Compsc sci 201 201 Colle lectio ions ns, Hashing hing, O - PowerPoint PPT Presentation

Compsc sci 201 201 Colle lectio ions ns, Hashing hing, O Objects Susan Rodger February 5, 2020 2/5/2020 CompSci 201, Spring 2020 1

Glitchy App? 2/5/2020 CompSci 201, Spring 2020 2

H is for … • Hashing • What better way to have a bucket list? • Hexadecimal • ABC is 10,11,12 • Base 16 > Base 2? 2/5/2020 CompSci 201, Spring 2020 4

Announcements • Assig ignm nment nt P P2 out l lat ater er t this is w week • APT APT-3 d due ue T Tue ues, F Feb eb 4, 4, Extended t to Thur Thurs F Feb eb 6 6 • Last chance to turn in Friday til 11:59pm • Dis iscussion 5 5 on n Feb eb 10 10 • Prepare for exam • Exam am nex next w week eek, F Feb eb 14 14 2/5/2020 CompSci 201, Spring 2020 5

PFWBVDW • Interfaces aces: L List, S , Set, a , and M Map • When it makes sense to use general type • Empirical and Analytical measures of efficiency • Maps: API and P Problem lem S Solving ving • Keys and Values • Big ig-Oh and and O O-Not otation • Building a mathematical formalism with intuition 2/5/2020 CompSci 201, Spring 2020 6

Midterm Coming Feb 14 • How much ch c code e have you w u written w en with p h paper er a and a writi ting u ute tensil? il? • Tests should measure what you've practiced • Practice writing code on paper! • Midterm r m revie iew a and p d previo ious te tests ts • These are the best practice available • Will practice in Discussion • Logistic tics • Start on time, end on time, accommodations • 1 page front and back of notes you bring and leave 2/5/2020 CompSci 201, Spring 2020 7

Breakfast 201 was yummy! • Wed ed. F Feb eb 5 5 9:30 9:30am • 30 30 minu inutes, d dis iscuss w wha hatever w wit ith h me • Enjo joy br breakfast • More re b bre reakfas asts coming ingl… 2/5/2020 CompSci 201, Spring 2020 8

The hashCode contract • Every o object t has s .hashCode() metho hod • Inherited from Object, but typically overridden • Use @Override and read online • Must r st resp spect . t .equals( s(): I If a.equals(b) ? • a.hashCode() == b.hashCode() • Converse not true! There will be collisions 2/5/2020 CompSci 201, Spring 2020 9

When Strings Collide • Gene enerate strings t tha hat w will c ill coll llid ide • Find ind s suc uch s strings in in the he wild ild String hashCode String hashCode ayay 3009136 buzzards -931102253 ayBZ 3009136 righto -931102253 bZay 3009136 snitz 109586548 bZbZ 3009136 unprecludible 109586548 2/5/2020 CompSci 201, Spring 2020 10

Default: Object.equals, .hashCode When you do not override… • For O Obj bjects ts p and q d q: • p.equals(q) is the same as p == q • Do p and q reference/point to same object • For O Obj bject p t p • p.hashCode() is location in memory of object • Th Thus: i if f p == q then en • p.hashCode() == q.hashCode() 2/5/2020 CompSci 201, Spring 2020 11

Summary: ArrayList and HashSet • Both h hav ave . e .add, .addAll, and mor more • Both iterable: for(Elt e : collection) • Both h h have . .cont ntains lever eraging . .equal uals • HashSet also uses .hashCode to reduce the collection iterated over: locker collisions • Object ect h hygien ene w e when en devel eloping y your ur c classes es • .toString(), .equals(), .hashCode() 2/5/2020 CompSci 201, Spring 2020 13

When Strings Collide https://www.youtube.com/watch?v=HeTShE2PiQI 2/5/2020 CompSci 201, Spring 2020 14

When Strings Collide • Gene enerate strings t tha hat w will c ill coll llid ide • Find ind s suc uch s strings in in the he wild ild String hashCode String hashCode ayay 3009136 buzzards -931102253 ayBZ 3009136 righto -931102253 bZay 3009136 snitz 109586548 bZbZ 3009136 unprecludible 109586548 2/5/2020 CompSci 201, Spring 2020 15

Concept: Inheritance • In Java, e ever ery c class e extend ends O Object ect • Gets methods by default: .toString, .hashCode, .equals, and more • Inherit metho hod + + implement entatio ion • Subclass can n overri rride bas ase c class metho hods • Make .equals work for Point class 2/5/2020 CompSci 201, Spring 2020 16

Work in 201 • How w impo portant a are APT APTs? • How important are APT quizzes? • How important nt a are as e assig ignm nment ents? • Earlier assignments, later assignments? • How im important: r read eadin ing and and W WOTO in in-class • How important is reading? 2/5/2020 CompSci 201, Spring 2020 17

