How to fix 'string index out of range' error in a function that ciphers a string

Question

I don't know how to efficiently go back to the beginning of the alphabet, when (index of a letter + 13) is out of range

I've written a function that only works if the (index of a letter + 13) is in range.

def rot13(message):
    letters = [i for i in message]
    for i in letters:
        if i.isupper():
            letters[letters.index(i)] = 
string.ascii_uppercase[string.ascii_uppercase.index(i) + 13]
        elif i.islower():
            letters[letters.index(i)] = 
string.ascii_lowercase[string.ascii_lowercase.index(i) + 13]
        else:
            continue
    return ''.join(letters)

When I call, e.g. rot13('Test'), of course I get the 'string index is out of range' error, how should i go about that problem?

Perhaps this might be of help too @hubvoy stackoverflow.com/questions/38558688/python-character-rotation — Devesh Kumar Singh
– Devesh Kumar Singh, Commented May 29, 2019 at 14:02

Florian H · Accepted Answer · 2019-05-29 13:35:58Z

1

Use the modulo operator %:

Modulo devides the number by the given factor and keeps the rest, eg:

27 % 26 = 1

In your case it would be this two lines:

string.ascii_uppercase[(string.ascii_uppercase.index(i) + 13) % 26]

string.ascii_lowercase[(string.ascii_uppercase.index(i) + 13) % 26]

answered May 29, 2019 at 13:35

Florian H

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