CS120: Lecture 13

1
CS120 Lecture 13

• MP Johnson
• Hunter
• mpjohnson_at_gmail.com

2
Agenda

• Go over MT
• Review ord-of-mag est
• Programming
• Algorithms ? programs
• Pseudocode
• JavaScript

3
Future Agenda

• More algs
• Sorting
• Gcd
• Poly eval?
• Recursion
• Towers of Hanoi
• Programming Java

4
Sorting

• Ideas?
• Selection sort
• Worst-case time?
• Is it ever faster?
• Insertion sort?
• playing cards intuition
• Worst-case time?
• Is it ever faster?
• How to get faster? Later

5
Sorting

• Very important problem
• Maybe single most important
• Sorting searching
• Although fashionable idea Search, dont sort
• Gmail, Msfts Apples future OSs
• Very well studied problem
• Will talk about several algs
• Insertion sort
• Selection sort
• Worst-case v. best-case

6
Why important?

• Sorting is very common
• Supposedly mainframes spent 25 of all time
  sorting
• Sort email, sort files (by name/date), etc.
• Sorting makes other ops/algs easier
• After sort
• searches take logn
• Eg Search phonebook by name v. by number
• Search for song in record store
• Find median is easy (how?)
• Find if array has duplicates is easy (how?)
• Find mode is easy (how?)
• 5. How to create convex hull?
• Given unordered set of pts, hard
• But sort by x value ? now easy

7
Menu of sorting algs

• Th(N2) is easy, but slow
• Insertion sort
• Selection sort
• Bubble sort
• First major sub-quadratic alg Shellsort
• Donald Shell
• O(N1.5)
• Took many years to analyze time
• best nlogn
• Merge sort
• Heapsort
• Treesort
• Quicksort (in practice)
• Will prove these are optimal
• Special bucket/radix n

8
Abstraction simulation

• Same logic with M-W and students-hospitals
• Meaning doesnt matter
• Computer only needs the logic/alg
• Computers can play chess, without understanding
  the game
• Computers can simulate the weather, without
  getting wet
• Computers can simulate intelligence, without
  being intelligent (?)
• Chess, help with hw, make jokes, debate the movie

9
Next Programming

• Algorithms ? programs
• Can express in pseudocode
• But must code in some prog lang
• This week lang JavaScript in webpages