the expected sum of the first 𝑛n natural numbers and subtracts the sum of the array from this expected sum. The result is the missing number.

Program 2: Using Sorting

By sorting the array, you can simply iterate through and identify where the numbers skip.

def get_missing_number(arr):
    arr.sort()
    for i in range(len(arr)):
        if arr[i] != i + 1:
            return i + 1
    return len(arr) + 1

array = [1, 2, 3, 4]
print("Missing number =", get_missing_number(array))  

Output

Missing number = 5

Explanation

After sorting the array, the program checks each element. If any element doesn’t match its index + 1 (since array indices start at 0), it means that the current index + 1 is the missing number. If the loop completes, the missing number is n+1.

Program 3: Using Set for Direct Lookup

This method involves creating a set from the array, then checking which number from 1 to n is not in the set.

array = [1, 2, 3, 4]

num_set = set(array)

n = len(array) + 1

missing_number = None

for number in range(1, n + 1):
    if number not in num_set:
        missing_number = number
        break  

if missing_number is not None:
    print("Missing number =", missing_number)
else:
    print("No missing number, the sequence is complete.")

Output

Missing number = 5

Explanation:

This method checks each number from 1 to n against a set of numbers in the array. The first number not found in the set is the missing number.