MCS-170 The Nature of Computer Science

Alice Lecture 3

1. Program Control Structures - Sequencing, Conditionals, Loops

• Overview
• Review -- World Methods vs Class Methods
  
• Sequencing Statements
• Conditional Statements
  
• Loops
  

• World-Level Methods vs Class-Level Methods
  
• World-Level Methods are actions that a director would give to all cast members, or are accessible to anyone. 
  
• Example: Squash1.a2w The World has the World.myFirstMethod plus two other methods.  Note that these two methods reference multiple characters.
  
• World.myFirstMethod is always the first method run be any senario.  We build up other methods from within this global method. 
  

• Class-Level Methods reference only a particular type of actor. For example, any instance of a Monkey class can move, turn, etc. 
  
• Example: Squash2.a2w  Here the monkey actions are defined at the Class level, with a parameter called victim used to squash each bunny. (more on parameters next week).  Discuss how sounds are used.
  

• Sequencing Statements
  
• We use sequencing statements to control how program instructions are carried out. There are two possible choices:
• Do in order -- this is used for sequential execution, each instruction within this tile is carried out in order, one at a time.
  
• Do together -- this is used for simultaneous execution, all instructions within this tile are carried out in parallel, all happen at the same time.
  

• Example: Squash2.a2w  Look at monkeyJump method.  Use simultaneous actions to make monkey appear to be jumping.  (Play with changing time durations in
  one of the sections - what effect does this have?)
  

• If/Else Statements (Conditional Execution)
• A conditional statement depends on a decision - if a condition is met we do task A , otherwise we do task B.

• Conditional statments rely on decisions that are either True or False, that is they rely on Boolean values.
  

• In Alice, conditionals are implemented with the If/Else tile (found on the bottom of the screen).

• Example: Recall our penguin story. Let's add to the storyboard as follows:
  

(new code)

If penguin is within 2 meters of shark
    Do the following steps in order
	penguin rises into the air to the height of the palm tree
	penguin moves to the top of the palm tree
Else
    Do nothing
shark swims to palm tree
Loop 4 times
    Do the following steps in order
	shark moves toward the palm tree
	shark turns 180 degrees
	shark moves away from the palm tree
	shark turns 180 degrees

• Here's the Alice implementation -- Penguins and Sharks

• Note: We often use Numerical Relational Operators in conditionals. These can be accessed as functions in the World Object
• a == b
• a != b
• a > b
• a >= b
• a < b
• a <= b

• We create True/Flase condtions be using these with simple numbers, and then replacing by numerical functions (like distance, etc)