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

2/5/2020 CompSci 201, Spring 2020 1

Susan Rodger February 5, 2020

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

• tation
• 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
• bject

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

2/5/2020 CompSci 201, Spring 2020 10

String hashCode ayay 3009136 ayBZ 3009136 bZay 3009136 bZbZ 3009136 String hashCode buzzards

• 931102253

righto

• 931102253

snitz 109586548 unprecludible 109586548

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

2/5/2020 CompSci 201, Spring 2020 14

https://www.youtube.com/watch?v=HeTShE2PiQI

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

2/5/2020 CompSci 201, Spring 2020 15

String hashCode ayay 3009136 ayBZ 3009136 bZay 3009136 bZbZ 3009136 String hashCode buzzards

• 931102253

righto

• 931102253

snitz 109586548 unprecludible 109586548

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 int[] allcha lchars = new i int[256] 256]; int int nex nextLet is is 'a' String ans answer = = ""; ""; messa ssage i is feed ans answer is is ch ch is is

2/5/2020 CompSci 201, Spring 2020 19

96 97 98 99 100 ‘a’ 101 102 103 ‘f’ ‘e’ ‘d’ ‘b’ ‘c’ ‘g’

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

• ne 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

• f 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)

2/5/2020 CompSci 201, Spring 2020 51

• Wha

hat is is “faster”? L Link inkedList o

• r Arra

rrayList

list.remove(0) – where called

2/5/2020 CompSci 201, Spring 2020 52

list.remove(0)

2/5/2020 CompSci 201, Spring 2020 53

• Wha

hat is is “faster”? LinkedLis ist or

• r Arra

rrayList

y = -4E-05x + 0.0009 y = 0.0064x2 - 0.0156x + 0.0238 R² = 0.9984 0.2 0.4 0.6 0.8 1 1.2 1.4

RemoveFirst

linked array Linear (linked)

• Poly. (array)

Access all elements randomly

2/5/2020 CompSci 201, Spring 2020 54

• Wha

hat is is “faster”? L Link inkedList o

• r Arra

rrayList

Access all elements randomly

2/5/2020 CompSci 201, Spring 2020 55

• Wha

hat is is “faster”? LinkedLis ist or

• r Arra

rrayList

y = 0.1292x2 - 0.7137x + 1.3337 R² = 0.9889 y = 0.0002x + 5E-05 R² = 0.8169 5 10 15 20 25 10000 20000 30000 40000 50000 60000 70000 80000 90000 100000110000120000130000140000150000

Random Access

linked array

• Poly. (linked)

Linear (array)