Sum of Digits: A Beginner’s Guide to String Iteration in Python

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When you’re learning to code, one of the first problems you’ll encounter is calculating the sum of individual digits within a number. It seems simple on the surface: take 123, add 1 + 2 + 3, and you get 6. But how do you actually make a computer do this? The answer introduces you to some fundamental Python concepts that you’ll use throughout your programming journey.

I initially thought about using the split() method but later learned that the split() method only removes spaces and commas.

The Problem

Let’s say we want to write a function that takes any integer and returns the sum of its digits. For instance:

  • sum_of_digits(123) should return 6
  • sum_of_digits(9999) should return 36

This might seem tricky at first because numbers are mathematical objects, not sequences we can easily break apart. The key insight is converting the number into a string—something we can iterate over character by character.

The Solution

Here’s an elegant solution in Python: 

def sum_of_digits(n):
    total = 0
    for digit in str(n):
        total += int(digit)
    return total

print(sum_of_digits(144))  # Output: 9
print(sum_of_digits(9999)) # Output: 36

How It Works

Let’s break down this code step by step:

Step 1: Initialize a Counter 

total = 0

We start by creating a variable called total to keep track of our running sum. It begins at zero, and we’ll add to it as we process each digit.

Step 2: Convert to String and Iterate 

for digit in str(n):

This is where the magic happens. The str(n) function converts our number into a string. When you convert 123 to a string, you get '123'—a sequence of characters. Python can iterate over strings character by character, so this loop will process each digit separately: first '1', then '2', then '3'.

Step 3: Convert Back and Accumulate 

total += int(digit)

For each character (which represents a digit), we use int(digit) to convert it back from a string to an actual integer. We then add it to our running total using the += operator.

Step 4: Return the Result 

return total

Once we’ve processed all digits, we return the final sum.

Why We Don’t Need split()

You might wonder: why not use split() to break apart the digits? This is a common question for beginners, and it’s worth addressing.

The split() method is designed to break strings into pieces based on a delimiter—usually whitespace, commas, or some other separator. When we convert a number to a string using str(n), we don’t have any delimiters between the digits. What we have is already a sequence of characters that we can iterate over directly.

For example, with n = 123:

  • str(123) gives us '123'
  • We can iterate directly over this: '1', '2', '3'
  • Using split() would actually be unnecessary and less efficient

The string is already a sequence in Python, so we can loop through it without any additional parsing.

The Python Way

The solution shown above is what Pythonic code looks like: clear, concise, and efficient. It leverages Python’s built-in strengths—string iteration and type conversion—to solve the problem elegantly. This same pattern of converting numbers to strings for digit manipulation appears frequently in real-world programming, so mastering it now will serve you well.

Try It Yourself

Want to test this? Run the code and try different numbers: 

print(sum_of_digits(144))  # 1 + 4 + 4 = 9
print(sum_of_digits(9999)) # 9 + 9 + 9 + 9 = 36
print(sum_of_digits(42))   # 4 + 2 = 6

The beauty of this approach is that it works for any integer you throw at it, regardless of how many digits it has. It’s a small function, but it teaches you something important: sometimes the simplest solution comes from thinking about how to transform your data into a form that’s easier to work with.

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