CS320n

1
CS320n Elements of Visual Programming

• Lists
• Mike Scott
• (Slides 9-1)

Thanks to Wanda Dann, Steve Cooper, and Susan
Rodger for slide ideas.
2
Collections

• In some animations, several objects must perform
  the same actions
• Example
• Cars in lanes from Frogger
• In such situations, it is convenient to
  collect all the objects into a group (a
  collection)
• Major benefit of Lists is we can write code for
  each object in the list
• rather than separate code for each object

3
List

• One way to organize objects into a collection is
  to use a list.
• We use lists to help us organize things. For
  example,
• assignments list
• shopping list
• email inbox
• In programming, a list is known as a data
  structure.
• an object that is used to group other objects
  together

4
Types of Lists

• In Alice, a list can be a list of numbers or a
  list of objects, colors, or other type .
• In this session, we will use an example of a list
  of objects.

5
Example

• Create a List of chickens
• add 5 chicken objects to the world

6
Creating a list Step 1
Create the initial world and add five chicken
objects to the world. Then, create a new
world-level variable in the properties panel.
(Why should it be world-level rather than
class-level?)
7
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 chickens, one at
a time
8
Resulting List

• No change in the scene
• We have simply added them to a list variable

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 powerful operations with a list
  is to repeatedly perform some action with each
  item in the list.
• This is called iteration or "iterating through a
  list."

10
Iterating through a list

• Iteration can be done in two ways
• in order (one at a time)
• all together (simultaneously)

11
Example/Demo Iteration in Order

• Have each chicken say hello
• Have each chicken kicks its legs

12
The Objects part named X function

• One problem when passing Objects as parameters is
  how do I affect a sub part?
• If the parameter type is simply Object subparts
  are unknown
• Same problem with elements of a list
• Use the Objects part named X function

13
Part named function

• Get "in order" operation for list
• Drop in a Chicken object
• Pick functions for Chicken Object and drag the
  part named function to replace the Chicken object

14
Part Named Function

• Select which part
• Pick other and type in name of sub part

15
Part Named function

• Replace Chicken object with item_from_chickens
• Final version

16
Example / Demo Iteration Together

• Change the previous animation so that the
  chickens move their legs together