COMP 105 Introduction to Computer Science

Kalamazoo College Computer Science Program Fall 2008

Mini-Lab 5: Loops and Lists

Introduction

The purpose of this mini-lab is to practice working with while loops (introduced in Chapter 7) and working with lists (Chapter 9).

Exercises

• The Birds *

  In this exercise we will recreate an excised scene from Alfred Hitchcock's classic film "The Birds". In this scene a group of birds slowly and methodically moves toward their hapless victim.

  Start out by adding three or more birds (available in the web gallery) to an otherwise empty world. Create a list containing all of the birds that you added. Now create an animation in which all of the birds take turns moving toward the camera by a small amount. Don't move each of the birds individually in your code. Instead, iterate through the list of birds using a "For all in order" block to create the animation.

  Until now we have used the "loop" construct to create infinite loops. Another possibility is to use a while loop with the condition set to "true". Put your "For all in order" block inside an infinite while loop so the birds keep moving toward the camera until the animation is terminated by the user. THIS WILL BE IMPORTANT IN THE NEXT EXERCISE.

  The effect should be that the birds are moved a bit closer to the camera each time through the list.

• Revenge of the Birds *

  The animation from the previous exercise doesn't have a very terrifying conclusion. The birds eventually reach the camera and then just bounce back and forth. In this exercise we will modify the animation so that once the first bird reaches the camera, there is a scary red screen effect and the animation terminates. In order to do this, create a world level function, "nearVictim", that will check to see whether any of the birds have reached the camera. This function should take the list of birds as a parameter. It should then iterate through the list, checking each bird to see whether it is within some small distance of the camera. If it ever finds such a bird it should immediately return true, otherwise it should return false. Keep in mind it only makes sense to return false (there are no birds near the victim), after all of the birds have been checked; Be careful that you don't return false immediately after checking the first bird.

  Once this function is written, you can easily modify your animation from the previous exercise so that it terminates when the first bird reaches the camera. Just replace the "true" in the while loop with a call to your new function. (You might also have to throw in a "not" from the world's list of functions. The loop should continue while the birds have not reached the camera.)
  
  In order to create the scary red screen effect, add an instruction after your while loop that changes the light's color to red.