(us)acm working group on algorithmic transparency and...

(US)ACMWorkingGrouponAlgorithmicTransparencyandAccountability

“...[discussing]thegrowingimpactofalgorithmicdecision-makingonoursociety...”

or,inotherwords,whyyoushouldbeconcernedaboutcomputeralgorithmsdeciding...• whetheryourresumeisselectedortrashed• whatwebsitesshouldbedisplayedinyoursearchresults• howmuchyoushouldpayforcarinsurance• howlikelyyouaretocommitacrime• ...etc

readthestatementhttp://www.acm.org/public-policy/algorithmic-panel

watchthepanelhttps://www.youtube.com/watch?v=DDW-nM8idgg&feature=youtu.be

Today’sbigideas

• Aniterator isanobjectthatreturnsoneelementatatime.Itcanreturnelementsfromadatastructureoranysequencewewant.

• BymakingIteratoraninterface,wecanwritecodethatusesaniteratorbutdoesn’tneedtoknowwhatdatastructuretheelementscamefrom

• Itishelpfultoidentifyaninvariantwhenimplementinganiterator.

• Wecanusegenerictypestowriteadatastructureonceandhaveitworkforanyelementtype

whyyoushouldcareaboutIterators

class ListPrinter {public static void print(LinkedList ls) {

for (int i=0; i<ls.size(); i++) {System.out.print(ls.get(i));System.out.print(",");

}}

}

IfNisthesizeofthelinkedlist,givethetightestupperboundonrunningtimeinbig-Oh.a) 𝑂(1)b) 𝑂(𝑁)c) 𝑂(𝑁𝑙𝑜𝑔𝑁)d) 𝑂(𝑁-)e) 𝑂(𝑁/)

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Iterator

iterator:Anobjectthatprovideselementsoneatatimeuntiltherearenomore

public interface Iterator {// return the next element or throw error if there is no next elementint next();

// return true if the Iterator has more elementsboolean hasNext();

}

searchenginesendsme15searchresultsthenIscrolltothebottomofthepage...

...andgetthenext15...

CanthinkofthesearchresultsbeingprovidedbyanIterator.Eachcalltonext()returns15moreresultsfromthelistofthousandsofresults

Example:Listiterator

listis[1,5,7,8,14]

Iteratoriter =list.iterator()iter.hasNext()=>trueiter.next()=>1iter.hasNext()=>trueiter.next()=>5iter.hasNext()=>trueiter.next()=>7iter.hasNext()=>trueiter.next()=>8iter.hasNext()=>trueiter.next()=>14iter.hasNext()=>false

Peerinstruction

listis[1,5,8,14]

SupposethatEvens,isanIteratorthatreturnstheevendataofthegivenlistinorder.Whatarethereturnvaluesofthelast5linesofcode?

a) true,8,true,14,falseb) 1,5,8,14,nullc) true,8,true,14,ERRORd) false,null,false,null,truee) false,1,false,5,truef) true,1,true,5,trueg) 8,14,null,null,nullh) 8,14,ERROR

Iteartor iter =newEvens(list);iter.hasNext()iter.next()iter.hasNext()iter.next()iter.hasNext()

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Iteratorsprovideelement-at-a-timeaccessto“streams”Example:Thestreamofprimenumbers

primes=[2,3,5,7,11,13,17,...]

infinitesequence,sowecannotstoreitinadatastructure,butwecanpretend...

justcalculatethenextprimewhensomeoneasksforit

Whatdoesthiscodeprint?public class Streams2 {

public static class PeerInstruction implements Iterator<Integer> {private int z;private int y;public PeerInstruction() {

z = 0;y = 1;

}public boolean hasNext() {

return true;}public Integer next() {

int t = z;z = y;y = y + t;return t;

}}

public static void main(String[] args) {Iterator<Integer> mystery = new PeerInstruction();while(mystery.hasNext()) {

System.out.print(mystery.next()); System.out.print(“,”);}

}}

a) 1,1,2,3,5,8,13,...b) 0,1,1,2,3,5c) 0,1,2,3,4,5,6,...d) 0,1,1,2,3,5,8,13,...e) 1,2,3,4,5,6

...meansneverends

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whyyoushouldcareaboutIteratorsPartII

Nisthesizeofthelinkedlist.Assumingareasonableimplementationoftheiterator,whichisthetightestupperboundofrunningtimeofListPrinter.print?a) 𝑂(1)b) 𝑂(𝑁)c) 𝑂(𝑁𝑙𝑜𝑔𝑁)d) 𝑂(𝑁-)e) 𝑂(𝑁/)

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class ListPrinter {public static void print(LinkedList ls) {Iterator iter = ls.iterator();

while (iter.hasNext()) {System.out.print(iter.next());

}}

}

Returnofinvariants

Whatnon-trivialinvariantsarethereinthefollowingcode?

