Sorting and Searching Lists

1. Sorting and Searching Lists September 26, 2011

2. Agenda • Review quiz #3 • Team assignment #1 due tonight • One per team • Arcade game • Searching • Linear • Binary • Sorting • Selection Search

3. What else can we do with a List? • What about a high score tracker for an arcade game? • What data needs to be stored? • Is the data sorted in some way? • Let’s try it out… • Movement • Add info to lists

4. Linear Search – High Score • Examine each component in the list • Similar to a checkout line • The line of customers is a list • The checkout clerk processes each customer’s purchase one at a time

5. Linear Search Algorithm – Searching for Highest Score Set largestIndex = 1 Set index = 1 Repeat for the length of L if L[index] > L[largestIndex] set largestIndex = index index = index + 1 Print L[largestIndex]

6. Example in Scratch

7. Binary Search • Phonebook example • Information must be sorted first! • How do we sort?

8. Sorting • How can we sort the scores from the highest score to the lowest score and the players alphabetically? • Selection Sort • Find the largest number in the list • Swap it with the item in the first position • Repeat starting with the second element in the list • What do we need? • Swap method • Sort method

9. Selection Sort • Similar to a linear search • Find the smallest (or largest) value and place it in the first slot • Repeat this for each position in the list • Animated example • http://www.cs.oswego.edu/~mohammad/classes/csc241/samples/sort/Sort2-E.html

10. Selection Sort – Players

11. Binary Search • Phonebook • Look at the middle of the list • If the value is larger than the item in the middle of the list • Look at the last half • Otherwise • Look at the first half

12. Binary Search Repeat until found = true or min > max mid = max+min/2 if value > L[mid] min = mid + 1 else if value < L[mid] max = mid – 1 else found = true if found = false mid = -1 print mid

13. Binary Search - BYOB