Sum of Digits of a Number in Python

The question

Read a positive integer and print the sum of its digits. Use a while loop with % 10 and // 10.

The code

n = int(input())
total = 0
while n > 0:
    total += n % 10
    n //= 10
print("Sum of digits =", total)

How it works

1. 1n % 10 gives the last digit (1234 % 10 = 4).
2. 2Add that digit to total.
3. 3n //= 10 removes the last digit (1234 becomes 123).
4. 4Repeat while n is greater than 0; when every digit is used, n becomes 0 and the loop ends.

Dry run for 1234

n becomes 0 after four passes, so the loop stops and the sum of digits is 10.

Sum of digits using a for loop and a string

Leave the input as text. A for loop over a string gives one character at a time, and int(ch) turns each one into a digit.

n = input().strip()
total = 0
for ch in n:
    total += int(ch)
print("Sum of digits =", total)

1234

Sum of digits = 10

Sum of digits in one line using sum() and map()

map(int, text) turns every character into a digit and sum() adds them. Handy outside the exam; when the question says "use a while loop", write the loop.

n = input().strip()
print("Sum of digits =", sum(map(int, n)))

1234

Sum of digits = 10

Sum of digits using recursion

The last digit plus the sum of digits of the rest: digit_sum(1234) = 4 + digit_sum(123). The recursion stops when n becomes 0.

def digit_sum(n):
    if n == 0:
        return 0
    return n % 10 + digit_sum(n // 10)

n = int(input())
print("Sum of digits =", digit_sum(n))

1234

Sum of digits = 10

Questions students ask

What is 1234 // 10 in Python?

123. The // operator is floor division: it divides and throws away the remainder, so the last digit disappears. 1234 / 10 with a single slash gives 123.4.

What is 1234 % 10 in Python?

4. The % operator gives the remainder, and the remainder after dividing by 10 is always the last digit.

What does n //= 10 mean in Python?

It is short for n = n // 10 — divide n by 10, drop the remainder and store the result back in n. In a digit loop it removes the last digit on every pass until n becomes 0.

More in while loop

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