Programming with Alice
Computing Institute for K-12 Teachers
Summer 2011 Workshop
Session 5
Methods and Functions

Often, in programming, you will encounter sections of code
that you will want to use more than once. In these cases it
is extremely helpful to place the code in a container called
a method or a function. This allows the group of blocks to
be ran or “called” using only a single block.

Methods and functions serve two main purposes in
programming: to simplify code and make it easier to
change (see the presentation notes for a detailed
explanation of these benefits).

Methods and functions, while identical in other
programming languages, do have one distinct difference in
Alice; methods are a collection of blocks that do things,
while functions are a collection of blocks that return a
value. Functions do not directly affect the objects in the
world.
Methods

Methods can be created by selecting an object, clicking the
methods tab, and clicking “create new method.”

You are then prompted to
enter a name for the new method:

I recommend you choose a name that briefly describes
what the method does. (i.e. A good name for a method
that moves a character while animating its legs could be
called “walk”)
Methods - example

Let’s construct a quick example: create a method that
simulates a strobe effect using the existing light in the world.

In the object tree, select the light object and click the
methods tab. Create a new method called “strobe.”

Move to the properties tab and drag the brightness block to
the body area of the new strobe method and enter 0 for the
value. This will turn the light off.

Call your new strobe method by clicking the methods tab and
dragging the strobe method to replace “world.my first
method” in the Events area.
Methods – Parameter

Drag another brightness block to the strobe method just
below the previous one, only this time set the brightness to
1.

If the method was called right now it would turn the light
off and then turn it back on once. To make it alternate
continuously, we’re going to use an infinite loop and place
both blocks inside.

The strobe is probably slower than you expect; we need a
way of easily changing the speed or delay between the on
and off states of the light.

With the strobe method open, click “create new parameter”
on the right side of the window, name it “delay”, set type
to number, and click ok. Set the duration of both set
brightness blocks to the delay parameter by clicking more,
mouse-over duration, mouse-over expressions, and click
delay.
Methods – Parameter cont.

Now, whenever you call the strobe method the delay
parameter must be supplied.

This is what your finished strobe method should look like:
Ownership

Methods and functions are, in a sense, “owned” by the
object that is selected when they are created. This can be
restrictive if the method affects an object other than the
one that “owns” it.

For example, if you are creating a method that makes an
airplane fly, you would want to make sure the method is
owned by the airplane and not the runway or other object
in the scene.

However, methods owned by the world object are
accessible by any object in the scene. Subsequently,
methods and functions unrelated to any specific object
should be owned by world.
Functions

Functions are used to calculate and return a value.

A real life example of a function would be finding the area
of a rectangle. To find the area of a rectangle, you first
need two values: the length and the width. Once those
values are known, simply multiply them together and the
result is the area. This task could easily be made into a
function called “findAreaOfRectangle” and would expect
two parameters called “length” and “width.” inside the body
of the function, the two parameter variables would be
multiplied and stored in a third variable called “area.” The
function would then return the value of the area back to
the location that the function was called from.

Although most functions will make use of parameters, they
aren’t always necessary as evidenced by the existence of
default Alice functions such as “mouse distance from left
edge” or “random number.”
Functions

Functions, like methods, are created by selecting an object,
clicking the functions tab, and clicking “create new
function.

Unlike methods, functions require a return type to be
selected upon creation.
Functions

Since functions must return something, they must return a
specific type of something. If we created a function that
added two numbers and returned the result, we would
expect it to return a number, not a string, Boolean, or
other value.

Functions can not affect the world directly. You can not call
methods from functions but you can call functions from
methods. Functions exist only to return a value.
Methods and Functions Example

In session 0, you created a car that moved across the
screen while it’s wheels rotated. Now, we’re going to
rewrite that program using methods and functions.

If you can’t find your saved project from session 0, please
go back and recreate it. It’s small and, hopefully, shouldn’t
take too long.

First, we’re going to create a method called “drive” for the
car. Make sure the car is selected then create the method.
Using the clipboard, move all the blocks to this new
method and replace “world.myfirstmethod” with a call to
the drive method in the Events area.
Methods and Functions Example

We want to be able to control the speed and distance
traveled by the car so let’s add those as parameters to the
drive method. With the drive method open in the method
editor, click the “create new parameter” button on the right
side. Both parameters will be numbers (speed and
distance) so create both of type Number.

Replace the number of meters in the move block with the
distance parameter from the top of the method editor.
Methods and Functions Example

We need to determine a duration for the blocks in the drive
method. All we know is the speed and distance, but
duration can easily be found by dividing distance by speed.

Do this for all five blocks. The clipboard may speed up the
process.

Now we need to calculate the rotation of the wheels.

Create a new function for the vehicle object named
“angularDistance” of type Number. Add the two
parameters, speed and distance, like you did for the drive
method.
Methods and Functions Example

In order to determine how many times the wheels should rotate
for the given distance, we need to first find the circumference of
the wheel. If we remember that circumference is pi * diameter,
this will be easy to calculate.

By selecting one of the wheels, you can access a function called
“object’s width” or “object’s height.” Since a wheel is round,
either of these functions should return a value that will suffice for
the diameter (these number are not exact which may lead to
inaccurate wheel rotation speed).

Now that we have the circumference of the wheel, we will now be
able to calculate the total number of rotations.
Methods and Functions Example

Now that the function is complete, we can use it in the
drive method. Place the new angular distance function in
the revolutions area of all four wheel turn methods. Set the
distance parameter of the function to the distance variable
in the drive method. This finished drive method should look
similar to this:

By changing the parameters in the drive method call in the
Events area, we can easily affect the vehicle’s movements.
Project 5

Add an additional parameter to the drive method named
“direction” that will accept either forward or backward and
affect the object in the expected way.

(Optional) Create a function to calculate duration based on
distance and speed and use it in the last example in this
session.
Day 3 Questions

What would you use to store a response from the user?

How does a function differ from a method in Alice?

Parameters can be used with both methods and functions.
( true/__false)

What type of variable would you use to store the value
returned by the function named “isWorking”?

What should be used to group the blocks that make a
character throw a ball?

Are methods and functions absolutely necessary or do they
just simplify programming?
Answer Key

What would you use to store a response from the user?
variable

How does a function differ from a method in Alice? a
function returns a value where as a method does
something, functions cannot affect the world

Parameters can be used with both methods and functions.
( X true/__false)

What type of variable would you use to store the value
returned by the function named “isWorking”? boolean

What should be used to group the blocks that make a
character throw a ball? method

Are methods and functions absolutely necessary or do they
just simplify programming? simplify programming