1. Compsci 201, Analysis+Markov
Owen Astrachan
Jeff Forbes
September 22, 2017
9/22/17
Compsci 201, Fall 2017, Analysis+Markov

2. Compsci 201, Fall 2017, Analysis+Markov
H is for …
Hashing
What better way to have a bucket list?
HTTP
A protocol we use every day
HackDuke
Oct 28/29, hackduke.org
Hexadecimal
0x001A57 is Duke Blue
9/22/17
Compsci 201, Fall 2017, Analysis+Markov

3. Compsci 201, Fall 2017, Analysis+Markov
Plan for the Day
Review Anonymous APT
Motivate Maps + Efficiency, prelude to Markov
Reminder of APTs, policies, APT Quizzes
Comparing elements, toward sorting and trees
Efficiency has a cost, when is it “worthwhile”?
Background for Markov: JUnit + Inheritance
9/22/17
Compsci 201, Fall 2017, Analysis+Markov

4. Redesign for Efficiency
Consider APT anonymous
Green solution
for each message:
If count(message,’a’) <= count(headline,’a’) OK
Repeat for ‘b’, ‘c’, …, ‘z’
Rescan the headline text for ‘a’, … ‘z’ AND for every message
Is this a concern?
9/22/17
Compsci 201, Fall 2017, Analysis+Markov

5. Compsci 201, Fall 2017, Analysis+Markov
Quantifying Concerns
Counting # occurrences of ‘a’ in headlines requires looking at all Hsize characters
We do this 26 times, that’s 26* Hsize (why)
We do this for M messages, that’s 26*Hsize*M
Also 26*Msize to count chars in each message
If we scan headline once to create map: Hsize
Process M messages is then 26*M + 26Msize
Is 26M + Hsize better than 26*Hsize*M ?
9/22/17
Compsci 201, Fall 2017, Analysis+Markov

6. Using Numbers to Understand
Is 26M + Hsize better than 26*Hsize*M ?
Suppose we have 100 messages with 1,000 characters in headlines
____ 2,600,000
Change with 10,000 characters in headlines
,000 ___ 26,000,000
If M or Hsize is constant, how does time change?
Is M + N better than MN?
9/22/17
Compsci 201, Fall 2017, Analysis+Markov

7. Compsci 201, Fall 2017, Analysis+Markov
Making Maps
In discussion saw int[] counters[256]
Can use Map<Character,Integer> to
for(char ch : str.toCharArray()){
counters[ch] += 1; }
for(char ch : str.toCharArray()){
if (! map.containsKey(ch)) map.put(ch,0);
map.put(ch, map.get(ch) + 1); }
9/22/17
Compsci 201, Fall 2017, Analysis+Markov

8. Compsci 201, Fall 2017, Analysis+Markov
Which map is best?
We can use both array and Map because char acts like an int
The “key” in some contexts won’t be an index
9/22/17
Compsci 201, Fall 2017, Analysis+Markov

9. Refactoring Anonymous
We had an all-green solution and we changed it!?
Didn't change what the code did
Did change how the code worked/performance
Refactoring: don't add functionality!
Correct code might break, tests to the rescue!
9/22/17
Compsci 201, Fall 2017, Analysis+Markov

10. Compsci 201, Fall 2017, Analysis+Markov
WOTO
Naïve or simple algorithms work well for small problems
“We should forget about small efficiencies, say about 97% of the time: premature optimization is the root of all evil. Yet we should not pass up our opportunities in that critical 3%”
Donald Knuth
Structured Programming with go to Statements". ACM Computing Surveys. 6 (4): 268
9/22/17
Compsci 201, Fall 2017, Analysis+Markov

11. Compsci 201, Fall 2017, Analysis+Markov
Donald Knuth
aka “The Donald”
Turing, Hopper, von Neumman awards
Author of “The Art of Computer Programming”
Arguably most important book written in Computer Science
First publication: Mad Magazine
(author of TeX) “I can’t go to a restaurant and order food because I keep looking at the fonts on the menu.”
9/22/17
Compsci 201, Fall 2017, Analysis+Markov

12. Compsci 201, Fall 2017, Analysis+Markov
Comparing and Sorting
Arrays.sort, Collections.sort
{“ant”, “bat”, “cat”, “dog”}
What algorithm is used in sorting?
How to change to sort-in-reverse or other order
Java 8 API helps here (study on your own)
TreeSet and TreeMap
Require comparisons
9/22/17
Compsci 201, Fall 2017, Analysis+Markov

13. Strings are Comparable
Compare strings lexicographically, natural ordering, dictionary order
“zebra” > “aardvark” but “Zebra” < “aardvark”
Conceptual, cannot use < or > or ==
“yak”.compareTo(s) returns < 0, == 0, > 0
s is “zebra”, “yak”, and “toad”, respectively
The int convention also used in C++, C, others
9/22/17
Compsci 201, Fall 2017, Analysis+Markov

14. Not Everything is Comparable
9/22/17
Compsci 201, Fall 2017, Analysis+Markov

15. Compsci 201, Fall 2017, Analysis+Markov
(x,y) < (z,w)
Can we compare Point objects?
To be “comparable”, implement the Comparable interface, supply .compareTo(..) method
9/22/17
Compsci 201, Fall 2017, Analysis+Markov

