[python] wrap-around Caesar cipher

[memilanuk]

Hello all,

Started working my way through Zelle's book (for the second time, got side-tracked), and I'm working on the exercises at the end of Chapter 4. #9 in particular is giving me some grief. If someone could nudge me in the right direction, I'd appreciate it.

Basically expanding on a Caesar cipher (previous problem), now the task is to make the values 'wrap around' from z to a rather than 'falling off' when at the end of the alphabet.

I got most of it myself, did a little digging on the 'Net for similar problems and found the rest... The main thing that is bugging me a bit is that with this implemenation 'a' and 'z' end up having the same character substituted in for them, which could make for some really interesting deciphered text ;)

Here's the code I have thus far:



# caesar2.py
# Ch. 4 Exercise #9
# Write a program that can encode and decode Caesar ciphers. The input
# to the program will be a string of plain-text and the value of the key.
# The output will be an encoded message where each character in the original
# message is replaced by shifting it 'key' # of characters in the ASCII
# character set. For example, if 'ch' is a character in the string and 'key'
# is the amount to shift, then the character that replaces 'ch' can be
# calculated as: chr(ord(ch)+key).
#
# Expanded to enable circular operation so as to prevent 'dropping off'
# near the end of the alphabet.
#

import string

def main():
# inform user of the purpose of the program
print "This program will take plain-text plus a key and encode the"
print "text using a Caesar cipher."
print

# get input from user
plaintext = raw_input("Enter the plain-text phrase: ")
key = input("Enter the key value (i.e. how many characters to shift): ")

# initiate empty list
msgList = []

for ch in plaintext: # for each character in plaintext
code = ord(ch) + key # convert to ASCII num, add key value
if ch.isalpha(): # check if character is letter or other
if code > ord('z'): # if ascii num for code value is > ord('z')
code = code - (ord('z') - ord('a')) # wrap back around to ord('a')
msgList.append(chr(code)) # then append the shifted char to the list
else:
msgList.append(ch) # if not alpha, don't bother converting

ciphertext = string.join(msgList,"") # concatenate all the char in the list

# output results back to user, using string formatting
print "The resulting cipher-text would be '%s'." % (ciphertext)

main()



If you have any suggestions on a better way to do the wrap-around so 'a' and 'z' don't get mixed up, I'm all ears.

TIA,

Monte

[ssam]

code = code - (ord('z') - ord('a'))

is a transformation that will map 'z' into 'a'. imagine the letters are numbers from a=1 to z=26.


code = code - (26 - 1)

will transform 26 (z) into 1 (a).

what you want is to assume a cycle. so that 27 would be 'a' again. so really you need a transform that turns 27 into 1. to express that in your code you could use:


code = code - (ord('z') + 1 - ord('a'))

[memilanuk]

Cool! That does seem to have fixed it.

Thanks,

Monte