1. Lists and Iteration in Alice
Stephen Cooper
Wanda Dann
Randy Pausch
Barb Ericson
Oct 2009
ListsAndIterationInAlice

2. ListsAndIterationInAlice
Learning Goals
Introduce the concept of a collection
And the need for one
Introduce the concept of a list
Introduce iteration
Show how to create a variable
Show how to create a list
Show how to iterate over a list
ListsAndIterationInAlice

3. ListsAndIterationInAlice
Collections
In some animations, several objects must perform the same actions
Example:
A marching band, where the band members are performing the same marching steps.
You could tell each object to perform each action
Your program will get very long very quickly
In such situations, it is convenient to collect all the objects into a group
ListsAndIterationInAlice

4. ListsAndIterationInAlice
One way to organize objects into a collection is to use a list.
In our daily lives, we use lists to help us organize things. For example,
assignments list
shopping list
In programming, a list is known as a data structure
Way to structure our data
ListsAndIterationInAlice

5. ListsAndIterationInAlice
Types of Lists
In Alice, a list can be a list of numbers or a list of objects, colors, or a specified type ….
In this session, we will use an example of a list of objects.
ListsAndIterationInAlice

6. ListsAndIterationInAlice
Example
The famous Rockettes® are preparing for their winter holiday show. You have been hired to create an animation for the holiday show that will be used on a web site where people can purchase tickets for the show.
ListsAndIterationInAlice

7. ListsAndIterationInAlice
Creating a list: Step 1
Create the initial world and add five rockette objects to the world.
Create a new variable in the properties panel for the world
Why should the world keep track of the list of rockettes?
ListsAndIterationInAlice

8. ListsAndIterationInAlice
Creating a list: Step 2
In the popup dialog box,
type in a name
select Object type
check “make a List” box
click new item button 5
times to select each
of the 5 rockettes,
one at a time
ListsAndIterationInAlice

9. Programming with a list
Now that a list has been created, how do we use the list in a program?
One of the most useful operations with a list is to repeatedly perform some action with each item in the list. We call this iteration or "iterating through a list."
ListsAndIterationInAlice

10. Iterating through a list
Iteration can be done in two ways:
in order (one at a time)
all together (simultaneously)
ListsAndIterationInAlice

11. Example: Iteration in Order
A typical chorus line dance involves each dancer (one after the other) kicking up one leg.
Possible storyboard:
kickUpRightLeg
Parameter: whichRockette
Do in order
Do together
whichRockette right thigh turn back
whichRockette right calf turn forward
whichRockette right calf turn back
For all dancers in order
item_from_dancers kickUpRightLeg
ListsAndIterationInAlice

12. ListsAndIterationInAlice
Challenge
Modify 07Rockettes.a2w
Have all of the dancers put down their legs one at a time
And then have them all lift them at the same time
And then put them all down at the same time
ListsAndIterationInAlice

13. ListsAndIterationInAlice
Things to Notice
The method that lifts up the rockettes leg is defined in the world class not in the rockette class
And it takes as a parameter of which rockette
This is due to an a problem in the current version of Alice
Methods don’t automatically apply to the object the method was called on
Like in Java with the implicit parameter “this”
It should be a method on the rockette class!
And it can be, but it must take a parameter of which rockette
ListsAndIterationInAlice

14. ListsAndIterationInAlice
Summary
You can create a collection of objects
A list
You can repeat a series of steps for each object in the list
Iterate over the list
You can have the items in the list do the action one at a time
Or at the same time
ListsAndIterationInAlice