Transcript Lecture notes (4-1)

CS320n –Visual Programming
Classes, Objects, and
World- Level Methods
Mike Scott
(Slides 4-1)
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
Visual Programming
Classes, Objects, and World Level
Methods
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Fish Circling Around Island
Smooth
Jagged
Play Movie
Visual Programming
Classes, Objects, and World Level
Methods
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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
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Classes, Objects, and World Level
Methods
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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)
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Classes, Objects, and World Level
Methods
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•
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)
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Classes, Objects, and World Level
Methods
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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…
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Classes, Objects, and World Level
Methods
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Snowman Program
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Classes, Objects, and World Level
Methods
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Snowman Program
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Classes, Objects, and World Level
Methods
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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
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Classes, Objects, and World Level
Methods
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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
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Classes, Objects, and World Level
Methods
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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
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Classes, Objects, and World Level
Methods
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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
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Classes, Objects, and World Level
Methods
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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
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Classes, Objects, and World Level
Methods
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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
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Classes, Objects, and World Level
Methods
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Demo
• A demonstration of writing the
catchAttention method
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Classes, Objects, and World Level
Methods
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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
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Classes, Objects, and World Level
Methods
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Using the catchAttention method
The catchAttention method is executed by calling
(invoking) the method from my first method
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Classes, Objects, and World Level
Methods
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Testing Method Independently
• Can test method by adding event to the
program
• disable other methods
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Classes, Objects, and World Level
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"
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Classes, Objects, and World Level
Methods
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Exercise for Class
• Do problem Chapter 4, exercise 2,
Confused Kanga on page 110 of the book
– see web for description
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Classes, Objects, and World Level
Methods
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