For Python Beginners: Two Ways to Convert a String into a List of Digits (List Comprehension vs. For Loop)

Introduction: How Do You Convert a String to Numbers?

When getting user input in Python, we usually use the input() function. However, input() always returns a string (str type)—even if the user types 12345, what your program actually receives is the string "12345".

So you might be wondering: “Wait, does that mean I can’t perform calculations with it?”
No worries! In this article, I’ll show you two methods to convert a string into a list of integers, digit by digit:

• Using list comprehension
• Using a for loop to process each character

These two techniques are essential for learning Python basics, especially in mastering string manipulation, type conversion, and list generation. We’ll walk through code examples and break down when to use each approach.

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The Basics of Input and String Handling in Python

Here are some key points you need to know:

• The return value of input() is always a string
• To treat it as a number, you need to convert it using int()
• To access each digit, you must process the string character by character

For example, to convert "12345" into [1, 2, 3, 4, 5], you need to extract each character and convert it into an integer with int().

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Method 1: Converting Using List Comprehension

✅ Code Example

x = input("Enter an integer: ")  # Example input: "12345"
digits = [int(ch) for ch in x]
print(digits)  # Output: [1, 2, 3, 4, 5]

✅ Explanation

In this code, we use list comprehension [int(ch) for ch in x] to:

• Loop through each character ch in the string x
• Convert each character to an integer using int(ch)
• Collect them into a new list

✅ Pros

• Concise and readable for simple cases
• Entire operation is done in one line

✅ Cons

• May be hard to grasp for beginners
• Becomes less readable if the logic gets complex

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Method 2: Converting Using a For Loop

✅ Code Example

x = input("Enter an integer: ")  # Example input: "12345"
digits = []

for ch in x:
    num = int(ch)
    digits.append(num)

print(digits)  # Output: [1, 2, 3, 4, 5]

✅ Explanation

This more traditional method lays out each step explicitly:

• Initialize an empty list digits
• Iterate over each character in x
• Convert each character to an integer and append it to the list

✅ Pros

• Easier to understand, especially for beginners
• More flexible when adding conditions or handling errors

✅ Cons

• More lines of code for simple operations

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Deepening Your Understanding Through Practical Examples

Calculating the Sum of All Digits

Using List Comprehension:

x = input("Enter an integer: ")
digits = [int(ch) for ch in x]
total = sum(digits)
print("Sum of digits:", total)

Using a For Loop:

x = input("Enter an integer: ")
digits = []
total = 0

for ch in x:
    num = int(ch)
    digits.append(num)
    total += num

print("Sum of digits:", total)

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Extracting Even Digits Only

x = input("Enter an integer: ")
even_digits = [int(ch) for ch in x if int(ch) % 2 == 0]
print("Even digits:", even_digits)

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Which Method Should You Use? Key Considerations

Criteria	List Comprehension	For Loop
Readability (for beginners)	△ Slightly challenging	◎ Very easy to follow
Flexibility (e.g. conditions)	△ Limited	◎ Highly adaptable
Conciseness	◎ Compact	△ Verbose
Step-by-step clarity	△ Packed into one line	◎ Clearly separated steps

Conclusion:
Use list comprehension for simple, one-liner transformations.
Use a for loop when the process is more complex or requires conditional logic.

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Summary: Master the Basics for Future Growth

Converting a string to a list of integers is a fundamental yet powerful technique in Python. It lays the groundwork for more advanced data processing and algorithm development.

• List comprehension is great for simple, clean code.
• For loops are perfect when you need clarity or flexibility.

By mastering both approaches, you’ll gain the confidence and skills to tackle a wide variety of problems in Python development.