CS320n –Visual Programming Classes, Objects, and World- Level Methods Mike Scott (Slides 4-1)

CS320n –Visual Programming

Classes, Objects, and World- Level Methods

Mike Scott(Slides 4-1)

Visual Programming Classes, Objects, and World Level Methods

2

What We Will Do Today• Look at ways of doing the fish problem

from last time

• Define Classes, Objects, and Methods in Alice

• Define and create world level methods• more complicated snowman movie


3

Fish Circling Around Island

Smooth

Jagged

Play Movie


4

Larger Programs• As you become more skilled in writing programs,

you will find that your programs quickly begin to increase to many, many lines of code.– Fish program about 30 already– How many things can you keep in short term memory?

• Games and other "real world" software applications can have thousands, even millions of lines of code.

• Want to organize large programs into small manageable pieces– big rocks into little rocks


5

Classes, Objects, and Methods• Alice uses classes, objects, and methods

as basic programming components.– it is just one way to think about programming

• object oriented

– there are many others

• These components help you to– organize a large program into small pieces– design and think about an intricate program– find and remove errors (bugs)


6

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.

Class: Fish (Uppercase name)

Objects: fish, fish1, fish2, fish3 (lowercase names)


7

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…


8

Snowman Program


9

Snowman Program


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More on the Snowman Program

• Snowwoman blushes and then turns back to her friend

• Snowman turns away and shakes his head

• Show Movie


11

Modifying the program• Modify program to get snowman to try to

get snowwoman’s attention twice• To make this modification must add more

lines of code– program getting longer, more complicated,

more difficult to think about and understand, harder to make changes

– program is suffering a “code explosion”

• Show movie of trying to get attention twice


12

Try twice to get attention• Notice the wait

command in between the two attempts to get the snowwoman’s attention– wait is one of the

commands at the bottom of the method editor


13

Why is this “Bad”?• Not hard to copy and paste code, right?

• Easy to simply say “make copy” of the Do together block and insert a wait

• … but, getting a little harder to maneuver around the program with all that code

• … and what if we want to change something about the getting attention behavior?– blink eyes twice or change duration of blink


14

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


15

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


16

Demo• A demonstration of writing the

catchAttention method


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World-level method• catchAttention is a world-level method

because it-is defined as a method for World

• if the method has instructions that involve more than one object it must be a world level method

• This method only involves the snowman so we could have made it a method for the snowman


18

Using the catchAttention method

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


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Testing Method Independently• Can test method by adding event to the

program

• disable other methods


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Why?

• Why do we want to write our own methods?– saves time -- we can call the method again and again

without reconstructing code– Easy to make changes to behavior

• only have to change in one place– 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 “say “Toot” moving eyes up and down"

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


21

Exercise for Class• Do problem Chapter 4, exercise 2,

Confused Kanga on page 110 of the book– see web for description

CS320n –Visual Programming Classes, Objects, and World- Level Methods Mike Scott (Slides 4-1)

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snowpeople world chapter

program code