CS320n

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CS320n Visual Programming

• Class-level Methods and Inheritance Part 1
• Mike Scott
• (Slides 4-3-1)

Thanks to Wanda Dann, Steve Cooper, and Susan
Rodger for slides
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Reminders

• Assignment due tonight by 11 pm

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What We Will Do Today

• Learn about class level methods and inheritance
• class work

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Animated Actions

• Some actions are more naturally associated with a
  specific class of objects rather than with the
  overall world.
• Examples
• A person walking
• A wheel rolling
• A fish swimming
• If the action involves 2 or more objects in the
  world it should be a world level method

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Class-level Methods

• Write a method to add abilities / functions /
  actions / behaviors to a specific class of
  objects
• class-level method
• NOT world-level methods
• Demo how to build class-level method

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An Example

• How can we create a skate method for ice skater
  objects?

• We need to
• create a new method for the ice skater
• add code to the new method to make it look like
  the skater skates

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Demo The solution

• First, to associate the behavior with the ice
  skater
• select the iceSkater tile in the Object Tree
• select the methods tab in the details area
• click on the "create new method" button

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Storyboard for skate
Skate Do together move skater
forward 2 meters Do in order slide
on left leg slide on right leg

• The slide actions
• require several motion instructions
• so well break these two actions into smaller
  pieces
• a technique known as stepwise refinement (and
  abstraction)

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An Aside

• Shouldnt the skater already know how to skate?
• Should people already know how to walk?
• The objects in Alice are simply 3D models created
  in other programs
• articulating, coloring, setting center point
• When added to Alice all objects have certain
  basic behaviors, move, turn, roll,
• programmers must write code for more complex
  behaviors
• CLASS-LEVEL METHODS!

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But This Seems So Hard

• Anecdote 1 Show Kelly Scott anything cool the
  computer does
• Anecdote 2 Finding Nemo. Making the viewer
  believe Dory can barely say what she is thinking
  and how sad she is
• It is hard. It is complex. Take a look at the
  penguin method walk

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Penguin Method Walk
and there is more after this. Bottom line.
Interesting behaviors are usually complex
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Refined storyboard for skate

Refinement of slideLeft Do in order Lift
right leg and turn upper body forward Lower
right leg and return body upright
Skate Do together 1) move forward 2 meters
2) Do in order slideLeft
slideRight
Refinement of slideRight Do in order Lift
left leg and turn upper body forward Lower
left leg and return body upright
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Writing the code

• The next step is to translate the design into
  program code.
• For the slideLeft method, a possible translation
  is
• Design step Instruction
• Lift the right leg turn the right
  thigh forward
• Turn upper body forward turn the upper body
  forward
• Lower the right leg turn the right thigh
  backward
• Return the body upright turn the upper body
  backward

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Demo

• Demonstration of
• slideLeft
• slideRight
• skate
• Could we parametize slideLeft and slideRight?
• should we?

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Correspondence of design to code
Skate Do together move skater
forward 2 meters Do in order slide
on left leg slide on right leg

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Question

• Writing the methods to make an ice skater perform
  a skating motion is an intricate task.
• Most likely, you would like to reuse these new
  methods in other worlds.
• How can you make the skate method available for
  an ice skater in a different world?
• any other ice skaters in this world DO NOT know
  how to skate

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Answer Save out as a new class
1) Rename iceSkater as advancedSkater. 2) Save
out as a new class. Alice saves the new class as
AdcancedSkater.a2c
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Inheritance

• The AdvancedSkater class
• inherits all the properties and methods from the
  original IceSkater class, and also
• has the newly defined methods (skate, slideLeft,
  slideRight)
• In other programming languages, the concept of
  creating a new class based on a previously
  defined class is called inheritance.
• object oriented programming. As opposed to
  procedural programming or functional programming
  o or

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Using CleverSkater

• An instance of the CleverSkater class can be
  added to a new world use FileImport.

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Benefits of Inheritance

• Inheritance supports
• the reuse of program code -- allows the
  programmer to write code once and use it again
  later in different programs.
• sharing code with others in a team project
• The best code in your program is the code you
  didnt have to write!