Alphabetical Order • En Encryption? Maybe n not • https://www2.cs.duke.edu/csed/newapt/encryption.html • Think about high-level algorithm • Apply your algorithm to: "pop", "array", "deeds" • Wha hat do w we nee e need to do t to code alg algorithm? • Recall: 'b' + 1 == 'c' • Recall: array['h'] is allowed, 'h' can be index 2/5/2020 CompSci 201, Spring 2020 18

Idea with Encryption APT int[] allcha int lchars = new i int[256] 256]; 96 0 int int nex nextLet is is 'a' 97 0 ‘a’ String ans answer = = ""; ""; 98 0 ‘b’ 0 99 ‘c’ messa ssage i is feed 100 0 ‘d’ ‘e’ 101 0 answer is ans is 0 ‘f’ 102 103 0 ‘g’ ch is ch is 2/5/2020 CompSci 201, Spring 2020 19

How often does a string occur? • Stri rings gs s stored i in Arra rrayList? • Call Collections.frequency(list,word) • If in array a rather than ArrayList? Collections.frequency(Arrays.asList(a), word) ArrayList<String> list is [“cat”, “cat”, “dog”, “fish”, “dog”, “cat”] Collections.frequency(list, “dog”) is Collections.frequency(list, “cat”) is 2/5/2020 CompSci 201, Spring 2020 36

How often does a string occur? • Is Is Collec lectio ions ns.f .freq eque uency ncy ef efficient icient? D Does i it mat atter? • Use Collections.frequency • Can create parallel arrays or use HashMap • Keep count[k] # occurrences of word[k] • Use HashMap if you know that 2/5/2020 CompSci 201, Spring 2020 38

WOTO (correctness counts) http:// //bi bit.ly/2 /201spr pring20-02 0205 05-1 2/5/2020 CompSci 201, Spring 2020 39

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 know of no other way to end up doing creative work 2/5/2020 CompSci 201, Spring 2020 40

Why use an interface? 2/5/2020 CompSci 201, Spring 2020 41

What is a Java Interface? • An enforcea ceable a le abstractio ion: n: m metho hods r requir uired ed • Set, Map, List interfaces • Can an im imple lement m more t e tha han o one int ne interface • Can extend only one base-class! • Argua uable le: M Mammal i al is a an inter erface ace • Do NOT inhe T inherit method implementations • Do inherit methods (names, types, etc.) 2/5/2020 CompSci 201, Spring 2020 43

Analogy: Mammals • Drago gon? ? • Mammals • Birth to live young • Hair • Warm-blooded • Lik ike an an int interface! 2/5/2020 CompSci 201, Spring 2020 44

Why use an Interface? • Work w with f h frameworks, e e.g., ., java.ut util il.Collect llection • Iterable, Serializable, and more – use with Java • Arra rrayList, , LinkedLis ist, , TreeS eSet, , Has ashS hSet all all … • .clear(), .contains(o), .addAll(..) , .size(), … .toArray() https:// ://docs cs.o .oracle.co com/en/ /en/java/j /javase/11/ /11/docs cs/ap api/ i/j ava.base/ e/java/ a/ut util/ il/Collec llectio ion.ht n.html 2/5/2020 CompSci 201, Spring 2020 46

There are two kinds … • Ther There ar are 10 10 kind inds o of peo eople in in the he w world ld … … • Those who understand binary and … • Is this funny? • Has ashS hSet/HashMap a and TreeS eSet/TreeMap ap • Tradeoffs in efficiency, organization • LinkedLis ist/Arra rrayList • Tradeoffs in efficiency, organization 2/5/2020 CompSci 201, Spring 2020 47

Link v Array • Get etting ng b between t en two e element ents • Unsnap/Snap v Shift/Insert 2/5/2020 CompSci 201, Spring 2020 48

Preliminaries • List<..> is is an an int interface in in java.ut util il • LinkedList<..> and ArrayList<..> • Implement the interface • Wha hat is is null null? • Variable value • No object referenced 2/5/2020 CompSci 201, Spring 2020 49

Benchmark: Empirical Analysis https://c //course sework.cs.d s.duke.e .edu/2 /201sp spring20/classc sscode/ • • In c class ss ListSplic icer er, m metho hod removeFirst • List<String> parameter • ArrayList<String> argument passed • LinkedList<String> argument passed • Only ly c call L ll List<..> ..> i interface ace m met etho hods • At runtime, call the actual object method • LinkedList.add v ArrayList.add 2/5/2020 CompSci 201, Spring 2020 50

list.remove(0) • Wha hat is is “faster”? L Link inkedList o or Arra rrayList 2/5/2020 CompSci 201, Spring 2020 51

list.remove(0) – where called 2/5/2020 CompSci 201, Spring 2020 52