1. Variables and Functions Alice

2. Naming is Important If you get a new pet one of the first things you do is name it Gives you a way to refer to the new pet without saying Please take that dog we got yesterday for a walk. Please take Fifi for a walk. In programming we name things we want to refer to again Gives us a way to work with them Like the World object This is called declaring a variable

3. Declaring a Variable To be able to refer to an object again we need to specify what type of thing it is and give it a name This is also called declaring a variable Type name; OR Type name = new Class(value, value, …); The equal sign doesn’t mean equal But assign the value of the variable on the left to the result of the stuff on the right The following creates a variable named earth which refers to a World object created on the right World earth = new World();

4. Declaring Variables Variables are names associated with values If the type of the variable is null It doesn’t refer to an object yet Variables can be reused World earth = null; earth = new World(); earth null earth World Object 1 earth World Object 2

5. A Variable Associates a Name with Space A variable is like a box with a label on it You can put something in a box You can take something out of a box You can even change what is in the box The size of the box restricts what you can put in it Hat Box

6. Limits on Declaring Variables You can't declare two variables with the same name! > World earth = new World(); Error: Redefinition of 'earth' You can change what an object variable refers to > World earth = new World(); > earth = new World();

7. Declaring Variables and Creating Objects You can declare a variable and assign it to refer to a new object in one statement World earth1 = new World(); Turtle tommy = new Turtle(earth1); Declaration of variables Creating the objects

8. Storing a Value in Variable x Alice x.set( value, something )‏ Java x = something; object number string some computation

9. Functions in Alice A method in Alice is called a void function in other languages, because it returns nothing. Returns? What's “returns”? A non-void function returns information In Alice, this is a function

10. How a function works A function receives value(s), performs some computation on the value(s), and returns (sends back) a value.

11. Types of Functions The type of a function depends on the type of value it returns. a number a specific object a color a Boolean (true or false)‏ other property values…

12. SumOfSquares Function The sumOfSquares function receives two value(s) x and y, performs the computation x 2 + y 2 on these value(s), and returns (sends back) this result. x 2 + y 2 X = 3, y = 5 34 x = 1, y = 4 17

13. SumFrom1To Function The sumFrom1To function receives a value N, and performs the computation 1 + 2 + … + N, and returns this sum. 1 + 2 + …+N N = 3 6 N = 100 5050

14. Using a Built-in Function The “distanceTo( ) dest_object” function receives an object and determines the distance from the dest_object and the object and sends the value back to the caller. distanceTo sheriff (this)‏ cowboy 1.3

16. Give Function Demo

17. A new example A common task in sports game programs is to bounce a ball. To illustrate how this is done, let's bounce the ball over the net. (The ball has been positioned 1 meter from the net.)‏ The ball should move up and forward and then down and forward in a pattern that looks something like this: Note: This looks easy – but do not be deceived!

18. Design Storyboard A design for a World-level method: World.ballOverNet: Do in order toyball turn to face the net Do together toyball move up toyball move forward Do together toyball move down toyball move forward

19. Demo Concepts illustrated in this example: Movement of an object is sometimes relative to another object. In this example, a built-in height function is used to determine the height of the net. Then the ball moves up enough to clear the net. the toyball's up and down movements are relative to the ground. An orient To method (alternatively stand Up) is used because it is not easy to tell "which way is up."

20. Rolling the ball Another sports game action involves rolling a ball along the ground. We want a realistic motion rather than a slide. The ball must simultaneously move and roll. realisticRoll Do together move ball forward 1 meter turn ball forward 1 revolution

21. Demo Our design assumed that the ball's motion is relative to the ground. But the ball turns relative to it's own sense of direction. AsSeenBy ground can be used to make the ball turn relative to the ground.

22. Revising the approach The ball is made to roll 1 revolution. Suppose we want the ball to roll a certain distance (say 3 or 4 or even 10 meters) along the ground. How many times should the ball turn 1 complete revolution?

23. Number of revolutions The number of revolutions depends on the size of the ball The number of revolutions is distance/( Pi * diameter)‏ But there is no built-in function to return the number of revolutions We will write our own! one revolution four revolutions

24. Parameters We want to return the value computed as distance / (  * diameter)‏ Obviously, what is needed is the ball’s diameter the ball object has a built-in width function the distance the ball is to travel can be sent as a parameter to the function

25. Demo Concepts illustrated in this example A function must have a return statement to send back a computed value. In this example, a math expression is created to compute a number value. The order of evaluation is indicated by the use of parentheses, as in traditional math expressions.

26. Calling the function test values We should run the animation with several test values to be sure it works as expected. What happens if you use a negative value?

27. Levels of Functions As with methods, functions can be either class-level or world-level. (The function just presented was class-level.)‏ The guidelines for class-level methods also apply to class-level functions: No references to other objects. No references to world-level functions you have written (built-in world-level functions are fine to use).