16. Compsci 201, Fall 2017, Analysis+Markov
Points
We’ll look at this to explore concepts in Java and Software Design
9/22/17
Compsci 201, Fall 2017, Analysis+Markov

17. Compsci 201, Fall 2017, Analysis+Markov
Build on What You Know
How does .equals work?
Make sure you have the correct type
Cast, compare
public boolean equals(Object o) {
if (o == null || ! (o instanceof Point)) {
return false; }
Point p = (Point) o;
return p.x == x && p.y == y;
9/22/17
Compsci 201, Fall 2017, Analysis+Markov

18. Compsci 201, Fall 2017, Analysis+Markov
Extend what you know
This is method in Point class
Point implements Comparable<Point>
public int compareTo(Point p) {
if (this.x < p.x) return -1;
if (this.x > p.x) return 1;
if (this.y < p.y) return -1;
if (this.y > p.y) return 1
return 0; }
9/22/17
Compsci 201, Fall 2017, Analysis+Markov

19. Developing and Testing
How did we test the Planet class? How do we test APTs?
Who writes the tests, how do we run them?
Unit test: single method, one step in development
JUnit is common library (also other languages)
Goal: all green, but red expected as you go!
9/22/17
Compsci 201, Fall 2017, Analysis+Markov

20. Compsci 201, Fall 2017, Analysis+Markov
Design, Build, Run Tests
9/22/17
Compsci 201, Fall 2017, Analysis+Markov

21. Compsci 201, Fall 2017, Analysis+Markov
Adding Tests
We will test compareTo
What are some good tests here
How will we implement them
What are some JUnit conventions
@Test
public void testMethod
assertEquals
9/22/17
Compsci 201, Fall 2017, Analysis+Markov

22. Compsci 201, Fall 2017, Analysis+Markov
Shafi Goldwasser
2012 Turing Award Winner
RCS professor of computer science at MIT
Twice Godel Prize winner
Grace Murray Hopper Award
National Academy
Co-inventor of zero-knowledge proof protocols
Work on what you like, what feels right, I now of no other way to end up doing creative work
9/22/17
Compsci 201, Fall 2017, Analysis+Markov

23. Compsci 201, Fall 2017, Analysis+Markov
Pause and Reflect
Ask someone near you that you don't know what their name is and where they are from
Move if you have to
What's the purpose of in-class WOTO questions?
9/22/17
Compsci 201, Fall 2017, Analysis+Markov

24. Compsci 201, Fall 2017, Analysis+Markov
Markov Assignment
Markov Models are used in many applications/areas
9/22/17
Compsci 201, Fall 2017, Analysis+Markov

25. What is a Java Interface?
An enforceable abstraction: methods required
Set and Map interfaces
Comparable interface
If Set<String> is parameter then can pass …
HashSet<String> or TreeSet<String>
Can sort String or anything that’s Comparable
Call .compareTo(..) method
9/22/17
Compsci 201, Fall 2017, Analysis+Markov

26. Compsci 201, Fall 2017, Analysis+Markov
Why use an interface?
9/22/17
Compsci 201, Fall 2017, Analysis+Markov

27. Compsci 201, Fall 2017, Analysis+Markov
Why use an Interface?
Work with frameworks, e.g., java.util.Collection
Iterable, Serializable, and more – use with Java
ArrayList, LinkedList, TreeSet, HashSet all …
.clear(), .contains(o), .addAll(..), .size(), … .toArray()
9/22/17
Compsci 201, Fall 2017, Analysis+Markov

28. Compsci 201, Fall 2017, Analysis+Markov
Developer Time
Making changes in timeSet and timeUtilSet
Change the call, don’t change method: util
TreeSet, HashSet implement same interface
Change the call, change method signature
Same methods: ArraySet, SimpleHashSet, but no common interface explicit
9/22/17
Compsci 201, Fall 2017, Analysis+Markov

29. Compsci 201, Fall 2017, Analysis+Markov
Concept: Inheritance
In Java, every class extends Object
Gets methods by default: .toString, .hashCode, .equals, and more
Subclass can override baseclass methods
Make .equals work for String
9/22/17
Compsci 201, Fall 2017, Analysis+Markov

30. Compsci 201, Fall 2017, Analysis+Markov
Shape is a base class
We'll show Rectangle and Circle subclasses
We can add new classes without recompiling or touching existing and tested classes
Subclasses can access protected data/methods
Cannot access private state/behavior
9/22/17
Compsci 201, Fall 2017, Analysis+Markov

31. Compsci 201, Fall 2017, Analysis+Markov
MakeShapes.java
public class MakeShapes {
public static void main(String[] args) {
ArrayList<Shape> shapes = new ArrayList<>();
shapes.add(new Rectangle(3,5));
shapes.add(new Circle(1.0));
shapes.add(new Rectangle(5,3));
for(Shape s : shapes) {
System.out.printf("%s\t%1.3f\n", s,s.area()); }
Inheritance models "is-a" relationships
9/22/17
Compsci 201, Fall 2017, Analysis+Markov

32. Compsci 201, Fall 2017, Analysis+Markov
Power and Simplicity
when changing a method from parent class
Helps compiler and developers
Java isn't the only Object Oriented language
Swift, Objective-C, C++, C#, Ruby, Python, ..
Java requires OO, not all languages do
9/22/17
Compsci 201, Fall 2017, Analysis+Markov