I'm getting a typeError: string indices must be integers, not type

Question

alpha = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
key = "XPMGTDHLYONZBWEARKJUFSCIQV" 

def encode():
    alpha[""] = key["x"]

def decode():
    key[""] = alpha[""] 

def menu():
    response = raw_input("""Crypto Menu
        quit(0)
        encode(1)
        decode(2)""")
    return response

def main():   
    keepGoing = True
    while keepGoing:
    response = menu()
    if response == "1":
        plain = raw_input("text to be encoded: ")
        print encode()
    elif response == "2":
        coded = raw_input("code to be decyphered: ")
        print decode()
    elif response == "0":
        print "Thanks for doing secret spy stuff with me."
        keepGoing = False
    else:
        print "I don't know what you want to do..."

print main()

I keep getting a TypeError saying string indices must be integers, not type. Not sure how to correct this, it is highlighting the decode and encode variables.

Answer 1

I think your problem is here:

alpha = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
key = "XPMGTDHLYONZBWEARKJUFSCIQV" 

def encode():
    alpha[""] = key["x"]

def decode():
    key[""] = alpha[""] 

I think you misunderstand how indexing in strings works. So let me try to correct this:

Take a string, like x = "hello". The reference x["h"] is meaningless. There's no way for Python to interpret this. On the other hand, x[0] is meaningful. It returns the element of x at index 0. That's "h", in our case.

Similarly, alpha[""] doesn't mean anything. When you use the square brackets, you are trying to specify an index in the string alpha. But the indices of alpha are integers. alpha[0] returns "A". alpha[1] returns "B". alpha[25] returns "Z".

So you need to use integers for your indices. Notation like key["x"] doesn't mean anything, and that raises errors.

Answer 2

There's a lot going on here, but you're definitely having issues with your encode and decode functions. If I understand what you're trying to do, you could rewrite them as follows:

def encode(string):
    alpha = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
    key = 'XPMGTDHLYONZBWEARKJUFSCIQV'
    encoded = []
    for character in string:
        character_index = alpha.index(character)
        encoded.append(key[character_index])
    return ''.join(encoded)

def decode(string):
    alpha = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
    key = 'XPMGTDHLYONZBWEARKJUFSCIQV'
    decoded = []
    for character in string:
        character_index = key.index(character)
        decoded.append(alpha[character_index])
    return ''.join(decoded)

Each function is doing essentially the same thing (here's what encode is doing:

1. Creating an empty list called encoded. I'll use this to store each character, translated, in order.
2. Loop through the characters of the string passed in.
3. At each iteration, find its index in the string alpha.
4. Find the character in key at that same index and append that character to the list encoded
5. Once all the characters have been translated, join them into a string and return that string.

Note: This will fail if a character in the string argument is not found in the alpha string. You could add some error checking.

You could make this even more general if you wanted to allow for different keys. You could write a translate function like this:

def translate(string, from_language_string, to_language_string):
    translated = []
    for character in string:
        character_index = from_language_string.index(character)
        translated.append(to_language_string[character_index])
    return ''.join(translated)

And then your encode and decode functions could be written like this:

def encode(string):
    return translate(string, 'ABCDEFGHIJKLMNOPQRSTUVWXYZ', 'XPMGTDHLYONZBWEARKJUFSCIQV')

def decode(string):
    return translate(string, 'XPMGTDHLYONZBWEARKJUFSCIQV', 'ABCDEFGHIJKLMNOPQRSTUVWXYZ')

To address what's going on in the rest of your code, for the conditionals in your main function, you'll just want to make sure to pass the strings read in from raw_input to the encode and decode functions as needed. Something like this:

if response == '1':
    plain = raw_input('text to be encoded: ')
    print encode(plain)
    # and so on

Good luck.