1. Week 3 - Friday
CS221

2. Last time What did we talk about last time?
Array implementation of a list

3. Questions?

4. Assignment 2

5. Project 1
Bitmap Manipulator

6. Assignment 1 issues
Most people messed up the add() method that takes an index
Most people didn't throw exceptions at quite the right times or didn’t throw them at all
There was a lot of inconsistent formatting
I don't specify what your formatting should be too closely, but it should be consistent
Check out the coding standards for the course:

7. Specific Assignment 1 don'ts
Don't create a new Node unless you have to:
Never look at the next thing unless you have to:
Node temp = new Node(); //wastes memory
temp = head;
public boolean contains(int element) {
Node temp = head;
while(temp.next != null){ //two different bugs
if( temp.value.equals(element) )
return true;
temp = temp.next; }
return false;

8. Specific Assignment 1 don'ts
Don't use == to compare two String objects!
public boolean contains(int element) {
Node temp = head;
while(temp != null){
// following might not work!
if( temp.value == element) )
return true;
temp = temp.next; }
return false;

9. String comparison What happens when you use == for String comparison?
String a = "Goats";
String b = "Goats";
String c = new String("Boats");
String d = new String("Boats");
if( a == b ) // prints
System.out.println("a == b");
if( c == d ) // doesn't print
System.out.println("c == d");

10. Specific Assignment 1 do's
Use other methods to simplify code:
Except when it doesn't make sense
Using get() in addAll() is inefficient
Keep it simple:
Test, test, test!
public boolean contains(int element) {
return indexOf(element) != -1; }
public boolean isEmpty() {
return size == 0; }

11. Stacks
Impromptu Student Lecture

12. Stacks

13. Stack
A stack is a simple (but useful) ADT that has three basic operations:
Push Put an item on the top of the stack
Pop Remove an item from the top of the stack
Top Return the item currently on the top of the stack (sometimes called peek)

14. Keeping track of things
When are stacks used?
Implicitly, in recursion (or in any function calls)
Explicitly, when turning recursive solutions into iterative solutions
When parsing programming languages
When converting infix to postfix

15. Implementations

16. Array implementations
Advantages:
Pop is Θ(1)
Top is Θ(1)
Disadvantages
Push is Θ(n) in the very worst case, but not in the amortized case

17. Array implementation
public class ArrayStack { private int[] data; private int size; public ArrayStack() {} public void push(int value) {} public int pop() {} public int peek() {} //instead of top public int size() {} }

18. Array Constructor

19. Array Push

20. Array Pop

21. Array Peek

22. Array Size

23. Upcoming

24. Next time…
Queue implementation with dynamic array
Linked lists

25. Reminders Keep reading section 1.3 Finish Assignment 2
Due tonight by 11:59pm
Keep working on Project 1
Due next Friday, September 22 by 11:59pm