CSC 162 Frogger Project

Spring 2010

Your task for this, the final project of the semester, is to complete the implementation of a simple version of the classic frogger game.  In this game, each of several frogs has to cross a busy road and a swift-flowing river to reach the safety of lily pads on the other side.  While crossing the road the frogs must avoid getting run over by cars; to cross the river they must hop from log to log (to avoid being eaten by crocodiles). 

We are providing you with starter code that uses the Tkinter graphics package and the threading concurrency package (both of which are standard in most installations of Python).  You’ll discover that cars and logs are implemented as threads, which execute concurrently (in parallel with one another).  Frogs, on the other hand, are implemented as event handlers, which execute in response to arrow key presses in the little game window. 

If you try out the code you’ll find that the cars, logs, and frogs all move, but they completely ignore one another.  Your task is to implement the interactions, so that careless frogs get run over—or die if they fall in the water—and frogs that land on logs move with them up or down the river. 

If you wish, you may work in teams of two on this project.  If you choose this option, I recommend that one of you implement the interaction between frogs and cars, and the other implement the interaction between frogs and logs. 


Right now, the locations of the cars, the logs, and the (single) active frog are kept in X and Y fields of the corresponding objects.  I recommend you break these out into a separate, global locations object, accessible to all your code.  This will make it easy for cars to tell when they run over a frog, for logs to move the active frog, and for the frog to tell when it has jumped under the wheels of a car or into the croc-infested water. 

Beware, however, that because this is a concurrent program, it is possible for, say, both frog code and log code to try to move the active frog at more or less the same time.  This can lead to subtle bugs.  You will want to learn about Lock or RLock objects (either will do), which can be used to force threads to take turns trying to change locations.  The most straightforward way to synchronize your program is probably to put a lock object in the locations object; define getter and setter methods that read, write, or update X and Y fields together; and protect the bodies of those methods with lock acquire and release operations.  The key is for each method call to move the world from one consistent state to another (e.g., “if the frog is on top of this log, move both of them together one unit up or down the river”). 

In certain cases (e.g., on some Linux distributions), Python may be installed with a non-thread-safe version of Tkinter.  If you encounter a strange “out of stack space” error message the moment you run your code, this may be what’s going on; check with the TA or professor for help.  (Standard PC and Mac installations of IDLE should be fine.) 


Please read the grading standards web page carefully and follow its instructions.  In particular, note that you will need to create a README.txt or README.pdf file, and you will need to turn your code in using Blackboard.  Many students have started too late on past assignments.  Don’t do that this time!  There isn't a huge amount of code to write, but it may take you several days of occasional study and intervening contemplation to understand what's going on. 

Extra Credit Suggestions

For extra credit (counted at the end of the semester; may raise your final grade), you might consider the following possibilities. 

Due Date:  Fri., Apr. 30 at 5:00 pm; no extensions.

Last Change:  12 April 2010 / Michael Scott's email address