Simple question about my ceasar cipher decoding loop

Language Help Python
1

It is an exercise in one of the Python courses. I don’t understand why this doesn’t work.
Caesar ciphers work by giving you an enconded message and a key, which is the number of times you have to shift the letter to find the correct one. In this case the key is 10.
The error i get when running this is for alphabet[index]:
IndexError: string index out of range

alphabet = "abcdefghijklmnopqrstuvwxyz"
message = "xuo jxuhu! jxyi yi qd unqcfbu ev q squiqh syfxuh. muhu oek qrbu je tusetu yj? y xefu ie! iudt cu q cuiiqwu rqsa myjx jxu iqcu evviuj"

new_message = ""
for character in message:
    if character in alphabet:
        index = alphabet.find(character)
        index = index + 10
        if index > len(alphabet):
            index = index - len(alphabet)
        new_message = new_message + alphabet[index]
    else:
        new_message = new_message + character
print(new_message)
2

I’m lazy, so I just popped in some debug code:

alphabet = "abcdefghijklmnopqrstuvwxyz"
message = "xuo jxuhu! jxyi yi qd unqcfbu ev q squiqh syfxuh. muhu oek qrbu je tusetu yj? y xefu ie! iudt cu q cuiiqwu rqsa myjx jxu iqcu evviuj"

new_message = ""
for character in message:
    if character in alphabet:
        print("character", character)
        index = alphabet.find(character)
        print("found", index)
        index = index + 10
        print("shift", index)
        if index > len(alphabet):
            index = index - len(alphabet)
            print("shift 2", index)
        new_message = new_message + alphabet[index]
    else:
        new_message = new_message + character
print(new_message)

Final fail:

character i
found 8
shift 18
character q
found 16
shift 26
Traceback (most recent call last):
  File "t.py", line 15, in <module>
    new_message = new_message + alphabet[index]
IndexError: string index out of range

Well, 26 is a wee bit over… try if index >= len(alphabet):

3

thanks! i also found i was supposed to use modulus 26, which works but i’m not sure how. my way was more… artisanal

4

Actually, I was going to mention the mod thing. To give an idea, this

print([x % 3 for x in (0,1,2,3,4,5,6)])

Yields:

[0, 1, 2, 0, 1, 2, 0]

Note the because find returns a -1 index on fail, you needn’t bother with the in check.

I took a stab at this and came up with:

def ceasar(msg, shift = 3, domain = "abcdefghijklmnopqrstuvwxyz"):
    def enc_char(c):
        index = domain.find(c)
        if index == -1:
            return c
        return domain[(index + shift) % len(domain)]
    return "".join(enc_char(c) for c in msg)

message = "xuo jxuhu! jxyi yi qd unqcfbu ev q squiqh syfxuh. muhu oek qrbu je tusetu yj? y xefu ie! iudt cu q cuiiqwu rqsa myjx jxu iqcu evviuj"

print(ceasar(message, 10))