By the time you have completed this lab, you and your lab partner should be able to
Select whichever partner was pilot first last time to be navigator first this time. This lab's pilot should log in and create a lab04 directory under your cs8 folder. You will work in this account for the rest of the lab. If your partner is more than 5 minutes late, ask the TA to pair you with someone else for this week.
Start IDLE. Then select "File=>New Window" and add a comment at the top of this new file like the following (with the proper substitutions of course):
# lab04.py # Your name(s), today's date # a cipher function and test cases
Then, save it as lab04.py inside your lab04 directory.
You have been hired as a programmer! Your mission is to write a program that encrypts a string of text. From Exercise 3.26 on page 108 of the text (rot is short for rotate):
Encryption often involves the Caesar cipher - named after Julius Caesar, who used the system to encrypt military messages. Many early Internet users also adopted this cipher. Called rot13, the cipher encrypts a message by rotating the plaintext character by 13 positions in the alphabet. For example, "a" becomes "n" and likewise "n" becomes "a". The nice thing about rot13 is that the same function can be used to encrypt and decrypt a message.
Write function rot13 - it takes a message as a parameter, and it returns an encrypted string in which all the characters in the message are rotated by 13 places. You may assume that all characters in the message are lower case letters (no upper case letters, spaces, digits or anything else). Here is a skeleton of the function. We have initialized an important variable, and given in-line comments to help you out.
def rot13(message):
result = "" # starts empty; add one character at a time below
# loop through each character
# shift it by 13 places (may wrap around to front of alphabet)
# add it to the result string
Of course you should return the result at the end too.
Hints: Remember the chr() and ord() functions from lecture. Find a mathematical formula that will take the number of one character and transform it to the number of the desired character. You may find the '%' (modulus) operator helpful in your formula.
Switch roles between pilot and navigator. The new pilot's job is to create test strings to test the function. What characters should be in each string in order to fully test the encryption function? How can you test that the function will encrypt a string, and then accurately decrypt the resulting string?
Write the test code below the function definition - not as an indented part of the function itself. This way by running the module, your test cases will execute automatically. Make sure you print to the screen both the original string and the encrypted string.
Be ready to demonstrate your function and test cases to a TA.
| Note: you don't have to finish Step 4 to earn full credit. Please begin and try to finish, but if you are running out of time before lab ends, then move on to Step 5 - the TA will check off your work as complete anyway. |
Write and test an encryption function that shifts by a variable number of positions rather than always 13. Just name it rot, but have it take two arguments:
def rot(n, string):The first parameter, n is the number of positions by which to shift. So if if n is 13, then rot produces the same result as rot13. The results are different for other values of n though. Here are two example runs from our solution:
>>> rot(1,'apple') 'bqqmf' >>> rot(2,'apple') 'crrng'By the way, with our solution, a string that was previously encrypted by shifting n positions can be decrypted by passing -n to rot:
>>> rot(-2,'crrng') 'apple'It's not required, but does your solution do that too?
If you did not finish Step 4, be ready to show evidence that you tried it and to answer any questions your TA might pose about your efforts. Also insure a copy of lab04.py is open, and that the module has been run since you last changed it. The TA may want to see you demonstrate its functions.
| Get your TA's attention to inspect your work, and to record your lab completion. |
Don't leave early though ... see challenge problems below.
Step 5a. ONLY IF YOU RAN OUT OF TIME TO HAVE THE TA INSPECT YOUR WORK
If you must complete this assignment at CSIL, then submit it with the turnin program. You MUST have both your name and your partner's name in the file in order to receive credit. Remember that the original pilot needs to do this step, since that is whose account you have been using in Phelps 3525.
Bring up a terminal window on CSIL, and cd into the original pilot's cs8 directory, and cd again into the lab04 directory (because there is just one file to turn in this time). Then type the following:
turnin Lab04@cs8c lab04.pyRespond "yes" when the program asks if you want to turn in (be sure to read the list of files you are turning in), and then wait for the message indicating success.
Optional Extra Challenge
- Write and test an encryption function that shifts each character of the string one more than the last. For example, shiftEncrypt("apple",1) shifts the 'a' by 1 position, the first 'p' by 2 positions, and so on. Here is that example and another run from our solution:
>>> shiftEncrypt(1,'apple') 'brspj' >>> shiftEncrypt(3,'apple') 'dturl'- Did the prior challenge, and still got some time? Write and test a decryption function for shiftEncrypt named shiftDecrypt:
>>> shiftDecrypt(1,'brspj') 'apple' >>> shiftDecrypt(3, 'dturl') 'apple' >>> shiftDecrypt(7, shiftEncrypt(7, 'anylowercasestring')) 'anylowercasestring'Just wondering ... did you make use of your rot function to simplify these challenges?
Prepared for Computer Science 8 by Diana Franklin and Michael Costanzo.