Programming MethodologyLecture23.docx

Programming MethodologyLecture23.docx

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ProgrammingMethodologyLecture23

ProgrammingMethodology-Lecture23

Instructor（MehranSahami）:

Sowelcometothe106AlecturejustbeforeThanksgiving.Ithoughtattendancemightnotbespectaculartoday.Peoplearestillfilteringin,butit抯goodtoseesomanyofyouhere,though.I抦suresomeofyouhavealreadyheadedhome.

YoumaybewonderingwhoIam.Iwouldhavethoughtbeforewegotbackmid-quarterevaluationsthatyoustoodachanceofrecognizingmeastheTAofyourclass,butthecommentofmorethanhalfofthepeoplewhorespondedtothequestionofhowisBendoingwas,揑don抰knowBen.I抳eneverinteractedwithBen.SoIassumehe抯doingagreatjob.?

Mostofyoujumpedtothatconclusion.But?

soIsortofchasedatthis,butnotmuch.Irealize,though,thatIwouldenjoytheopportunitytodevelopalittlemoreexposure.AndsoMaronandIdecidedtoswitchrolesfortoday.Infact,he抯makingcopiesaswespeak,andthere抯goingtobeonehandoutthat抯sortofpertinenttowhatyou抣lbedoingfor[inaudible]thenextassignment.Andheshouldbebackanytime,butIcertainlycan抰blamehimforbeing?

runningalittlelatesinceIwassupposedtomakethecopiesbeforeclassmyself.Soanyway,inmycapacityasMaron,Idon抰hopetobeasspontaneousorasfunnyasheis,butIdohopetotalkaboutsomethingthatisalittlebitoffofthesortofchartedpathofCS106A抯curriculum.

Soonewayofdescribingthethingsthatyou抮elearningin106Aisthatyou抮elearninghowtotakethewaysthatyoualreadyknowofdoingthingsthatmaytakeyousometimeandmaybeerror-proneinthedoingofthemandtellacomputerpreciselyhowtodothemsothatyoureapthebenefitsofamachinebeingabletoexecutethoseinstructions.Andthey抣lbefaster.There抣lbefewererrorsaslongasyougettheinstructionsrightinthefirstplace.Andthat抯gottobeanexhilaratingfeeling.It抯gottobeempowering,oratleastitwaswhenIwassittinginyourplaceasafreshman,toknowthat,allofasudden,ifyouhaveagoodidea?

ifyoucancomeupwithaprocedureforsolvingaproblemthatyou抳ealwayshadandtellacomputerhowtodoit,thenthecomputerwillbeabletodothatoverandoverandoveragain.Allright?

Sothat抯106Ainanutshell.Soit抯sortof?

it抯notaperfectmetaphor.Whatdoesitmeantorunbreakoutbyhands?

Idon抰know.

SowhatI?

thedistinctionI抦tryingtomakebetweenwhatyou抳ebeenlearningsofarandwhatyouwilllearn,hopefully,inthislecture?

andcertainlyifyougoontoCS106B?

isthattherearethingsthatcomputerscandothatyoucouldn抰possiblyhavesimulatedbyhand.Computerscanhandlesomuchmoredatathanyoucouldevermanagetosortthroughonyourownmanuallythatit抯worthyou抮elearningabitabouthowtomakethemdothat?

howtothinkaboutinstructingacomputertodosomethingthatyoucouldn抰possiblyhavedone.Sotoday,we抮egoingtotalkabouttwoofthemostimportantkindsofthoseproblemsincomputerscience,andI抦goingtostopjusttalkingatyouandshowyousomeexamplesinasecond.Butthosetwotopicsaresearchingandsorting.Howdoyoufindsomethinginasetofthingsthatyouhave?

Andhow,ifyoucouldimposesomestructureonthatset,wouldyoubothfinditmorequickly,and?

IguessIgotaheadofmyself?

howwouldyougoaboutimposingthesortofstructurethatwouldhelpyoufinditmorequickly?

Allright.SoifI抦seemingtoramble,thatstopsnow.

Sosearchingandsorting?

Isaytheyareimportant,andIhopeyoubelieveme,andyouthinkthatforthatreason,it抯worthtalkingaboutforanentirelecture.Buttheyreallyarejustawayofgettingtotalkaboutsomethingalittledeeper,whichisthisconceptofalgorithmicefficiencythatwehaven抰saidmuchaboutintheclasssofar.Sothat抯thethirdpartofthistwo-partlecture.What抯thedealwithsearching?

Well,searchingdoesn抰quitefitthemoldassomethingyoucouldn抰dobyhand.Youfindthingsallthetime.Chapter12ofthebooklooksattwooperationsofthesearchingandsorting.Thisisprettygeneric.Allright.Sosearchingissimpler.Youcandefineasearchproblem?

sayyouhaveanarrayorsomeothercollectionofthings,andyouhavesomethingyouwanttofind.Thetypicalwaythisisdoneisthatyouwanttofindtheindexintothearray?

wherethatelementwas?

or,inamoregenericcase,youwanttofindoutifthatelementiseveninthearray.Soyouwanttohavesomewayofdeterminingthatitwasn抰actuallyfound.

