字符串索引超出范围（Python）

Question

I'm writing a program to encode, decode and crack with the Caesar Cipher.

I have this function that shifts the letters in a string along by a specified amount:

def shift(data, shifter):
    alphabet = "abcdefghijklmnopqrstuvwxyz"
    data = list(data)
    counter = 0  #  we will use this to modify the list while we iterate over it
    for letter in data:
        letter = letter.lower()
        if letter not in alphabet:
            counter += 1
            continue
        lPos = alphabet.find(letter)
        if shifter >= 0:
            shiftedPos = lPos + (0 - shifter)
        else:
            shiftedPos = lPos + abs(shifter)
        if shiftedPos >= len(alphabet) - 1: shiftedPos -= len(alphabet)
        data[counter] = alphabet[shiftedPos]  #  update the letter
        counter += 1  # advance
    data = ''.join(data)  # make it into a string again
    return data

And I have this function to crack a ciphered string:

def crack(decryptor=None, tries=None):
    if decryptor is None and tries is None:
        task = getValidInput("Get data from a [f]ile or [s]tdin?  >", "Please give either 'f' or 's'.", 'f', 's')
        if task == "f":  # it's a file
            dataFile = getValidFile()  # get an open file object
            data = dataFile.read()  # get the data from the text file. hopefully it's ascii text!
        elif task == "s":  # we need to get data from stdin
            data = input("Enter data to crack  >")
        tries = getValidInt("Enter tries per sweep  >")
    else:
        data = decryptor

    retry = True
    shifter = 0
    while retry:
        for i in range(0, tries):
            oput = "Try " + str(i) + ": "
            posData = shift(data, shifter)
            negData = shift(data, 0 - shifter)
            # semitry 1 - positive
            oput += posData + ", "
            # semitry 2 - negative
            negData = ''.join(negData)  # make it into a string again
            oput += negData

            print(oput)

            shifter += 1

        doRetry = getValidInput("Keep trying (y/n)? > ", "Invalid!", 'y', 'n')
        if doRetry == 'n': retry = False

However, after selecting 'y' to continue a few times, I get the following IndexError :

Traceback (most recent call last):
  File "CeaserCypher.py", line 152, in <module>
    crack()
  File "CeaserCypher.py", line 131, in crack
    negData = shift(data, 0 - shifter)
  File "CeaserCypher.py", line 60, in shift
    print(alphabet[shiftedPos])
IndexError: string index out of range

Why am I getting this error and how can I fix it?

Answer 1

IndexError means that the index you are trying to access does not exist. In a string, that means you're trying to get a character from the string at a given point. If that given point does not exist, then you will be trying to get a character that is not inside of the string.

"0123456"[7] tries to get the 7th character in the string, but that index does not exist so "IndexError" is raised.

All valid indexes on a string are less than the length of the string (when you do len(string)). In your case, alphabet[shiftedPos] raises IndexError because shiftedPos is greater than or equal to the length of the string "alphabet".

To my understanding, what you want to do is loop back over the string when you go out of bounds like this. "z" (character 25) gets incrimented by say 2 and becomes character 27. You want that to now become character 2 (letter "b") in this case. Hence, you should use modulo. replace "alphabet[shiftedPos]" with "alphabet[shiftedPos%len(alphabet)]" and I believe this will solve this problem.

Modulo, btw, divides a number by n and gives you the remainder. Effectively, it will subtract n until the number is less than n (so it will always be in the range you want it to be in).