classes, objects, and world-level methods alice. programming in alice© 2006 dr. tim margush2 class...

Classes, Objects, and World-level Methods

Alice

Programming in Alice © 2006 Dr. Tim Margush 2

Class / Object

ClassA template describing the characteristics (properties, methods, and functions) of an object of a specific type

ObjectAn instance of a class; each object of one class has identical characteristics, but property details may vary

Programming in Alice © 2006 Dr. Tim Margush 3

Method / Function

MethodA programming unit that can be executed on demand by using its name; corresponds to a sequence of actions

FunctionA programming unit that can be executed on demand by using its name; represents a value

Classes, Objects, & Methods

Object-oriented programming uses classes, objects, and methods as basic programming components.

These components help you to design programs

think about an intricate program

test programs

find and remove errors (bugs)

In your programs,you have been using…

Classes In Alice, classes are predefined as 3D models

Objects An object is an instance of a class.

Capitalization is a matter of Style

Classes Classes: Penguin (Uppercase name)

Objects Objects: penguin, penguin1, penguin2, opus (lowercase names)

In your programs,you have also used…

built-in (predefined) methods Examples: move, turn to face, say

World.my first method Example:

In the snowpeople world (Chapter 2), we wrote program code where the snowman tried to get the attention of a snowwoman.

As a reminder, see next slide…

Elements of Visual Programming

Animation Programs 8

Adding Instructions to world.my first method

The method should be open in the method editor window of Alice. (bottom right)

if not select the world from the object tree, the method tab, and click the edit button next to my first method



Method Editor Window



Step 1 – Snowman turnsSelect the object you want to perform the objectSelect the method / action you want the object to perform

could use turn or turn to faceoften many ways to accomplish

the same task

Click and drag it to the method editor window



Snowman TurnsCan adjust aspects of how the snowman turns to face the snowwoman

click the more option

right now duration and style are the only things you should alter



Step 2 – Combined ActionWe want the snowman to say “ahem” and blink at the same timeactions are normally sequentialto do actions together, at the same time, use a “Do together” blockClick and drag “Dotogether” block intothe method



Affecting subpartsThe snowman does not have “blink eyes” methodCan accomplish a blink by affecting subpartsSelect snowman object from object tree and expand subparts

expand the headnow we can give commands to individual

parts, in this case the eyeshave eyes move up and downspecify direction and distance of move



Step 2 – First Attemptworld.my first method looks like this

TEST the methodplay the movie. Does it do what we want?



Logic ErrorThe program works, but does not do what we intended.

This is an example of a logic errorvery easy in Alice to see logic errorsthe movie does not do what we wanted

What’s the problem?



Do together and Do in orderAll commands in the Do together block are executed simultaneouslySo what is the result if you move an eye up .1 meters and down .1 meters at the same time?

Apparently nothing

So while we want the eyes to move together and to say “ahem” we want the eyes to first move up and then downUse a Do in order block inside the Do together block



Corrected Do Together

Larger ProgramsAs you become more skilled in writing programs, you will find that your programs quickly begin to increase to many, many lines of code.Games and other "real world" software applications can have thousands, even millions of lines of code. In this session, we begin to look at organizing large programs into small manageable pieces.

Goals of OOP

Organize a large program into small pieces Design and think about an intricate program

Reusable code allows for well written piece of code to work in more than one program

Test find and remove errors (bugs)

Modifying the programTo make the snowpeople animation more realistic, we might want the snowman to be a little less shy. In our original program, the snowman tries to catch the snowwoman's attention only once. Perhaps he could try to get her attention more than once.

To make this modification, additional lines would be added to the code.

This would make the program code longer and more difficult to read and think about.

A Solution

A solution to the problem is to define our own method name the new method catchAttention

Then, we can drag-and-drop the catchAttention method into the edit box just like the built-in methods

Demo: The solution

First, to associate the new method with the World

• select the World tile in the Object Tree

•select the methods tab in the details area

•click on the "create new method" button

Demo

A demonstration of writing the catchAttention method

World-level method

catchAttention is a world-level method because it is defined as a method for World

has instructions that involve more than one object (snowman, snowwoman, camera)

Using the catchAttention method

The catchAttention method is executed by calling (invoking) the method from my first method

Why?

Why do we want to write our own methods? saves time -- we can call the method again and again without reconstructing code

reduces code size – we call the method rather than writing the instructions again and again

allows us to "think at a higher level"can think “catchAttention" instead of

“turn head to face the camera, then say ‘Ahem’ while moving eyes up and down"

the technical term for "think at a higher level" is "abstraction"

Assignment

Read Tutorial04 World-level Methods

How to create them

How to call them

When it is appropriate to use them

Lab04

classes, objects, and world-level methods alice. programming in alice© 2006 dr. tim margush2 class...

Documents

slide slide

snowman object

method action

method tab

method example

value slide

programming unit

blink eyes method