Alphabet

Rotational Cipher
Rotational Cipher in Python 
AlPHABET = "abcdefghijklmnopqrstuvwxyz"

def rotate(text, key):
    result = ""
    for letter in text:
        if letter.isalpha():
            if letter.isupper():
                result += AlPHABET[(AlPHABET.index(letter.lower()) + key) % 26].upper()
            else:
                result += AlPHABET[(AlPHABET.index(letter) + key) % 26]
        else:
            result += letter
    return result

The approach starts with defining the constant ALPHABET which is a string of all lowercase letters. The function rotate() is then declared, and a variable result is defined as an empty string.

The text argument is then iterated over via a for loop. Each element is checked to make sure it is a letter, and subsequently checked if it is uppercase or lowercase. Uppercase letters are converted to lowercase. Then the index of each letter is found in the AlPHABET constant. The numeric key value is added to the letter index and modulo (%) 26 is used on the result. Finally, the new number is used as an index into the AlPHABET constant, and the resulting letter is converted back to uppercase.

Lowercase letters follow the same process without the conversion steps.

If the element is not a letter (for example, space or punctuation) then it is added directly to the result string. The result string is returned once the loop finishes.

If only English letters are needed, the constant string.ascii_lowercase can be imported from the string module.

from string import ascii_lowercase

AlPHABET = ascii_lowercase
18th Sep 2024 · Found it useful?

Other Approaches to Rotational Cipher in Python

Other ways our community solved this exercise
for letter in text:
    if letter.isalpha():
        if letter.isupper():
            result += chr((ord(letter) - 65 + key) % 26 + 65)
        else:
            result += chr((ord(letter) - 97 + key) % 26 + 97)
    else:
        result += letter
ASCII values

Use letters ascii value to rotate the alphabet 

AlPHABET = "abcdefghijklmnopqrstuvwxyz"

def rotate(text, key):
    translator = AlPHABET[key:] + AlPHABET[:key]
    return text.translate(str.maketrans(AlPHABET + AlPHABET.upper(), translator + translator.upper()))
Str Translate

Using str.translate to rotate the alphabet 

first_letter, rest = text[0], text[1:]
if first_letter.isalpha():
    if first_letter.isupper():
        return AlPHABET[(AlPHABET.index(first_letter.lower()) + key) % 26].upper() + rotate(rest, key)
    else:
        return AlPHABET[(AlPHABET.index(first_letter) + key) % 26] + rotate(rest, key)
else:
    return first_letter + rotate(rest, key)
Recursion

Using Recursion to rotate the alphabet