CSC1401

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CSC1401

• Animation
• Creating methods/instructions (class-level)

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Recall.

• Princess Escape
• A princess has been grounded by her father (a
  wizard) and kept inside the castle. Being a
  rather rebellious princess, she has emailed the
  local dragon taxi service. The dragon will fly to
  the princess and she will climb aboard the dragon
  to escape from the castle to meet some friends
  at the village dance club.

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Textual Storyboard(pseudo code)

Do in order dragon takes off dragon flies
to princess princess climbs on dragon's back
dragon and princess escape knight shakes his
arm (and sword) in protest
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In our programs,we have been using

• Classes
• In Alice, classes are predefined as 3D models
• Objects
• An object is an instance of a class.
• Class Frog (Uppercase name)
• Objects frog, frog1, frog2, frog3
• (lowercase names)

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We have also used

• built-in (predefined) methods
• Examples move, turn to face, say
• World.my first method
• Example
• In the Dragon world, we wrote program code
  where a dragon took off (as the first step in our
  animation).
  
• All the program code was written in this one
  method, see next slide
  

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Larger Programs

• As you become more skilled in writing programs,
  you will find that programs quickly increase to
  many, many lines of code.
  
• Games and other "real world" software
  applications can have thousands, even millions of
  lines of code.

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Potential Problem

• The program code just seemed to grow and grow.
• If we continue to write programs this way the
  programs will become longer and more difficult to
  read and think about.

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Solution

• A solution is to organize the instructions into
  smaller tasks.
  

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Stepwise Refinement

• The process of breaking a problem down into large
  tasks and then breaking each task down into
  simpler steps is called stepwise refinement.
  
• Once the storyboard is completed, we write a
  method for each task.

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Stepwise refinement - 1
Do in order dragon takes off dragon flies
to princess princess climbs on dragon's back
dragon and princess escape knight shakes his
arm (and sword) in protest

• How can a dragon "take off"?

Do together dragon moves up dragon flaps w
ings
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Stepwise refinement - 2

• How can a dragon flap its wings?

Do together dragon moves up dragon flaps w
ings
Do together dragon flap left wing dragon f
lap right wing
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Stepwise refinement - 3
Do together dragon flap left wing dragon f
lap right wing

• How can a dragon flap its left wing?

Do in order Do together dragon close left
wing rolls right dragon far left wing rolls
right Do together dragon close left wing roll
s left dragon far left wing rolls left
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Demo Create a class-level method
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Drag the code into the editor
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The complete method
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Calling a method
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Demo

• Create the flapLeftWing method
• Create an analogous flapRightWing method

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Create a flapWings method
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Demo

• Create the flapWings method
• Create a takeOff method, where the dragon moves
  up 2 meters and flaps its wings twice
  
• What changes will you need to make to the
  duration parameter to get the animation working?

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Summary

• 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 surprise instead of
• The alien moves up and says Slithy toves? and
  then the robot's head turns around. "
  
• the technical term for "think at a higher level"
  is "abstraction"

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Summary Classes, Objects, Methods

• Object-oriented programming uses classes,
  objects, and methods as basic programming
  components.
  
• These components help to
• organize a large program into small modules
• design and think about an intricate program
• find and remove errors (bugs)

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Assignment

• Read Chapter 4, Section 3
• Class-level Methods
• How to create them
• How to call them