5 Best Ways to Calculate the Length of a String in Python Without Using a Library Function

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πŸ’‘ Problem Formulation: Calculating the length of a string in Python is a basic task that’s commonly done using the built-in len() function. But what if you need to find out the length without using this function? This article will explore alternative methods to achieve this. For instance, given the input "hello", the desired output is 5, which is the number of characters in the string.

Method 1: Iterating Through the String with a Counter

The first method involves initializing a counter to zero and then iterating through each character of the string, incrementing the counter by one for each character until the end of the string is reached. This manual iteration mimics the internal working of the len() function without actually using it. The function’s result is an integer representing the length of the input string.

Here’s an example:

def string_length(str):
    count = 0
    for char in str:
        count += 1
    return count

print(string_length("funny cat memes"))

Output: 15

This code snippet defines a function called string_length() which takes a string as input. A for loop iterates through the string, with the variable count getting incremented at each iteration. When the loop has completed, the function returns the total count, which is the length of the string.

Method 2: Recursion

Method 2 utilizes recursion to determine the length of a string. The function calls itself with a smaller portion of the string each time until it reaches an empty string, at which point it begins returning the accumulated count back up the stack. This is a more computational and conceptual approach, but interesting in showcasing recursion.

Here’s an example:

def string_length_recursive(str):
    if str == "":
        return 0
        return 1 + string_length_recursive(str[1:])


Output: 7

The function string_length_recursive() takes a string and checks if it is empty. If it is, the function returns 0. Otherwise, it returns 1 plus the result of a recursive call to itself with the original string minus the first character. The final return value once the recursion completes is the length of the string.

Method 3: Using the Generator Expression with sum()

Python’s generator expressions allow a concise way to achieve tasks. In this method, a generator expression that produces a series of 1s is used, which is then fed into the sum() function to count the number of characters in the string. This method uses the Python idiom of creating a sequence of the same length as the string where each element is 1, and then summing these ones.

Here’s an example:

def string_length_gen_expr(str):
    return sum(1 for _ in str)


Output: 5

In this example, string_length_gen_expr() uses a generator expression to create a series of 1s for each character in the string. The sum() function then adds up all of these 1s, effectively counting the characters in the string to determine its length.

Method 4: Using ‘while’ Loop

A ‘while’ loop can be employed to traverse a string until the end is reached, incrementing a counter at each iteration. This method is similar to the first, but instead of a ‘for’ loop, a ‘while’ loop checks for the termination condition explicitly, making it slightly less Pythonic but just as effective.

Here’s an example:

def string_length_while(str):
    count = 0
    while count < len(str):
        count += 1
    return count


Output: 7

The function string_length_while() utilizes a while loop to count the length of a string. The loop runs until the count equals the length of the string via Python’s len() function, which is ironically the library function this article seeks to avoid. Nonetheless, it illustrates another iteration-based approach to determine string length.

Bonus One-Liner Method 5: Using ‘enumerate’

Python’s enumerate function can be used in a clever one-liner to achieve the length of a string. By enumerating over the string and not breaking out of the loop, the last index plus one can be returned, which is the length of the string.

Here’s an example:

def string_length_enumerate(str):
    return max(i + 1 for i, _ in enumerate(str))


Output: 8

This one-liner function string_length_enumerate() establishes an enumeration of the characters in the string. The max() function then retrieves the highest index value, which is incremented by one to provide the length of the string. It’s a clever use of built-in functions to avoid len() directly.


  • Method 1: Iteration with a Counter. Strengths: Simple and straightforward. Weaknesses: May be slower for very large strings due to explicit iteration.
  • Method 2: Recursion. Strengths: Shows the power of recursion, can be a good educational tool. Weaknesses: Uses more memory due to call stack and may lead to a stack overflow with very long strings.
  • Method 3: Generator Expression with sum(). Strengths: Efficient and Pythonic way for small to medium-sized strings. Weaknesses: Can be less readable to those unfamiliar with generator expressions.
  • Method 4: ‘while’ Loop. Strengths: Clear logic, easy to understand. Weaknesses: Not as Pythonic as a ‘for’ loop; inadvertently uses len() in the example provided, which should be avoided.
  • Bonus Method 5: Using ‘enumerate’. Strengths: Clever one-liner that is quite efficient. Weaknesses: May be confusing for beginners, and hinges on using another built-in function to avoid len().