1. Learn about functions in Alice
What we will do today
Learn about functions in Alice

2. Functionality A function asks a question and give an answer
Receives value(s)
Performs computation on value(s)
Returns (sends back) a value

3. Types of functions
The type of a function depends on the type of value it returns
a calculated value (a number)
a specific object
a color
etc. 42

4. Built-in/user defined functions
A built-in function is a previously existing function.
You do not need to create Built-in functions.
A User defined function is a function that you need to design and implement.
You are adding functionality to the class when you add a user defined function.
Distance between center points. 4

5. Built-in functions Used one of Alice’s built-in functions
skateAround method for the cleverSkater
asked for the distance between the advanced skater and the object, target. (In the specific example shown, target was a penguin.)
Distance between center points.

6. Example How do we bounce a ball? Let’s bounce a ball over a net
Ball is 1 meter from the net to start
Ball should move up and forward, then down and forward
Note: Looks easy – but do not be deceived!

7. Design Storyboard A possible storyboard To reach the top of the net,
we know the ball should move forward 1 meter (we positioned the ball 1 meter in front of the net)
but how far upward should the ball move to clear the net?
World.ballOverNet:
Do in order
toyball turn to face the net
Do together
toyball move up
toyball move forward
toyball move down

8. Height
Can use the built-in height function to determine the height of the net and move the ball up that distance
Demo – ballOverNetWork – what happens?

9. Problem The ball does not bounce over the net
The problem – cannot tell “which way is up” from the perspective of the ball

10. Solution
We think “up” and “down” relative to the ground – so can orient the ball (and net) with the ground
Now, ball will bounce over the net

11. Realistic Motion Turn the camera to look at the bounce from the side.
Is it realistic?
The default style for motion is “gently”
begin and end gently
can get more realistic by
making forward motion abrupt,
up motion end gently, and
down motion begin gently

12. Adjusting Style
adjusting the style of motion makes the bounce more realistic

13. Rolling the ball How do we roll a ball along the ground?
Want a realistic motion rather than a slide
The ball must simultaneously move and roll.

14. Demo: A first attempt

15. Revising the Approach The ball is made to roll 1 revolution.
What if we want the ball to roll a certain distance?
How can we make the ball roll the correct number of revolutions to cover a given distance along the ground?

16. Math to the rescue!
The number of revolutions depends on the size of the ball
The number of revolutions is
distance / (Pi * diameter)
There is no built-in function to return the number of revolutions
Must write it!

17. Parameters We want to return the value computed as
Distance / Pi * diameter
Obviously, what is needed
The ball’s diameter
The ball object has a built-in width question
The distance the ball is to travel
Can be sent as a parameter to the question

18. Parameters/Arguments
Arguments is the Object/value passed to the function
Parameter is the alias (for the argument) used by the function. 18

19. numberOfRevolutions function
distance is the parameter

20. Building a numberOfRevolutions function
Step 1
Step 2: Drag distance parameter to replace 1 in return

21. The numberOfRevolutions function
Step 3: Select math and divide.
Select other and key
in 3.14

22. The numberOfRevolutions function
Step 4: Select the 3.14.
Pick math and 3.14 *
Select 1

23. The numberOfRevolutions function
Step 5: Replace the 1 with
the width of the ball
from the functions

24. Demo: Calling the Function
10 is a test value.
We should run the animation with several test values to be sure it works as expected.
What happens if you use a negative value?

25. Creating a Roll Method Create a class level method for realistic roll
problem: amount of time to roll. (Demo)
Elements of Visual Programming
Functions

26. Fixing Realistic roll Set duration of to function of distance.
Try 2 meters per second
Fixes one problem, but what happens with negative numbers?
What is result of -5 / 2?
Fix, using a world level method that finds the absolute value of a number

27. Levels of functions
As with methods, you can write functions as either class-level or world-level. (what was the function we just wrote?)
Guidelines for class-level methods apply to class-level functions:
No references to other objects
No references to world-level functions
Built-in world-level functions are ok

28. Classwork today

29. Labs Car Pyramid Make sure all tires rotate the appropriate amount
Have the mummy approach the pyramid
Have the mummy scale the side the pyramid
Have the mummy stop at the top