CS320n Visual Programming

1
CS320n Visual Programming

• Execution Control with If / Else and Boolean
  Functions
• (Slides 6-2-2)

Thanks to Wanda Dann, Steve Cooper, and Susan
Rodger for slide ideas.
2
What We Will Do Today

• Work more with if / else statements using the
  Zeus lighting bolt world

3
The Zeus world revisited

• Recall the Zeus world
• Testing this world, we found three significant
  problems
• Anything the user clicked got zapped by Zeus
  bolt not just philosophers!
• Philosophers could get zapped with lightning more
  than once!
• if we clicked on Zeus the program halted

4
Checking One Philosopher

• Check to see if who is homer
• add an if / else to the shootBolt method
• Drag in the who tile then select and homer
• is a relational operator meaning is equal to

5
Logical Operators

• It is possible to check more than one condition
  in a boolean statement

6
Philosophers Only

• We only want to shoot the bolt when one of the 4
  philosopher's is clicked
• could make 4 separate events like in the fire
  truck saving people from the burning building
• OR could check in the shootBolt method if
  parameter is one of the philosophers
• if who is a philosopher shoot them
• else do nothing

7
Checking Multiple Conditions

• drag boolean logic, either or to replace
  condition on if
• set both values to true initially

8
changing condition

• right now the condition is always true
• change it so it checks if the parameter who is
  equal to Homer or Plato.
• Drag who into the condition

9
Adding More Philosophers

• If who is equal to plato or homer we will shoot
  them
• add in the other philosophers by nesting logical
  operators
• Replace trues with a check for who being equal to
  the other three philosophers

10
Checking All Four Philosophers

• long, but does check all 4 philosophers
• DEMO

11
Complexity

• The boolean condition in the if statement that
  checks to see if who is a philosopher is rather
  complex
• someone reading the code may not understand
  exactly what the purpose of that condition is
• create a function to check if who is a
  philosopher
• make complex things appear simple abstraction

12
isPhilosopher function

• isPhilosopher is still complex but this makes
  shootBolt look a lot simpler

13
Demo

• A demonstration of the Zeus world with If
  statement and logic operators.
• What problem have we fixed?
• If we want to add another person to the scene
  that Zeus can shoot what changes must be made to
  the program?
• What problem still exists?

14
Completing the Zeus world

• The second problem is how to prevent lightning
  striking the same philosopher more than once.
• How do we know if a philosopher has already been
  struck by lightning?
• When a lightning bolt strikes, the philosopher
  is zapped and the his color property is set to
  black.
• Checking color is a convenient way to check
  for a previous lightning strike.

15
Preventing a duplicate strike

• We only need to check for a possible
  duplicate-strike if we already know that a
  philosopher is clicked
• the way to handle this is nest a second If
  statement inside the first

! is a relational operator meaning is not
equal to
16
Alternative to Nested if

• Instead of using a nested if we could the logical
  expression a and b as the test for the if
• a is the parameter who is a philospher
• b is the parameter whos color is not black

17
Demo

• A demonstration of completed Zeus world
• Does it matter if use nested if or a more complex
  boolean condition?
• May be easier to use nesting to achieve certain
  behaviors
• example, if a philosopher has already been shot,
  he says Leave me alone!