Python | Find number of lists in a tuple

Sum of number digits in List in Python

Last Updated : 03 May, 2025

Our goal is to calculate the sum of digits for each number in a list in Python. This can be done by iterating through each number, converting it to a string, and summing its digits individually. We can achieve this using Python’s built-in functions like sum(), map(), and list comprehensions. For example, given the list [123, 456, 789], we will calculate the sum of digits for each number, resulting in a list of sums such as [6, 15, 24].

Using Loops

This code calculates the sum of digits for each number in the list a using a while loop and stores the results in the res list.

Python

a = [123, 456, 789]
res = []

for val in a:
    total = 0
    while val > 0:
        total += val % 10
        val //= 10
    res.append(total)

print(res)

Output

[6, 15, 24]

Explanation:

The code loops through each number in the list a, extracting and summing its digits.
It uses the modulo operator (%) to get the last digit and integer division (//) to remove the last digit, continuing this until the number is reduced to zero.
The sum of digits for each number is stored in the list res.

Using List Comprehension

List comprehension offers a more concise way to sum digits. This code calculates the sum of digits for each number in the list a using a list comprehension and stores the results in the res list.

Python

a = [123, 456, 789]

res = [sum(int(digit) for digit in str(val)) for val in a]
print(res)

Output

[6, 15, 24]

Explanation:

str(val) converts the number to a string so we can loop through its digits.
int(digit) converts each string digit back to an integer for summing.
The list comprehension sums the digits of each number in the list.

Using map Function

map() function is a functional approach that applies a function to each element in the list.

Python

a = [123, 456, 789]

res = list(map(lambda val: sum(int(digit) for digit in str(val)), a))
print(res)

Output

[6, 15, 24]

Explanation: The lambda function directly computes the sum of digits and map() function is used to map this lambda function for each value in the list “a”.

Related Articles:

Python While Loop
List Comprehension in Python
Python map() function

Python | Find number of lists in a tuple

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