COMP 105 Introduction to Computer Science

Kalamazoo College Computer Science Program Fall 2008

Mini-Lab 2: Functions, If/Else and Loops

Introduction

In this mini-lab we will be exploring several new programming constructs introduced in Chapter 3 of the Alice textbook. We will expand and modify some of this code during the next class, so be sure to include comments and to save frequently.

Exercises

• Net Jumper *

  In this scenario the mad scientist is testing his flubber augmented jumping skills by jumping over a volleyball net. He turns to face the net, jumps over it, then turns around and repeats the process. In all, he should jump over the net five times. The net should be moved upward one meter between each jump. You should write your code so that the scientist is able to jump the net successfully even if it is positioned arbitrarily high above the ground.

  Keep in mind that the "height" function associated with the volleyball net does not reflect how far the net is above the ground. It indicates how tall the net is, regardless of where it's located. Also, don't worry about the fact that the volleyball net will appear to float in midair after it is moved upwards.

• Fugu Chase *

  In this scenario, a shark is chasing a small fish: The fish darts about randomly, while the shark methodically moves toward it. If the shark ever comes close to making contact with the fish, then the fish expands to many times its original size and the shark swims away in fear.

  This is a fairly complicated animation. I suggest that you complete it by breaking the problem into several pieces, each of which should be solved and tested in turn:

  Step #1: Random Fish

  In order to create the effect of a randomly moving fish, you can use an If/Else statment inside a loop. The If/Else can be conditioned on the world level function choose true 50% of the time which randomly returns either true or false. If the result is true, then the fish moves foward and to the left, if the result is false, then the fish moves forward and to the right. HINT: Motions inside of a loop will look jerky as they start and stop each time through the loop. This can be addressed by making sure that the style of the motions is set to be "abruptly".

  Step #2: Terrifying Shark

  Every time the fish moves, the shark should simultaneously move toward him. This will look most realistic if the shark turns to face the fish and moves forward at the same time. In order to make for an exciting chase the shark should move some smallish fixed interval on every time step.

  Step #3: The Exciting Conclusion

  At this point, you should have a realistic looking chase, with an anticlimactic conclusion - nothing happens when the shark catches the fish. Your last step is to modify the code so that it checks to see if the shark has gotten sufficiently close to the fish, and generates the appropriate animation when it has. Your check for "sufficiently close" should take into account the sizes of the two animals. In other words, the animation should still look right if the fish and shark are re-sized.

  Remember that the distance between two objects is measured as the distance between the centers of the objects. You will not be able to get the fish to expand exactly when the two objects touch.

  Extras **

• Since the fish is moving moving randomly, it is possible that the action will leave the field of view of the camera. Add code to ensure that the camera is always pointed at the fish.
• Even if the camera is always pointed at the fish, it is possible that the fish will move so far away from the camera that the action becomes hard to follow. Add code to allow the camera to maintain a fixed distance from the fish.