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Need help with caesar cipher

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Join Date: Mar 2007

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pyguy25

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Need help with caesar cipher

Mar 7th, 2007

Hello. I was wondering if anyone could help me with a caesar cipher program I am attempting to create. I was asked to write a caesar cipher encoder program. Ok. No problem. This is what i got:

 Help with Code Tags 
 Python Syntax (Toggle Plain Text) 
import string
def main():
    print "This program will encode your messages using a Caesar Cipher"
    print
    key = input("Enter the key: ")
    message = raw_input("Enter the message: ")
    codedMessage = ""
    for ch in message:
            codedMessage = codedMessage + chr(ord(ch) + key)
    print "The coded message is:", codedMessage
 
main()
import string
def main():
    print "This program will encode your messages using a Caesar Cipher"
    print
    key = input("Enter the key: ")
    message = raw_input("Enter the message: ")
    codedMessage = ""
    for ch in message:
            codedMessage = codedMessage + chr(ord(ch) + key)
    print "The coded message is:", codedMessage
 
main()

This works fine. But then I am asked to modify it to make it circular, where after "z" it will return to "a". I am at a loss for how to do this. This is probably a stupid question, but I am a beginner to Python and desperately need assistance. Please help. Thanks.

Last edited by pyguy25; Mar 7th, 2007 at 7:24 pm.

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Join Date: Jul 2006

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jrcagle

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Re: Need help with caesar cipher

Mar 7th, 2007

That's a good start. I might do something like

 Help with Code Tags 
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...
for ch in message:
    code_val  = ord(ch) + key
    if code_val > ord('z'):
        code_val -= 26  # better: ord('z') - ord('a'), just in case len(alphabet) != 26
    codedMessage = codedMessage+chr(code_val)
...
...
for ch in message:
    code_val  = ord(ch) + key
    if code_val > ord('z'):
        code_val -= 26  # better: ord('z') - ord('a'), just in case len(alphabet) != 26
    codedMessage = codedMessage+chr(code_val)
...

Jeff

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Join Date: Mar 2007

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pyguy25

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Re: Need help with caesar cipher

Mar 7th, 2007

Thanks. That really helps. Would there also be a way for me to make it so non-alphabetic characters (spaces, punctuation, etc.) do not change with the rest of the text?

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mawe

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Re: Need help with caesar cipher

Mar 8th, 2007

 Help with Code Tags 
 Python Syntax (Toggle Plain Text) 
...
for char in message:
    # check if the character is alphabetic
    if char.isalpha():
        ...
...
for char in message:
    # check if the character is alphabetic
    if char.isalpha():
        ...

Regards, mawe

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Lardmeister Lardmeister is offline

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Re: Need help with caesar cipher

Mar 8th, 2007

Sounds interesting, so i put it all together:

 Help with Code Tags 
 Python Syntax (Toggle Plain Text) 
message = "just a simple test!"
key = 11
coded_message = ""
for ch in message:
    code_val  = ord(ch) + key
    if ch.isalpha():
        if code_val > ord('z'):
            code_val -= ord('z') - ord('a')
        coded_message = coded_message + chr(code_val)
    else:
        coded_message = coded_message + ch
print message
print coded_message
message = "just a simple test!"
key = 11
coded_message = ""
for ch in message:
    code_val  = ord(ch) + key
    if ch.isalpha():
        if code_val > ord('z'):
            code_val -= ord('z') - ord('a')
        coded_message = coded_message + chr(code_val)
    else:
        coded_message = coded_message + ch
print message
print coded_message

-- and I get this:

Help with Code Tags

Python Syntax (Toggle Plain Text)

just a simple test!
ugef l etxbwp fpef!

-- great, but how can I get the original message back?

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jrcagle

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Re: Need help with caesar cipher

Mar 8th, 2007

Well, what you're trying to do is invert the function that got you there to begin with. See if you can decode a couple of examples by hand and then go from there.

Jeff