How to reverse lookup characters in list?

[tjland@iserv.net]

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.appen d(word[x])
b = alphabet.index( word[x])
number_list.app end(b)

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

for x in range(0, len(number_list 2)):
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
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But defense wins championships!

[Keith Jones]

Re: How to reverse lookup characters in list?

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.lowercas e

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.lowercas e]

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.lowercas e.index(char)
position += rotation

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

new_word += string.lowercas e[position]

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

Hope that helps some.

On Mon, 28 Jul 2003 00:15:35 -0400, tjlan wrote:


[color=blue]
> ----------------------------------------------------------------------------
> 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....
>
> 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.appen d(word[x])
> b = alphabet.index( word[x])
> number_list.app end(b)
>
> for x in range(0, len(number_list )):
> c = number_list[x]
> c = c + v
> number_list2.ap pend(c)
>
> for x in range(0, len(number_list 2)):
> d = number_list2[x]
> new_word.append (d)
>
> for x in range(0,
> #Stopped here because dont know of way to switch back.[/color]

[Klaus Alexander Seistrup]

Re: How to reverse lookup characters in list?

Keith Jones wrote:
[color=blue]
> If you must ahve a list, you can do
>
> [a for a in string.lowercas e]
>
> to get a list of characters from a to z.[/color]

Or simply "list(string.lo wercase)".


// Klaus

--[color=blue]
><> unselfish actions pay back better[/color]

[Andrew Dalke]

Re: How to reverse lookup characters in list?

Keith Jones[color=blue]
> import string
> string.lowercas e
>
> that gives you a-z, lowercase..[/color]

Actually, use 'string.ascii_l owercase' because 'string.lowerca se' 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
[color=blue][color=green][color=darkred]
>>> s = "testing"
>>> v = 2
>>> letters = string.ascii_lo wercase
>>> t = ""
>>> for c in s:[/color][/color][/color]
.... t = t + letters[(letters.index( c)+v)%len(lette rs)]
....[color=blue][color=green][color=darkred]
>>> t[/color][/color][/color]
'vguvkpi'[color=blue][color=green][color=darkred]
>>> s = t
>>> v = -2
>>> t = ""
>>> for c in s:[/color][/color][/color]
.... t = t + letters[(letters.index( c)+v)%len(lette rs)]
....[color=blue][color=green][color=darkred]
>>> t[/color][/color][/color]
'testing'[color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

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_lo wercase
for i in range(len(lc)):
from_char = lc[i]
to_char = lc[(i+v) % len(lc)]
encoding_d[from_char] = to_char

uc = string.ascii_up percase
for i in range(len(uc)):
from_char = uc[i]
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.item s():
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_lo wercase + string.ascii_up percase
to_letters = _r(string.ascii _lowercase, n) + _r(string.ascii _uppercase,
n)
self.encode_tab le = string.maketran s(from_letters, to_letters)
self.decode_tab le = string.maketran s(to_letters, from_letters)
def encode(self, s):
return string.translat e(s, self.encode_tab le)
def decode(self, s):
return string.translat e(s, self.decode_tab le)

code = RotEncoder(2)
print code.encode("te sting")
print code.decode(cod e.encode("testi ng"))

Andrew
dalke@dalkescie ntific.com

[Bob Gailer]

Re: How to reverse lookup characters in list?

At 06:11 AM 7/28/2003 +0000, Keith Jones wrote:
[color=blue]
>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.lowerca se
>
>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.lowercas e][/color]

Or just list(string.low ercase])
[snip]

Bob Gailer
bgailer@alum.rp i.edu
303 442 2625


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[Keith Jones]

Re: How to reverse lookup characters in list?

[color=blue]
> Or just list(string.low ercase])
> [snip]
>[/color]

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