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Enhancing Control with If/Else & Boolean Functions in Event-Driven Programming

Learn to add complexity to programs with If/Else statements & Boolean functions. Explore decision-making in interactive worlds. Examples include air traffic collision detection & avoidance.

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Enhancing Control with If/Else & Boolean Functions in Event-Driven Programming

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  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 expressionthat 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 Do together 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

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