在python 3.4中将列表的一部分从字符串转换为整数

Question

I'm trying to create a simple Caesar Cypher for a class project, and I can't get past creating the cypher key. So far I have:

import sys
ALPHABET = ['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','']

def main(text, key):
    print("Encoding message '", text,"'with key",key)
    ALPHABET = generate_cypher(key)

def generate_cypher(ckey):
    cypher = []
    for i in ALPHABET:
        x = ALPHABET[i] + ckey
        cypher = cypher + x
    return cypher

main(sys.argv[1], int(sys.argv[2])

I keep getting this error: TypeError: list indices must be integers, not str

Answer 1

The line for i in ALPHABET is iterating over the elements in ALPHABET not the indices. So its going 'a', 'b', 'c', etc. You want to use for i in range(len(ALPHABET)) if you want to iterate over the indices.

The error is telling you that ALPHABET[i] is failing as i is not an integer. In this case the first iteration is the letter a so it is trying to do ALPHABET['a'] which won't work and returns the error. That is why you need to iterate over the indices not the elements.

Assuming ckey is intended to be an integer for the offset of the cypher you can fix you code with the following:

Credit to @AChampion on a nice little trick for the index length

def generate_cypher(ckey):
    cypher = []
    for i in range(len(ALPHABET)):
        x = ALPHABET[(i+ckey)%len(ALPHABET)]
        cypher.append(x)
    return cypher

Notice that I added some logic to fix the situation at the end of your iteration where the index will be beyond the available indices of the list. The cypher needs to wrap around to the beginning at this point thus the necessity of the modulo for ensuring that the index will never be larger than the size of ALPHABET.