Wrap list around in Python

Question

I'm making a cipher where a text input is given and the output is the input but shifted along 2 in the alphabet for example "hi" is turned into "jk". I'm having problems wrapping the list around so that "y" can turn into "b" and so on. Plaintext is a set input. Key is 2

charset=["A","B","C","D","E","F","G","H","I","J","K","L","M","N","O","P","Q","R","S","T","U","V","W","X","Y","Z"] # characters to be encrypted

def caesar_encrypt(plaintext,key):

    plaintext = plaintext.upper() # convert plaintext to upper case
    ciphertext = "" # initialise ciphertext as empty string

    for ch in plaintext:
        if ch == " ":
            pass
        else:
            index = charset.index(ch)
            newIndex = index + key
            shiftedCharacter = charset[newIndex]
            ciphertext += shiftedCharacter
            print(ciphertext)
    return ciphertext

try `newIndex = (index + key) % len(charset)` (meet _integer modulo_) — Jean-François Fabre, Oct 10 '17 at 19:35
The indentation in this is just plain messed up which can cause problems. — tadman, Oct 10 '17 at 19:36
On another note, `charset = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"` words just as well and is a lot more compact. You could even do `import string` and then use `string.ascii_uppercase`. — Izaak van Dongen, Oct 10 '17 at 19:52

score 2 · Accepted Answer · edited Oct 10 '17 at 19:46

2

Just change:

newIndex = index + key

To:

newIndex = (index + key) % len(charset)

This way, the values will wraparound gracefully

Modulo (%) documentation

edited Oct 10 '17 at 19:46

thumbtackthief

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answered Oct 10 '17 at 19:38

Daniel Trugman

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score 0 · Answer 2 · answered Oct 10 '17 at 19:36

0

To shift, you can try this:

import string

converter = {string.ascii_uppercase[i]:string.ascii_uppercase[i+2] for i in range(24)}

answered Oct 10 '17 at 19:36

Ajax1234

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As far as I can see this won't wrap around for `z`. You need modulo, which has been covered by other answers. – Izaak van Dongen Oct 10 '17 at 19:50

Wrap list around in Python

2 Answers2