Sum of digit of a number using recursion

Given a number, we need to find sum of its digits using recursion.

Examples: 

Input: 12345
Output: 15
Explanation: Sum of digits → 1 + 2 + 3 + 4 + 5 = 15

Input: 45632
Output: 20

Approach:

To understand the algorithm, consider the number 12345 and refer to the illustration below..

• Extract the last digit: 12345 % 10 = 5, pass 1234 to the next step.
• Extract next digit: 1234 % 10 = 4, pass 123 to the next step.
• Extract next digit: 123 % 10 = 3, pass 12 to the next step.
• Extract next digit: 12 % 10 = 2, pass 1 to the next step.
• Extract last digit: 1 % 10 = 1, pass 0, stopping recursion.

Each step adds the extracted digit to the sum, forming 1 + 2 + 3 + 4 + 5 = 15 

Note: Instead of if (n == 0) return 0;, we can use if (n < 10) return n;, eliminating extra function calls for single-digit numbers without changing the output.

// Recursive C++ program to find sum of digits 
// of a number 
#include <bits/stdc++.h> 
using namespace std; 

// Function to check sum of digit using recursion 
int sum_of_digit(int n) 
{ 
    if (n == 0) 
    return 0; 
    return (n % 10 + sum_of_digit(n / 10)); 
} 

// Driven code 
int main() 
{ 
    int num = 12345; 
    int result = sum_of_digit(num); 
    cout << result << endl; 
    return 0; 
} 

// Recursive C program to find sum of digits 
// of a number
#include <stdio.h>

// Function to check sum of digit using recursion
int sum_of_digit(int n)
{
    if (n == 0)
       return 0;
    return (n % 10 + sum_of_digit(n / 10));
}

// Driven Program to check above
int main()
{
    int num = 12345;
    int result = sum_of_digit(num);
    printf("%d", result);
    return 0;
}

import java.io.*;
class GfG
{
    // Function to check sum 
    // of digit using recursion
    static int sum_of_digit(int n)
    { 
        if (n == 0)
            return 0;
        return (n % 10 + sum_of_digit(n / 10));
    }

    // Driven Program to check above
    public static void main(String args[])
    {
        int num = 12345;
        int result = sum_of_digit(num);
        System.out.println(result);
    }
}

# Function to check sum of
# digit using recursion
def sum_of_digit( n ):
    if n == 0:
        return 0
    return (n % 10 + sum_of_digit(int(n / 10)))

# Driven code to check above
num = 12345
result = sum_of_digit(num)
print(result)

using System;
class GfG {
    
    // Function to check sum 
    // of digit using recursion
    static int sum_of_digit(int n)
    { 
        if (n == 0)
            return 0;
            
        return (n % 10 + sum_of_digit(n / 10));
    }

    // Driven Program to check above
    public static void Main()
    {
        int num = 12345;
        int result = sum_of_digit(num);
        Console.WriteLine(result);
    }
}

// Function to check sum of digit using recursion 
function sum_of_digit(n) 
{ 
    if (n == 0) 
    return 0; 
    return (n % 10 + sum_of_digit(parseInt(n / 10))); 
} 

// Driven code 
var num = 12345; 
var result = sum_of_digit(num); 
console.log(result ); 

15

Besides writing (n==0 , then return 0) in the code given above we can also write it in this manner , there will be no change in the output .

if (n <1 0) return n; by writing this there will be no need to call the function for the numbers which are less than 10

Time Complexity: O(log₁₀n), Traverse through all the digits, as there are log₁₀n bits.
Auxiliary Space: O(log₁₀n), due to recursive function calls stored in the call stack.