September 26, 2011 Sorting and Searching Lists

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

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

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

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]

Example in Scratch

Binary Search Phonebook example Information must be sorted first!  How do we sort?

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

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  sc241/samples/sort/Sort2-E.html

Selection Sort – Players

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

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

Binary Search - BYOB