List comprehension and ord() in Python to remove all characters other than alphabets

In this article, we will learn how to remove all non-alphabetic characters from a string using list comprehension and the ord() function in Python. This approach filters characters by checking their ASCII values.

Algorithm

The algorithm follows these steps:

1. Traverse the given string to check each character
2. Select characters that lie in the range [a-z] or [A-Z] using ord()
3. Use join() function to combine all valid characters into a string

Understanding ord() Function

The ord() function accepts a character as an argument and returns its corresponding ASCII value. This allows easy comparison with ASCII ranges ?

print("ASCII value of 'a':", ord('a'))
print("ASCII value of 'z':", ord('z'))
print("ASCII value of 'A':", ord('A'))
print("ASCII value of 'Z':", ord('Z'))

ASCII value of 'a': 97
ASCII value of 'z': 122
ASCII value of 'A': 65
ASCII value of 'Z': 90

Implementation Using List Comprehension

Here's the complete implementation that removes all non-alphabetic characters ?

def remchar(input_string):
    # checking uppercase and lowercase characters
    final = [ch for ch in input_string if
             (ord(ch) in range(ord('a'), ord('z')+1, 1)) or 
             (ord(ch) in range(ord('A'), ord('Z')+1, 1))]
    
    return ''.join(final)

# Driver program
if __name__ == "__main__":
    input_string = "Tutorials@point786._/?"
    print(remchar(input_string))

Tutorialspoint

Alternative Approaches

Using isalpha() Method

Python provides a simpler approach using the built-in isalpha() method ?

def remchar_simple(input_string):
    return ''.join([ch for ch in input_string if ch.isalpha()])

# Test the function
input_string = "Hello123World!@#"
print(remchar_simple(input_string))

HelloWorld

How It Works

The list comprehension approach works by:

• Iterating through each character in the input string
• Checking if the character's ASCII value falls within alphabetic ranges
• Filtering only valid alphabetic characters
• Joining the filtered characters back into a string

Comparison

Conclusion

List comprehension with ord() provides a clear way to filter alphabetic characters by checking ASCII ranges. While isalpha() is simpler, understanding the ord() approach helps grasp character encoding concepts in Python.