1. World-level Methods with Parameters
Alice

2. Larger Programs
As you become more skilled in writing programs, you will find that your programs quickly begin to increase to many, many lines of code.
Games and other "real world" software applications can have thousands, even millions of lines of code.
In this session, we begin to look at organizing large programs into small manageable pieces.

3. Goals of OOP organize a large program into small pieces
design and think about an intricate program
find and remove errors (bugs)

4. Modifying the program
To make the snowpeople animation more realistic, we might want the snowman to be a little less shy. In our original program, the snowman tries to catch the snowwoman's attention only once. Perhaps he could try to get her attention more than once.
To make this modification, additional lines would be added to the code.
This would make the program code longer and more difficult to read and think about.

5. A Solution A solution to the problem is to
define our own method
name the new method catchAttention
Then, we can drag-and-drop the catchAttention method into the edit box just like the built-in methods

6. Demo: The solution First, to associate the new method with the World
select the World tile in the Object Tree
select the methods tab in the details area
click on the "create new method" button

7. Create a World Method
Choose a meaningful name
Edit the method

8. Demo
A demonstration of writing the catchAttention method

9. World-level method catchAttention is a world-level method because it
is defined as a method for World
has instructions that involve more than one object (snowman, snowwoman, camera)

10. Using the catchAttention method
The catchAttention method is executed by calling (invoking) the method from my first method

11. Why? Why do we want to write our own methods?
saves time -- we can call the method again and again without reconstructing code
reduces code size – we call the method rather than writing the instructions again and again
allows us to "think at a higher level"
can think “catchAttention" instead of
“turn head to face the camera, then say ‘Ahem’ while moving eyes up and down"
the technical term for "think at a higher level" is "abstraction"

12. Adding spinning wheels would be fun, but complicate our example
moveInCircle
Another candidate for a world method is one to move an object in a circle
Here a world method does it to a car
Adding spinning wheels would be fun, but complicate our example
Programming in Alice
© 2006 Dr. Tim Margush

13. Using moveInCircle Here we just do the maneuver twice
What about moving different objects in a circle?
Create a method for each????
No! That's what parameters are for!
Programming in Alice
© 2006 Dr. Tim Margush

14. Add an Object Parameter
Edit the moveInCircle method and click the create new parameter button
Give it a name
Choose a type
OK?
Programming in Alice
© 2006 Dr. Tim Margush

15. moveInCircle(whichObject)
The parameter is named whichObject
It holds the place of a real object that will be substituted later
Replace all occurrences of car (in the method) with the parameter
Just drag it into place
drag
Programming in Alice
© 2006 Dr. Tim Margush

16. Click to select the argument
Test
I got this error message when I tested my program
The problem is in the call of this method
We added a parameter
We did not choose an argument
Click to select the argument
Programming in Alice
© 2006 Dr. Tim Margush

17. Other Arguments Several additional choices come to mind
How big of a circle?
How long should it take?
Which direction should it turn?
moveInCircle(whichObject, radius, duration, direction)
Add more parameters!
Programming in Alice
© 2006 Dr. Tim Margush

18. Adding Parameters Note the Types Number, Number, and Other
Under Other, find Direction so you will be able to choose up, down, left, etc.
Programming in Alice
© 2006 Dr. Tim Margush

19. Convert radius to circumference
Adapting the Code
The parameters will need to be dragged into the body of the method to replace the specific constants
Convert radius to circumference
Programming in Alice
© 2006 Dr. Tim Margush

20. Advantages of Methods Abstraction Code reuse Generality
Methods allow the programmer to organize details of a task under the method's name
Code reuse
Methods can be called several times without having to copy the details of the code
Generality
Parameters allow the same actions to be applied to a variety of objects
Debugging and maintenance
Fixing or changing part of a program may only require modification of a single method
Programming in Alice
© 2006 Dr. Tim Margush

21. Assignment World-level Methods Lab 05 How to create them
How to call them
When it is appropriate to use them
Lab 05