CS 40 - Lecture 11

Cover page image

Cay S. Horstmann

List

Collection of objects
Each object has position number (0, 1, 2, 3)
Can add, remove objects
Can do something to all objects in the list

For All In Order

Use For all in order to do something to all objects in a list
Three ingredients:
- a list (duh)
- a variable
- code
Variable is set to first element of the list, code is executed
Variable is set to next element of the list, code is executed
...and so on...
...until all list elements have been used

Lab 0: Make them Move

Populate a world with four or five people from the People, Egypt, Japan galleries
Click on the world and then the properties tab
Add a new variable called peopleList (or animalsList if you were a contrarian and used animals instead of people)
Make it a list of objects
When prompted, add the people (or animals) to the list.
To be continued...

Lab: Make them Move (Cont'd)

Edit world.my first method.
Drag a For all in order tile
Drag the [Obj] item_from_peopleList tile into the Do Nothing area below
Select “move forward one meter”
Play. What happens?
Why is this better than calling evilStepSister.move(), cinderella.move(), iceSkater.move() etc? ActiveLecture.org

Lining up Objects

Unless you were very careful, your objects are probably not perfectly aligned, and the movement wasn't all that smooth
The list doesn't do anything to visually align its items
Goal: world.lineUp method
Line up first person 1 meter next to a toaster. the second person 1 meter next to the first etc.
Outline of method is in next slide

Lining up Objects

Lab 1: Fix the method

Load LineUp.a2w
Add a call to world.lineUpPeople as the first statement in world.my first method
What happens? ActiveLecture.org
Fix the problem(s).
Make sure to move each item to the left of objectAtRight.
Update objectAtRight in each loop iteration.
Upload your world to eCampus

Using Walkthroughs to Understand Loops

Make a table
One column for each variable

item_from_peopleList objectAtRight

toaster
Simulate loop
One row for each loop iteration

Cross out old values when they change

item_from_peopleList	objectAtRight
	toaster
evilStepSister2	evilStepSister2
cinderella

Finding the Tallest Person

Your mission: Find the tallest person in peopleList and have her/him say “I am the tallest”.
Need a plan.
Have a variable for “the tallest person encountered so far”
Have a variable for the height of that person
Local variables of my first method
When are they updated? When we find a taller person.

Loop for Tallest Person

Lab 2. Tallest Person

Load FindTallest.a2w
When the current person is taller than the tallest person so far, do what needs to be done...
After the end of the loop, have the tallest person brag.
To make sure you did it right, double the size of one of your people and hit Play again. Does the newly tallest person brag now?
Submit your world.

Too Boring?

Now that you know how to find the tallest person, you can sort by height.
Find the tallest person. Add him/her to another list and remove from the original list.
Keep going until the original list is empty.
Now line up the items in the new list.
Congratulations! You are a computer scientist (not just a “software engineer”)

Reminders

Done with Alice
No class next week
Project proposals due March 25
Looking for teammates? Check out bios/photos in eCampus. Yours not there? Put it in.