1. Execution Control with If/Else and Boolean Functions
Alice

2. Adding More Complexity to Worlds
Initially programs were not interactive
program ran and user watched
Then learned how to make programs interactive
add events to program
when event occurs an event handler method is called
adds variability to programs
not always the same thing happening

3. Event Driven Programming
a whole philosophy of programming called “event driven” programming
flow of program controlled by external events as opposed to things internal to the program
very important in user interfaces
the parts of programs that interact with real users
think about using a computer
the operating system ( Windows XP, Mac OS X, etc. ) has a large component that is event driven
responding to your actions. (Move mouse, click, type)

4. Thinking About More Advanced Worlds
No doubt you have started to think about building animations like simulations and video games…
To build more advanced worlds, you will need to write code that involves decisions

5. Examples of Decisions A race car simulation
driver provides input from steering wheel, gas pedal, and brake
if car stays on road display next section of road
if car hits another car, crash
if car goes too far off road, crash
if pass checkpoints, more time

6. Logical Expressions
Program must make decision based on current conditions
on road?
time left?
intersected another car?

7. Logical Expressions
A condition is checked in a logical expression that evaluates to Boolean value
true or false (Boolean) value
car on road true
car over finish line false

8. If/Else
In Alice, a logical expression is used as the condition in an If/Else control structure.
Decisions (using If/Else) are used in
functions
methods

9. Example: Boolean Function
Suppose you are building a simulation system used to train air traffic controllers.
One of the tasks of an traffic controller is to be alert for possible collisions in the flight space.

10. Storyboard
One factor in determining whether two aircraft are in danger of collision is the vertical distance (difference in altitudes) between them.
We can write a function that checks the vertical distance against a minimum difference in altitudes.
The function returns true if they are too close, otherwise false.
isTooClose
Parameters: aircraftOne, aircraftTwo, minimumDistance
If the vertical distance between aircraftOne and aircraftTwo is less than minimumDistance
return true
Else
return false

11. Demo Lab12FlightCollision-V1 Concepts illustrated in this example
A world-level relational operator is used to create a Boolean expression as the condition.
The absolute value function is used to make sure the computed difference in altitude is not a negative number.

12. Storyboard
To avoid a collision, the aircraft that is above the other should move up and the lower aircraft should move down.
avoidCollision
Parameters: aircraftOne, aircraftTwo
If aircraftOne is above aircraftTwo
Do together
aircraftOne move up
aircraftTwo move down
Else
aircraftOne move down
aircraftTwo move up

13. Demo Lab12FlightCollision-V2 Concepts illustrated in this example
Decisions were made to
control whether a method is called
determine which set of instructions are immediately executed