adding string to a string

Question

I have a problem adding a string to another string. I am new to Python.

The string cannot remember the previous values I added.

Anyone who can help me? The following is the code snippet in Python.

My problem is in the while loop of encrypt().

THANKS IN ADVANCE.

class Cipher:


    def __init__(self):
        self.alphabet = "abcdefghijklmnopqrstuvwxyz1234567890 "
        self.mixedalpha = ""
        self.finmix = ""

    def encrypt(self, plaintext, pw):
        keyLength = len(pw)
        alphabetLength = len(self.alphabet)
        ciphertext = ""

        if len(self.mixedalpha) != len(self.alphabet):
            #print 'in while loop'

            x = 0
            **while x < len(self.alphabet):
                mixed = self.mixedalpha.__add__(pw)
                if mixed.__contains__(self.alphabet[x]):
                    print 'already in mixedalpha'
                else:
                    add = mixed.__add__(str(self.alphabet[x]))
                    lastIndex = len(add)-1
                    fin = add[lastIndex]
                    print 'fin: ', fin
                    self.finmix.__add__(fin)
                    print 'self.finmix: ', self.finmix
                x+=1**


        print 'self.finmix: ', self.finmix
        print 'self.mixedalpha: ', self.mixedalpha
        for pi in range(len(plaintext)):
            #looks for the letter of plaintext that matches the alphabet, ex: n is 13
            a  = self.alphabet.index(plaintext[pi])
            #print 'a: ',a
            b = pi % keyLength
            #print 'b: ',b
            #looks for the letter of pw that matches the alphabet, ex: e is 4
            c = self.alphabet.index(pw[b])
            #print 'c: ',c
            d = (a+c) % alphabetLength
            #print 'd: ',d
            ciphertext += self.alphabet[d]
            #print 'self.alphabet[d]: ', self.alphabet[d]
        return ciphertext

err... any chance you point out where you think the problem is? or maybe post a smaller example... or at the very least post the output and expected output from your program — Gareth Davis
– Gareth Davis, Commented Jun 10, 2011 at 6:16
Note that adding two strings is a rather expensive operation, since it involves creating a new string object and copying the contents of the two strings into it. If you do a lot of these operations and speed is of some importance, consider using a StringIO object from the cStringIO module instead. — Lauritz V. Thaulow
– Lauritz V. Thaulow, Commented Jun 10, 2011 at 9:42

tzot · Accepted Answer · 2011-06-10 08:59:32Z

3

Python strings are immutable, so you should reassign the variable name to a new string.

Functions with "__" in them are not normally what you really want to use. Let the interpreter make that call for you by using the built in operators/functions (in this case the "+" operator).

So, instead of:

self.finmix.__add__(fin)

I suggest you try:

self.finmix = self.finmix + fin

or the equivalent and terser:

self.finmix += fin

If you make this sort of change throughout, your problem will probably go away.

edited Jun 10, 2011 at 8:59

tzot

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answered Jun 10, 2011 at 6:21

John Mulder

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Comments

user1114 · Accepted Answer · 2011-06-10 06:21:29Z

1

I don't have a solution to your problem, but I have a couple of more general suggestions.

The private methods .__add__ and .__contains__ aren't meant to be used directly. You should use the the + and in operators directly instead.

Instead of going through the indexes of self.alphabet with a while loop...

while x < len(self.alphabet):
    print self.alphabet[x]
    x += 1

you could just iterate over the letters

for letter in self.alphabet:
    print letter

class Cipher: triggers a sort of backwards-compatibility mode that doesn't work with some newer features. Specifying class Cipher(object): would be better.

answered Jun 10, 2011 at 6:21

user1114

1

Comments

Gareth Davis · Accepted Answer · 2011-06-10 06:18:29Z

0

I'm guessing but the following:

self.finmix.__add__(fin)

#should be
self.finmix = self.finmix.__add__(fin)

answered Jun 10, 2011 at 6:18

Gareth Davis

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