2008 September 24 / jjddaavviiss@@ccaarrlleettoonn..eedduu

Assignment 2

Carleton College CS 201, Fall 2008, Prof. Joshua R. Davis

The goal of this assignment is to practice writing Python code — in particular, writing subclasses and using a graphics library. By the end, you will have a functioning virtual aquarium.

0. Start Your Engines

Download these files to your computer. Put them all in a single folder.

aquarium.py is the program you'll be editing.
graphics.py is the graphics library we'll use.
graphics.pdf is the manual for the graphics library.

Open a Terminal window and navigate to that folder. Open the aquarium.py file in Smultron. Notice that I've typed "!!" in four places. It's not because I'm excited (although I am); those "!!" indicate where you are going to make changes. The first one is where you type your name; do that now.

1. Write The `Fish` Class

Write a class named Fish. It should have the following methods.

The constructor should take in five arguments:
1. the length of the fish (in pixels),
2. the starting x location of the fish,
3. the starting y location of the fish,
4. the fish's color, and
5. the graphics window.
It should create the fish and draw it in the graphics window (using the simple shapes in the graphics library).
swim() should take in one argument:
1. the number of pixels that the fish should move forward.
It should move the fish forward that number of pixels and redraw the fish at the new location.

2. Test The `Fish` Class

Modify main() to instantiate three to five fish and make them swim from their starting location to the edge of the screen. (If they swim off the screen, that's okay.)

3. Write The `FancyFish` Class

Write a class named FancyFish that is a subclass of Fish. Fancy fish are fancy in two ways: they have stripes, and they swim in a complicated manner. Here are the methods for FancyFish.

Its constructor takes all five arguments that Fish's constructor takes, and a sixth argument: the color of the stripe. It should call Fish's constructor, then add the stripe and draw the fish with the stripe.
fancySwim() takes in a single argument — the number of pixels to swim — but it causes the fancy fish to swim up by that amount, then forward by that amount, then down by that amount. You should call Fish's swim() method to accomplish the forward swimming part.

4. Test Both Kinds Of Fish

Modify main() so that it instantiates three to five fish of each kind and makes them swim. If they swim off the screen, that's okay. On the other hand, if you have time, then perhaps you can improve this behavior?

5. Hand In Your Work

Your file aquarium.py will be graded on these points:

Does it work? Do both kinds of fish function correctly? (3 points)
Does FancyFish make proper use of its superclass Fish? (3 points)
Is the code clean and appropriately commented? (1 point)

Here's how you submit aquarium.py electronically. In the Terminal, make sure that you are in the directory where your aquarium.py file is. Then type

hsp aquarium.py cs201-00-f08

This command launches the CS department's homework submission program. It will ask for your usual Carleton password. If you have problems, contact Mike Tie in CMC 305.