int sum = 0;for (int i=0; i<5; i++) {

sum += i;}

invariant:anexpressionthatisalwaystrue(somethingthatdoesnotvaryisinvariant)

InvariantsaregoingtobeusefulwhenimplementingIterators

ImplementationofaniteratorforLinkedList

class LinkedList {private ListNode header;public LinkedList() {

header = new ListNode(0);}public Iterator iterator() {

return new LinkedListIterator(header.next);}private class LinkedListIterator implements Iterator {

private ListNode current;public ListIterator(ListNode first) {

this.current = first;}public boolean hasNext() {

return this.current != null;}public Object next() {

if (!hasNext()) throw new NoSuchElementException();

int result = (Integer) current.data;current = current.next;return result;

}}

}

WhatistheinvariantforaLinkedListIterator?(shouldbeintermsofcurrent)

class RepeatsIterator implements Iterator {private Iterator fromIter;private int count;private int currentElement;public RepeatsIterator(Iterator from) {

fromIter = from;count = 0;currentElement = 0; // okay because 0 is never returned findNext();

}private void findNext() {

while (count == currentElement && fromIter.hasNext()) {currentElement = (Integer) fromIter.next();count = 0;

}}public boolean hasNext() {

return count < currentElement;}public Object next() {

if (!hasNext()) throw new NoSuchElementException();int result = currentElement;count++; // invariant broken herefindNext(); // findNext() fixes the invariantreturn result;

}} WhatistheinvariantforaRepeatsIterator beforeandafter(butnot

necessarilyduring)eachcalltonext()?(shouldbeintermsoffromIter, count, currentElement)

RepeatsIterator returnsxcopiesofeachelementxforlist[3,1,2]theiteratorwillreturn3,3,3,1,2,2

PeerinstructionGreaterThanIterator willreturnonlytheelementsoflist whosevalueisgreaterthanK

private class GreaterThanIterator implements Iterator<Integer> {private int nextEle; // index of next element to returnprivate ArrayList<Integer> list; // list being iterated overprivate final int K;

}

Whichoftheseoptionswouldbethemostusefulinvariant?

a) list.get(nextEle) >K||nextEle >=list.size()b) list.get(nextEle)<=K&&nextEle <list.size()c) nextEle ==0d) nextEle >Ke) nextEle <list.size()f) nextEle >=list.size() https://b.socrative.com/login/student/

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Buildingpartofadatabasemanagementsystem

Queryprocessor(HW5)

“HowmanyflightsdidSkyWestAirlineshavein2015?”data

(textoratable)

Answer:45001

Thekey:Iteratorsdoalltheworkofprocessingthedata!

HW5

QueryprocessedbyachainofiteratorsHW5

Yourjob:• buildvariousiterators• usethemtoanswersomequeriesonrealdata

TextFileReader("flights.csv")

Filter(airline == "SkyWest" and year == 2015)

Count

while(iter.hasNext()){print(iter.next())

}

next

next

next

SkyWest, 2015, 4:45pmSkyWest, 2010, 12:02pmDelta, 2015, 11:20pm

SkyWest, 2015, 4:45pm

1

Generics:timetostopcasting

// B. compiles but poor code

// C. compiles and good code

// A. does not compile

// 1DogList x = DogLinkedList();x.add(new Dog("pug"));Dog element = x.get(0);CatList y = CatLinkedList();y.add(new Cat("Tabby"));Cat element2 = y.get(0);

// 2List x = LinkedList();x.add(new Dog("pug"));Dog element = x.get(0);List y = LinkedList();y.add(new Cat("tabby"));Cat element2 = y.get(0);

Scrambled:Matchthecodetothecomment// 3List x = LinkedList()x.add(new Dog("pug"));Dog element = (Dog) x.get(0);List y = LinkedList()y.add(new Cat("tabby"));Cat element2 = (Cat) y.get(0);

// 4List<Dog> x = LinkedList<>()x.add(new Dog("pug"));Dog element = x.get(0);List<Cat> y = LinkedList<>()y.add(new Cat("tabby"));Cat element = y.get(0);

(assumeallclassesexist)

Definingaclasses/interfaceswithagenerictype

interface List<T> {T get(int i);T remove(int i);void append(T element);

}

class LinkedList<T> implements List<T> {private ListNode<T> header;

private class ListNode<T> {public T data;public ListNode<T> next;

}T get(int i) { ... }T remove(int i) { ... }void append(T element) { ... }

}

Today’sbigideas

• Aniterator isanobjectthatreturnsoneelementatatime.Itcanreturnelementsfromadatastructureoranysequencewewant.

• BymakingIteratoraninterface,wecanwritecodethatusesaniteratorbutdoesn’tneedtoknowwhatdatastructuretheelementscamefrom

• Itishelpfultoidentifyaninvariantwhenimplementinganiterator.

• Wecanusegenerictypestowriteadatastructureonceandhaveitworkforanyelementtype