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Python - Sort list of numbers by sum of their digits

Sum the Digits of a Given Number – Python

Last Updated : 24 Feb, 2025

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The task of summing the digits of a given number in Python involves extracting each digit and computing their total . For example, given the number 12345, the sum of its digits is 1 + 2 + 3 + 4 + 5 = 15.

Using modulo (%)

This method efficiently extracts each digit using the modulus (%) and integer division (//) operations, avoiding unnecessary string conversions. It is the fastest and most memory-efficient approach, making it ideal for performance-focused applications.

Python

n = 12345 sum = 0  while n > 0:     sum += n % 10  # extract last digit     n //= 10       # remove last digit  print(sum)

Output

Explanation: This code iterates through the digits of n, extracting each last digit using % 10, adding it to sum and removing it with // 10 until n becomes 0.

Table of Content

Using recursion
Using map()
Using reduce

Using recursion

This method uses recursion to break the problem into smaller subproblems. It follows the same mathematical approach but introduces function call overhead, making it slightly less efficient than the iterative method. It is useful when a recursive solution is preferred.

Python

# recursive function  def fun(n):     if n == 0:         return 0  # base case     return (n % 10) + fun(n // 10)  n = 12345 print(fun(n))

Output

Explanation: fun(n) recursively sums the digits of n. If n is 0, it returns 0. Otherwise, it adds the last digit (% 10) to the sum of a recursive call on the remaining digits (// 10), repeating until n becomes 0.

Using map()

map() apply the int() conversion directly, reducing memory usage. It provides a more functional programming style but still involves string conversion and making it less efficient than mathematical methods.

Python

n = 12345 sum = sum(map(int, str(n))) print(sum)

Output

Explanation: This code converts n to a string, maps each digit to an integer and sums them using sum().

Using reduce

functools.reduce() applies a cumulative operation to the digits. Although some may prefer this approach, it adds unnecessary function call overhead, making it the least efficient method for this problem.

Python

from functools import reduce n = 12345 sum = reduce(lambda x, y: x + int(y), str(n), 0) print(sum)

Output

Explanation: This converts n to a string, then applies a lambda function that iteratively adds each digit (converted to an integer) to an accumulator, starting from 0.

Python - Sort list of numbers by sum of their digits

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