1. Alice in Action with Java
Chapter 5
Lists and Arrays

2. Objectives Use a list to store multiple items
Use Alice’s forAllInOrder and forAllTogether statements
Use random numbers to vary the behavior of a program
Use an array to store multiple items
Alice in Action with Java

3. Lists and Arrays Some variables only store a single value
Number, Boolean, String, Object, Sound, Color
Variables that can store multiple values
list: container that can dynamically grow and shrink
array: container whose size remains fixed at run-time
Benefits of using variables that store multiple values
Reduces amount of code that needs to be written
Makes a program more readable
Example: a variable playList that holds 12 songs
Pass all 12 songs to a method using one parameter
Alice in Action with Java

4. Lists and Arrays (continued)
Alice in Action with Java

5. List Example 1: Flight of the Bumble Bees
Scene 2: a dozen bees rise to defend queen’s honor
Set up the scene by adding 12 bees and 1 queenBee
Manipulating 12 individual bees leads to inefficiencies
Adding 12 bees to a list simplifies implementation
Form: Object[]bees = bee, bee2,... bee12;
Defining bees list variable in playScene2()
Open Create New Local Variable dialog box
Select Object type and then make a List
Click new item and add the first bee to the list
Continue adding new items until the list holds 12 bees
Alice in Action with Java

6. List of Bumble Bees
Alice in Action with Java

7. Processing a List of Bumble Bees
Processing list entries
Drag the forAllInOrder statement to editing area
Drop forAllInOrder, select expressionsbees
Construct move()using bee2 as a placeholder
Drag item_from_bees onto the placeholder and drop
Send turnToFace()message to the queenBee
Send say()message with command to queenBee
Simulated behavior
queenBee turns to each bee and orders it to rise
Each bee responds by rising upward 5 meters
Alice in Action with Java

8. List Operations forAllInOrder statement forAllTogether statement
Used to sequentially perform actions on list items
forAllTogether statement
Used to simultaneously perform actions on list items
Example: cause all bee objects to take off at once
Each item in list is accessed by index (position)
Messages can be sent to an entire list or to list items
Example 1: aList.clear(); (removes all items)
Example 2: aList.add(0, 1); (inserts 1 at list head)
Alice in Action with Java

9. doAllInOrder
Alice in Action with Java

10. doAllTogether
Alice in Action with Java

11. List Operations
Alice in Action with Java

12. List Operations A few list functions Using list functions
aList.size(): the number of items in aList
aList[i]: the value item i in aList
aList.getRandomItem(): random value in aList
Using list functions
Drag a list definition into the editing area
Drop list onto placeholder with function’s return type
Alice will display a list of messages to choose from
Alice in Action with Java

13. List Example 2: Buying Tickets
Scene 3: line of people waiting to buy tickets
Animating the scene using the list data structure
Create a new world with a ticket box office
Line up five people in front of the ticket window
Add five people to the personList
Use nested looping to advance the line
After an outer loop iteration, the line is reduced by one
Testing the animation
Compare positions of each person after each iteration
Alice in Action with Java

14. List Example 2: Buying Tickets (continued)
Alice in Action with Java

15. The Array Structure
Fixed-size container that can store a group of values
Data items are accessed by position (as in a list)
Values of data items can be changed at run-time
The capacity of an array cannot change at run-time
Two reasons for using an array instead of a list
The array is more memory-efficient than a list
An array is more time-efficient for accessing an item
Alice in Action with Java

16. Array Example 1: The Ants Go Marching
User story: soldier ant sings a song while marching
View the story as a counting problem
The ant song is 10 verses long
Most of the lines are repeated
A for loop can be used to iterate through 10 verses
Managing the differences in lines using parallel arrays
One String array stores the verse-specific numbers
Another String array stores verse-specific ant actions
Values at ith index of array are accessed with each loop
Enhance the program by adding a marching method
Alice in Action with Java

17. Array Example 1: The Ants Go Marching
Alice in Action with Java

18. Array Operations Array items are accessed by index (like lists)
Predefined operations for arrays
anArray[i] = val; (the subscript operation)
val.set(value, anArray[i]);
anArray.length
Two versions of the subscript operation
Write version: changes value of item i in the array
Read version: retrieves the value of item i in the array
Traverse an array using a for loop or while loop
forAllInOrder and forAllTogether not available
Alice in Action with Java

19. Array Example 2: Random Access
Scene: left castle door tells jokes to right castle door
Elements used to program random knock-knock jokes
Castle interior with camera positioned before doors
Parallel arrays for accessing name and punchline
Dialog including name and punchline arrays
Use of Random.nextDouble()to generate index
Random.nextDouble()details
Accessed in the functions pane of world’s details area
Set integerOnly attribute to true
Define a range of random values for indexing the array
Alice in Action with Java

20. Array Example 2: Random Access (continued)
Alice in Action with Java

21. Summary Data structure: structure storing a group of values
List: dynamic container that holds a group of values
Array: static container that holds a group of values
Item: individual variable stored by index in an array or list
forAllInOrder statement: loops through list items in sequence
Alice in Action with Java

22. Summary (continued)
forAllTogether statement: applies instructions to list items in parallel
Random.nextDouble(): generates random numbers
Alice usually displays <None> to indicate that an object value is null
Alice in Action with Java