MCS-170 The Nature of Computer Science

Alice Lecture 7

Suppose that we have created a character on the screen with many special methods. Also, suppose we wish to create multiple copies of this character, but preserving the new methods we have defined. We can accomplish this by creating a new class that will store these new methods.

Example: In this example (squash2.a2w) we have created a monkey along with 8 special methods. In order to create a new monkey class, we do the following:

1) Rename the Class. We have to give the class a new name (different than monkey). In our example, we have called the new monkey a "trainedMonkey."
2) Save the Class. Right-click on the class in the Object tree (upper right corner) and select save object. In the box that pops-up go to the directory where you wish to save the class and click Save. The class will be automatically saved with the new name, only capitalized, and given the extension ".a2c" (for Alice version 2.0 Class)

Once a new Class is created it can be used in a new world by selecting Import from the File menu. (Demo this)
Note 1: The new Class (TrainedMonkey) will inherit all of the methods from Monkey.
Note 2: Be careful when creating new Classes that you do not define Class methods that depend on other specific objects in a scene. For example, if we had defined a method forTrainedMonkey that used one of the bunnies, then this method might not work in other Alice worlds.

Indefinite Looping -- Recall that if we don't know beforehand how many times we want to repeat an action, then we can use a While statement to test if some condition is met to stop the repetition of the action. Today, we will look at a second technique for handling indefinite looping -- that of recursive programming.
Recursive Programming is our first example of a programming idea that is not listed as one of the tabs at the bottom of the Alice screen (Do in order, If/Else, etc). This is because recursion is not a labeled programming statement, but is a programming methodology, a way of creating a method that calls itself!

Example: We have two bunnies in a garden. We want them to hop toward each other as follows:

bunny and bunny2 turn to face each other
bunny and bunny2, repeatedly hop towards each other at the same time as long as they are > 3 meters apart. Once they are closer than 3 meters, bunny says "hello"
Here is a world (bunnyGarden.a2w) with the bunnies in place and the hopping towards each other assumed to take place in the World.BunniesMeet method.
Recursive Construction of World.BunniesMeet:

Note the top-down programming style -- have high-level methods that are repeatedly broken down into lower-level methods.

Class Work: Suppose you have a scene with a butterfly and a white rabbit. The butterfly should move to a random location (within 0.2 meters). Each time the butterfly moves, have the rabbit turn to face the butterfly and move 0.5 meters toward the butterfly. When the rabbit gets close enough, say "Gotcha!"