Reads contents from a text file and performs a rot13 encode on letters from the English alphabet.

rot13.py

def join_letters_to_string(chars):
    s = ""
    for c in chars:
        s += c
    return s
        
def rotate(letter, position):
    # Assumptions:
    #  -input is a letter (i.e. length = 1)
    #  -input is an alphabetical character (i.e. no punctuations)
    letter_unicode = ord(letter)
    
    letter_rotated_unicode = letter_unicode + position

    # Loopback after Z/z.
    if letter_rotated_unicode > ord("Z") and letter.isupper():
        letter_rotated_unicode = ( ((letter_unicode + position) - ord("Z")) + (ord("A") - 1) )
    elif letter_rotated_unicode > ord("z") and letter.islower():
        letter_rotated_unicode = ( ((letter_unicode + position) - ord("z")) + (ord("a") - 1) )

    letter_rotated = chr(letter_rotated_unicode)
    return letter_rotated

def rot_13(word):
    rotated_letters = [] 
    for letter in word:
        rotated_letter = rotate(letter, 13)
        rotated_letters.append(rotated_letter)
    
    return join_letters_to_string(rotated_letters)
    

def main():
    f = open("input.txt", "r")
    words = f.readline().strip().split()
    sentence = ""
    for i in range(len(words)):
        r_word = rot_13(words[i])
        if i != (len(words)-1):
            sentence += r_word + " "
        else:
            sentence += r_word
    print(sentence)
        
if __name__ == "__main__":
    main()