CS 111: Introduction to Computer Science

Winter 2017

HW04: Caesar cipher

Due: Wednesday, 01/18 at 22:00

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 & Zebra! would be transformed to the ciphertext message Bdn & Cheud!. Note that letters near the end of the alphabet wrap around to the beginning (e.g., Y + 3 = B), uppercase and lowercase is maintained, non-alphabet symbols are fixed. You can, of course, perform a Caesar cipher transformation using other values than 3 for the shift. Yak & Zebra! with a shift of 4, for example, becomes Ceo & 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.

Your task

Write a program that:

• Asks the user for an integer (the Caesar shift value) and a string (the plaintext),
• Calls a function named caesar (see below for details) to obtain the ciphertext, and
• Prints the ciphertext.

Your caesar function should look like this:

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

prompt$ python3 caesar.py
What is your plaintext? follow the elk
What shift do you want to use? 3
Your ciphertext is: iroorz wkh hon
prompt$

A little help

• To step one character at a time through a string, see the strings2.py sample from an in-class lab.
• There are several ways to shift a letter to another letter by some integer amount. One way is to use the Python functions ord and chr, which are described in Section 5.4.1 of your textbook.
• Think about edge cases. The specification above specifies that the user should input an integer. Make sure your program does something sensible for any possible integer input, not just numbers like 3.

Optional extension

Suppose you have written a program called something.py, and you type python3 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. Experiment with allowing the user to specify the shift value as an optional command-line argument.

Grading

Please name your file caesar.py. This makes it a little easier for the grader.

• Style/comments [3 points]
  • All the style guidelines from previous assignments.
  • Have a comment that specifies what the caesar function does. You may simply copy the one I provided above. In the future, when you write your own functions, always write a (short) comment describing the function.
  • Make sure your caesar function matches the description in the assignment: not changing the spelling or even the order of the arguments (plaintext, shift). This allows others to use your function as long as they know the "specification" of the function (pass in a plaintext and a shift, get back a cyphertext) without caring about its "implementation" (the code that you write to create the resulting cyphertext).
  • Try not to have mysterious "magic" numbers appearing in your program. I may understand what 26 is, but I may be confused as to what 97 is, for example.
• caesar function [5 points]
  • Correctly performs the Caesar cypher for a variety of test cases.
• main function [2 points]
  • The rest of your code should go in a main function, which prompts the user for input and calls caesar.
  • The last line of your file should be main() to run the main function.

Start early, have fun, and discuss questions on Moodle.