CS320n – Elements of Visual Programming Lists Mike Scott (Slides 9-1) Thanks to Wanda Dann, Steve Cooper, and Susan Rodger for slide ideas.

Visual ProgrammingAnimation Programs2 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

Visual ProgrammingAnimation Programs3 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 inbox In programming, a list is known as a data structure. –an object that is used to group other objects together

Visual ProgrammingAnimation Programs4 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.

Visual ProgrammingAnimation Programs5 Example Create a List of chickens add 5 chicken objects to the world

Visual ProgrammingAnimation Programs6 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?)

Visual ProgrammingAnimation Programs7 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

Visual ProgrammingAnimation Programs8 Resulting List No change in the scene We have simply added them to a list variable

Visual ProgrammingAnimation Programs9 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."

Visual ProgrammingAnimation Programs10 Iterating through a list Iteration can be done in two ways: – in order (one at a time) – all together (simultaneously)

Visual ProgrammingAnimation Programs11 Example/Demo: Iteration in Order Have each chicken say hello Have each chicken kicks its legs

Visual ProgrammingAnimation Programs12 The “Object’s 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 Object’s part named X function

Visual ProgrammingAnimation Programs13 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

Visual ProgrammingAnimation Programs14 Part Named Function Select which part Pick other and type in name of sub part

Visual ProgrammingAnimation Programs15 Part Named function Replace Chicken object with item_from_chickens Final version

Visual ProgrammingAnimation Programs16 Example / Demo: Iteration Together Change the previous animation so that the chickens move their legs together