Text Shift function in Python

Question

I\'m writing code so you can shift text two places along the alphabet: \'ab cd\' should become \'cd ef\'. I\'m using Python 2 and this is what I got so far:

Answer 1

You are looping over the list of characters, and i is thus a character. You then try to store that back into data using the i character as an index. That won't work.

Use enumerate() to get indexes and the values:

def shifttext(shift):
    input=raw_input('Input text here: ')
    data = list(input)
    for i, char in enumerate(data):
        data[i] = chr((ord(char) + shift) % 26)
    output = ''.join(data)
    return output

You can simplify this with a generator expression:

def shifttext(shift):
    input=raw_input('Input text here: ')
    return ''.join(chr((ord(char) + shift) % 26) for char in input)

But now you'll note that your % 26 won't work; the ASCII codepoints start after 26:

>>> ord('a')
97

You'll need to use the ord('a') value to be able to use a modulus instead; subtracting puts your values in the range 0-25, and you add it again afterwards:

    a = ord('a')
    return ''.join(chr((ord(char) - a + shift) % 26) + a) for char in input)

but that will only work for lower-case letters; which might be fine, but you can force that by lowercasing the input:

    a = ord('a')
    return ''.join(chr((ord(char) - a + shift) % 26 + a) for char in input.lower())

If we then move asking for the input out of the function to focus it on doing one job well, this becomes:

def shifttext(text, shift):
    a = ord('a')
    return ''.join(chr((ord(char) - a + shift) % 26 + a) for char in text.lower())

print shifttext(raw_input('Input text here: '), 3)

and using this on the interactive prompt I see:

>>> print shifttext(raw_input('Input text here: '), 3)
Input text here: Cesarsalad!
fhvduvdodgr

Of course, now punctuation is taken along. Last revision, now only shifting letters:

def shifttext(text, shift):
    a = ord('a')
    return ''.join(
        chr((ord(char) - a + shift) % 26 + a) if 'a' <= char <= 'z' else char
        for char in text.lower())

and we get:

>>> print shifttext(raw_input('Input text here: '), 3)
Input text here: Ceasarsalad!
fhdvduvdodg!