By Minh Vu
Updated Feb 08, 2026
Minh Vu
Software Engineer
Hi 👋, I'm a software engineer specializing in backend systems, distributed systems, and scalable architecture. My blog shares practical tutorials based on 3+ years of experience. LeetCode 1756 (Top 10%). Actively seeking SDE roles — let's get in touch!
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Sum of digits is a common problem where given a number, you need to find the sum of digits of that number.
It seems to be a basic problem, but very useful in some contexts. This tutorial will help you to solve this problem, let's get started!
The first method is to use the modulo operator (%) and division to extract each digit from the number and calculate their sum. Here's how it works:
num with the given number.sum with 0 to store the sum of digits.num becomes 0.num divided by 10 using the modulo operator and add it to sum.num by dividing it by 10 using integer division.num becomes 0.sum will be the sum of digits of the given number.Let's see some examples:
#include <stdc++.h> using namespace std; int main() { int num; cout << "Enter a number: "; cin >> num; int sum = 0; while (num != 0) { sum += num % 10; num /= 10; } cout << "Sum of digits: " << sum << '\n'; return 0; }
num = int(input('Enter a number: ')) sum = 0 while num != 0: sum += num % 10 num //= 10 print('Sum of digits:', sum)
import java.util.Scanner; public class Main { public static void main(String[] args) { Scanner input = new Scanner(System.in); System.out.print("Enter a number: "); int num = input.nextInt(); int sum = 0; while (num != 0) { sum += num % 10; num /= 10; } System.out.println("Sum of digits: " + sum); } }
const readline = require("readline").createInterface({ input: process.stdin, output: process.stdout }); readline.question("Enter a number: ", (num) => { let sum = 0; while (num != 0) { sum += num % 10; num = Math.floor(num / 10); } console.log("Sum of digits:", sum); readline.close(); });
Output:
Enter a number: 21032002 Sum of digits: 10
In this example, the given number is 21032002, so the sum of digits is 2 + 1 + 0 + 3 + 2 + 0 + 0 + 2 = 10.
Similar to the previous method, we will use recursion to loop through the digits of the number. Here's how it works:
sumOfDigits that takes an integer num as input.num is less than 10, return num as the sum of digits.num by adding the last digit (obtained using the modulo operator) to the sum of digits of the remaining digits (obtained using integer division).#include <stdc++.h> using namespace std; int sumOfDigits(int num) { if (num < 10) { return num; } return num % 10 + sumOfDigits(num / 10); } int main() { int num; cout << "Enter a number: "; cin >> num; cout << "Sum of digits: " << sumOfDigits(num) << '\n'; return 0; }
def sumOfDigits(num): if num < 10: return num return num % 10 + sumOfDigits(num // 10) num = int(input('Enter a number: ')) print('Sum of digits: ', sumOfDigits(num))
Another quick and efficient way:
num = int(input('Enter a number: ')) print('Sum of digits: ', sum(map(int, str(num))))
import java.util.Scanner; public class Main { public static int sumOfDigits(int num) { if (num < 10) { return num; } return num % 10 + sumOfDigits(num / 10); } public static void main(String[] args) { Scanner input = new Scanner(System.in); System.out.print("Enter a number: "); int num = input.nextInt(); System.out.println("Sum of digits: " + sumOfDigits(num)); } }
const readline = require("readline").createInterface({ input: process.stdin, output: process.stdout }); function sumOfDigits(num) { if (num < 10) { return num; } return (num % 10) + sumOfDigits(Math.floor(num / 10)); } readline.question("Enter a number: ", (num) => { console.log("Sum of digits:", sumOfDigits(num)); readline.close(); });
Output:
Enter a number: 9032000 Sum of digits: 14
In this example, we define a recursive function sumOfDigits and given the number is 9032000, so the sum of digits is 9 + 0 + 3 + 2 + 0 + 0 + 0 = 14.
Maybe you know or not, but such simple task like sum of digits is very useful in some contexts:
The Luhn algorithm is a simple checksum formula used to validate a variety of identification numbers, such as credit card numbers, IMEI numbers, National Provider Identifier numbers in the US, and Canadian Social Insurance Numbers. To learn more about the Luhn algorithm, you can read this article.
In the scope of this tutorial, we will learn how to use the Luhn algorithm to validate a simple 16-digit credit card number. Here's how it works:
See the following example:
#include <stdc++.h> using namespace std; bool luhnAlgorithm(string cardNumber) { int sum = 0; for (int i = 0; i < cardNumber.length(); i++) { int digit = cardNumber[i] - '0'; if (i % 2 == 0) { digit *= 2; if (digit > 9) { digit -= 9; } } sum += digit; } return sum % 10 == 0; } int main() { string cardNumber; cout << "Enter a credit card number: "; cin >> cardNumber; if (luhnAlgorithm(cardNumber)) { cout << "Valid credit card number\n"; } else { cout << "Invalid credit card number\n"; } return 0; }
def luhnAlgorithm(cardNumber): sum = 0 for i in range(len(cardNumber)): digit = int(cardNumber[i]) if i % 2 == 0: digit *= 2 if digit > 9: digit -= 9 sum += digit return sum % 10 == 0 cardNumber = input('Enter a credit card number: ') if luhnAlgorithm(cardNumber): print('Valid credit card number') else: print('Invalid credit card number')
import java.util.Scanner; public class Main { public static boolean luhnAlgorithm(String cardNumber) { int sum = 0; for (int i = 0; i < cardNumber.length(); i++) { int digit = cardNumber.charAt(i) - '0'; if (i % 2 == 0) { digit *= 2; if (digit > 9) { digit -= 9; } } sum += digit; } return sum % 10 == 0; } public static void main(String[] args) { Scanner input = new Scanner(System.in); System.out.print("Enter a credit card number: "); String cardNumber = input.nextLine(); if (luhnAlgorithm(cardNumber)) { System.out.println("Valid credit card number"); } else { System.out.println("Invalid credit card number"); } } }
const readline = require("readline").createInterface({ input: process.stdin, output: process.stdout }); function luhnAlgorithm(cardNumber) { let sum = 0; for (let i = 0; i < cardNumber.length; i++) { let digit = parseInt(cardNumber[i]); if (i % 2 == 0) { digit *= 2; if (digit > 9) { digit -= 9; } } sum += digit; } return sum % 10 == 0; } readline.question("Enter a credit card number: ", (cardNumber) => { if (luhnAlgorithm(cardNumber)) { console.log("Valid credit card number"); } else { console.log("Invalid credit card number"); } readline.close(); });
Output:
Enter a credit card number: 4137894711755904 Valid credit card number
Hope you find this useful, see you in the next tutorial!
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