COMP 105 Introduction to Computer Science

Kalamazoo College Computer Science Program Fall 2008

Mini-Lab 4: Events and User Defined Functions

Introduction

The goal of this mini-lab is to practice writing event driven programs and user defined functions.

Exercises

• Obstacle Course*

  The goal of this exercise is to create an Alice world in which the user attempts to navigate a character through an obstacle course. The first step is to add a character of your choice to the world, along with several appropriate obstacles and an object to represent the finish line.

  Character control will be implemented by writing event handling methods that are called when keys are pressed. You will need to implement at least three methods: one to move the character forward, one to turn her right, and one to turn her left. You may also want to implement a method that causes the character to jump. Each of these methods should do more than simply move the character to the appropriate position. The movement should be animated so that it looks realistic, or at least interesting. For example, you might animate the character's legs so that she appears to walk when moving forward. Just to be clear: you should not use Alice's built in "Let the arrow keys move..." event.

  Don't worry about handling collisions with obstacles, or about doing anything special when the character reaches the finish line. The player is on the honor system.

  Note: The "When a key is typed" event doesn't actually take place until the end of a key press. This can result in an annoying lag when these events are used to handle character control. One solution is to convert "When a key is typed" events to "While a key is pressed" events. This can be accomplished by first adding a "When a key..." event, then right clicking it and selecting the "Change to" option. Then, any method that is called from the "Begin:" position will be called as soon as the key is pressed.

• Checking for a Collision*

  Our obstacle course game would be more interesting if we could actually collide with the obstacles. For this exercise write a function called "checkForCollision" that checks for a collision between any two objects. The function should take two parameters representing the objects. It should return true if the obstacles are in contact, and false if they are not. Your function should take into consideration how far apart the two objects are as well as their sizes. In other words, two small objects need to be closer together to collide than two larger ones.

  Test your function by using it to check for collisions between your character and just one of your obstacles. You can set this up by placing an infinite loop in "world.my first method" that contains an "if/else" statement. If a collision is detected, the character should react somehow, if not nothing should happen.

• Finishing the Game**

  The last step is to set up your game so that the character can collide with any of the obstacles. You can do this by creating a new world level function that makes several calls to "checkForCollision", one call for each obstacle in the world. If any of those calls return "true" then this new function should return "true", if none of those functions return "true" then this new function should return "false".

  Test your new function by modifying "world.my first method" so that the character reacts whenever she collides with any obstacle.

  Extras

  • Modify your world so that there is a celebratory animation when the character reaches the finish line. Possibilities include a fireworks display or a short dance.**