CS 111: Introduction to Computer Science

Implementing the Caesar cipher

Due: 11:59PM Wednesday, September 23. Hand in as caesar.py.

The Caesar cipher is a very simple way of creating coded messages. The original cipher, attributed to Julius Caesar, involves shifting every letter in a plaintext message 3 positions further down in the alphabet to get the corresponding ciphertext message. For example, a Q in the plaintext message would be shifted by three postions to become a T in the ciphertext. A plaintext message of "YAK AND ZEBRA" would be transformed to the ciphertext message "BDN DQG CHEUD". Note that letters near the end of the alphabet wrap around to the beginning (e.g. Y + 3 = B). You can, of course, perform a Caesar cipher transformation using other values than 3 for the shift. "YAK AND ZEBRA" with a shift of 4, for example, becomes "CEO ERH DIFVE".

In this assignment, you will write a function that implements the Caesar cipher. This will give you some practice encapsulating a collection of operations into a Python function. This assignment will also introduce the use of command-line arguments in Python programs.

Your task

Write a program that:

Accepts an integer (the Caesar shift value) as its first command-line parameter, and a string (the plaintext) for its second,
Calls a function named caesar (see below for details) to obtain the ciphertext, and
Prints the ciphertext.

Your caesar function should look like this:

def caesar(shift, plaintext): '''Returns ciphertext obtained by transforming every letter in the plaintext by a Caesar cipher by the specified shift. Non-letter characters should remain unchanged in the ciphertext.''' [Your code goes here] return [something]

A typical session with your completed program might look like this:


prompt> python caesar.py 3 "follow the elk"
iroorz wkh hon
prompt>

Note that if you want your plaintext to include spaces, you need to enclose it in quotation marks so the shell will consider the plaintext to be a single command-line parameter.

A little help

To step one character at a time through a string, see the strings2.py sample from last Friday's lab.
To shift a letter to another letter by some integer amount, you will need to use the Python functions "ord" and "chr", which are described in Chapter 4 of your textbook, starting on page 88.
Suppose you have written a program called "something.py", and you type "python something.py pig cow goose" in the terminal window. In this situation, "something.py", "pig", "cow", and "goose" are all referred to as command-line arguments of the program something.py. Before the Python interpreter starts executing your program, it creates a list called "sys.argv" to contain the command-line arguments. In this assignment, your users will communicate information to caesar.py using command-line arguments. To see how such a thing is done, look at the commandline.py sample program.

We will discuss all of these ideas in class on Monday (9/21).

Start early, have fun, and ask questions.