How to reverse lookup characters in list?

[tjland]

Okay so im working on a very simple encryption method using just loops.
Kind of novel i think. Okay so first i set up a list of the alphabet with
just every seperate letter, then user is prompted for a word, this is not
user friendly just for me. Ok now as you can see below this is pretty
basic, if u can follow all the loops i get to a point where I have the
letter positions in the list for the final word but i dont know of a way
to come back with the corresponding letter to output to the user. Ummm
some help would be really appreciated. Sorry about the length!


----------------------------------------------------------------------------
from time import sleep
#Alphabet list used to move letters forward for simple encryption.
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"]

word_list = []
number_list = []
number_list2 = []
new_word = []
#variable for how far you want to switch
v = 2
word = raw_input("Word: ")
a = len(word)
for x in range(0, len(word)):
print word[x],
word_list.append(word[x])
b = alphabet.index(word[x])
number_list.append(b)

for x in range(0, len(number_list)):
c = number_list[x]
c = c + v
number_list2.append(c)

for x in range(0, len(number_list2)):
d = number_list2[x]
new_word.append(d)

for x in range(0,
#Stopped here because dont know of way to switch back.
----------------------------------------------------------------------------



When you see the net take the shot
When you miss your shot shoot again
When you make your shot you win!

Just remember
Offense sells tickets
But defense wins championships!

 

[Keith Jones]

okay, isn't the number in the list the index of the new letter? So you'd
just use alphabet[d].. though what you're doing can be done in a much
simpler manner..

for starters... check out the string module. you can do

import string
string.lowercase

that gives you a-z, lowercase.. I think you'll be fine with a string,
rather than a list (infact, it's better since it's immutable). If you must
ahve a list, you can do

[a for a in string.lowercase]

to get a list of characters from a to z.

Now, all you're really doing with all those lists and loops is just
mapping from one character to another. In fact, you're just doing a rotX,
where x is the value of your variable v

you could just do:

rotation = int(raw_input('rotation value: '))
word = raw_input('word: ')
.....................
new_word = ''
for char in word:
position = string.lowercase.index(char)
position += rotation

# this makes it wrap around. i.e. 'z'+2 = 'b'
position = position % 26

new_word += string.lowercase[position]

print 'your new word is', new_word
.....................

Hope that helps some.

 

[Klaus Alexander Seistrup]

If you must ahve a list, you can do

[a for a in string.lowercase]

to get a list of characters from a to z.

Or simply "list(string.lowercase)".


// Klaus

--

 

[Andrew Dalke]

Keith Jones

import string
string.lowercase

that gives you a-z, lowercase..

Actually, use 'string.ascii_lowercase' because 'string.lowercase' depends
on your locale.

On the other hand, for this application, string.lower might be the right
thing.

Also, the tjland? You need to change the

c = c + v
into
c = (c + v) % len(alphabet)

Suppose c is 'z' and v is 2. Then 25 (the position of the 'z') + 2 is 27,
which
is beyond the list of letters. You need some way to loop around to 0 once
you go over the end, and the '%' is the loop-around operator. (It's
actually
called the mod function.) It also works the other way, so "-1 % 26" loops
around to 25.

You might find this helpful
.... t = t + letters[(letters.index(c)+v)%len(letters)]
........ t = t + letters[(letters.index(c)+v)%len(letters)]
....
With a bit more experience, I think you'll find this also interesting

v = 2

# Start by mapping each letter to itself
# (There are 256 possible values in an 8-bit character.)
encoding_d = {}
for i in range(256):
c = chr(i)
encoding_d[c] = c

lc = string.ascii_lowercase
for i in range(len(lc)):
from_char = lc
to_char = lc[(i+v) % len(lc)]
encoding_d[from_char] = to_char

uc = string.ascii_uppercase
for i in range(len(uc)):
from_char = uc
to_char = uc[(i+v) % len(uc)]
encoding_d[from_char] = to_char

s = "This is a test."

# This is better written as:
# t = "".join([encoding_d[c] for c in s])
# but that's something to try out after you learn a
# bit more Python.

t = ""
for c in s:
t = t + encoding_d[c]

print t

# Make a decoding dictionary which is the opposite of the
# encoding dictionary
decoding_d = {}
for from_char, to_char in encoding_d.items():
decoding_d[to_char] = from_char

# or as: u = "".join([decoding_d[c] for c in t])
u = ""
for c in t:
u = u + decoding_d[c]

print u

With some more experience beyond that, you might write it like this
(It uses a very different style and implementation)

import string

def _r(s, n):
# rotate the characters left n positions
n %= len(s) # make sure it's properly within range
return s[n:] + s[:n]

class RotEncoder:
def __init__(self, n):
from_letters = string.ascii_lowercase + string.ascii_uppercase
to_letters = _r(string.ascii_lowercase, n) + _r(string.ascii_uppercase,
n)
self.encode_table = string.maketrans(from_letters, to_letters)
self.decode_table = string.maketrans(to_letters, from_letters)
def encode(self, s):
return string.translate(s, self.encode_table)
def decode(self, s):
return string.translate(s, self.decode_table)

code = RotEncoder(2)
print code.encode("testing")
print code.decode(code.encode("testing"))

Andrew
(e-mail address removed)

 

[Bob Gailer]

okay, isn't the number in the list the index of the new letter? So you'd
just use alphabet[d].. though what you're doing can be done in a much
simpler manner..

for starters... check out the string module. you can do

import string
string.lowercase

that gives you a-z, lowercase.. I think you'll be fine with a string,
rather than a list (infact, it's better since it's immutable). If you must
ahve a list, you can do

[a for a in string.lowercase]

Or just list(string.lowercase])
[snip]

Bob Gailer
(e-mail address removed)
303 442 2625

 

[Keith Jones]

Or just list(string.lowercase])
[snip]

Haha.. I KNEW there was a better way to do it! It was late, I was tired, I
had list comprehensions on my mind, <insert more inane excuses here>.


Keith