Using a caesarian cipher on a string of text in python?

Question

I'm trying to slowly knock out all of the intricacies of python. Basically, I'm looking for some way, in python, to take a string of characters and push them all over by 'x' characters.

For example, inputing abcdefg will give me cdefghi (if x is 2).

Thanks!

Answer 1

This solution works for both lowercase and uppercase:

from string import lowercase, uppercase

def caesar(text, key):
    result = []
    for c in text:
        if c in lowercase:
            idx = lowercase.index(c)
            idx = (idx + key) % 26
            result.append(lowercase[idx])
        elif c in uppercase:
            idx = uppercase.index(c)
            idx = (idx + key) % 26
            result.append(uppercase[idx])
        else:
            result.append(c)
    return "".join(result)

Here is a test:

>>> caesar("abcdefg", 2)
'cdefghi'
>>> caesar("z", 1)
'a'

Answer 2

I think your best bet is to look at string.translate. You may have to use make_trans to make the mapping you like.

Answer 3

My first version:

>>> key = 2
>>> msg = "abcdefg"
>>> ''.join( map(lambda c: chr(ord('a') +  (ord(c) - ord('a') + key)%26), msg) )
'cdefghi'
>>> msg = "uvwxyz"
>>> ''.join( map(lambda c: chr(ord('a') +  (ord(c) - ord('a') + key)%26), msg) )
'wxyzab'

(Of course it works as expected only if msg is lowercase...)

edit: I definitely second David Raznick's answer:

>>> import string
>>> alphabet = "abcdefghijklmnopqrstuvwxyz"
>>> key = 2
>>> tr = string.maketrans(alphabet, alphabet[key:] + alphabet[:key])
>>> "abcdefg".translate(tr)
'cdefghi'

Answer 4

I would do it this way (for conceptual simplicity):

def encode(s):
    l = [ord(i) for i in s]
    return ''.join([chr(i + 2) for i in l])

Point being that you convert the letter to ASCII, add 2 to that code, convert it back, and "cast" it into a string (create a new string object). This also makes no conversions based on "case" (upper vs. lower).

Potential optimizations/research areas:

• Use of StringIO module for large strings
• Apply this to Unicode (not sure how)

Answer 5

Another version. Allows for definition of your own alphabet, and doesn't translate any other characters (such as punctuation). The ugly part here is the loop, which might cause performance problems. I'm not sure about python but appending strings like this is a big no in other languages like Java and C#.

def rotate(data, n):
    alphabet = list("abcdefghijklmopqrstuvwxyz")

    n = n % len(alphabet)
    target = alphabet[n:] + alphabet[:n]

    translation = dict(zip(alphabet, target))
    result = ""
    for c in data:
        if translation.has_key(c):
            result += translation[c]
        else:
            result += c

    return result

print rotate("foobar", 1)    
print rotate("foobar", 2)    
print rotate("foobar", -1)
print rotate("foobar", -2)

Result:

gppcbs
hqqdct
emmazq
dllzyp

The make_trans() solution suggested by others is the way to go here.