Andthatmaybeallyoucareabout.Itmaybethecasethat,ifthesearchroutinereturnsanythingotherthannegativeone,youdon抰actuallycarewhatitsvaluewas.Butweadopt?

Iwilladopttheconventionforthislecturethatifyoudon抰findwhatyou抮esearchingfor,thenthemethodthatyouwrotetodothesearchshouldreturnanegativevaluesincethat抯notavalidindexintothearray.Allright.Soifyouhaveasetofthingsthatyou抮etryingtosearchthrough,theeasiestwayofdoingthatisjusttolookatallofthemandseeifeachone,inturn,iswhatyou抮elookingfor.That抯calledalinearsearchbecause,tosortofpre-figurewhatwe抮egoingtotalkaboutattheendofthelecture,thenumberofoperationsyouhavetodotofindtheelementthatyou抮elookingforislinearlyproportionaltothenumberofelementsthatyouhadtobeginwith.Nowitmaynotbeobviousrightnowthatthere抯somethingbetterthatyoucando,andwithanarbitrarysetofdata,thereisnotanythingbetterthatyoucando.ButthisprocedurethatI抳ewrittenuphereisanimplementationofwhatyoualreadycouldhavewritten?

aprocedureforfindinganelementinanarrayofintegers.

Sothere抯nothingverytrickyaboutthis.Ifithadbeenastring,youmighthaveevenavoidedhavingtowritethefunctionandcalleditsomethinglike,揑ndexof?

withacharacter.Andinsideof揑ndexof?

thoughyoudon抰havetowritethecode,itwouldhavehadafourloopthatiteratedovereachofthecharactersofthestring,testedeachoneforqualitywiththekeythatyouwerelookingforandthen,presumingitfoundonethatequaledthatcharacter?

orinthiscase,thatinteger?

itwouldreturntheindexofit.Andifthefourloopcompletes,thenyoumustnothaveexaminedanyelementsthatmatched,andsoyoureturnnegativeone.Okay?

Sohere抯asimulationofhowthatwouldwork,althoughthe?

leavingnothingtotheimaginationhere,thisisprettyeasytosortofenvision.

Sowehavethisarrayofprimes,andithas17init.Sowearegoingtoexpecttofindthatthislinearsearchshouldn抰returnnegativeone.Butthesecondone,we抮elookingfor27?

whichsortoflooksprime,butofcourse,it抯notbecauseit抯ninetimesthree.Allright?

Okay.Sowecalledlinearsearch,andhere抯ournewstatframe,whichhasthelocalvariablesIandtheparametersPandarrayinit.ThisissortofEricRobert抯?

whowrotethetextbooks?

wayofdisplayingtheexecutionmodelofJAVA.Allright?

Sowe抮ejustloopingoverit,andwe抮etestingasIequalszeroandthenoneandthentwoandthenthreeandfour,five,six?

whereshouldwefindit?

Well,wewerelookingforseven?

no.Rightnow,we抮elookingfor27.Sowe抮ejustgonnagotothe?

Ithinktherewasaproblemwiththewaythat?

well,anyway.Theconsolehereprintsoutthat,indeed,wefound17atpositionsix.TheremayhavebeenaproblemconvertingPowerPointslidestoKeynoteslides,soIapologizeforthesortofshakinessoftheseanimations.Butthecontentoftheslidesshouldbeadequate.Cool.

Allright.Sonowwe抮einoursecondcalledlinearsearch,andtheonlythingthat抯changedatthispointfromthebeginningofthefirstcalledwasthatthekeyisdifferent.SoIisstillgonnastartatzero,andit抯stillgonnaiterateoverallofthepositionsofthearray.Butwe抮egonnagothroughtheentirearray,whichmeansgoingallthewayuptotenwhereIisnolongerlessthanten.Andwedidn抰find27anywhere.SoIhopethisisallfairlyclear.Butthesearetheresultsthatyouwouldhaveexpected.Sowefoundthepositionof17,andthenwelookedfor27butdidn抰finditandreturnednegativeone.Cool.

Howmanypeoplethinktheycouldhavewrittenthatintheirsleep?

Thatthatwastoomuchtimetospendonsuchasimpleidea?

Allright.Well,itgetsmoreinterestingfromhereonout,but?

sotalktome.Whatistheproblemwithlinearsearch?

Student:

Ittakesalotoftime.

Instructor（MehranSahami）:

Ittakesalotoftime,right?

Ittakesasmuchtimeasyouhaveelements.SoifIaskedaquestion,whichwas?

ifIknewtheSUIDnumberofoneofyouintheroom,andIasked,揑sthisyourSUIDnumber?

Andyousay,揘o.?

AndIaskyou,揑s05179164yourSUIDnumber?

?

Isityours?

Bekind.Tellmeit抯yours.

Student:

[Inaudible].

Instructor（MehranSahami）:

It抯not.Soitactuallyturnsoutthatit抯mine,soIwouldhavehadtosearchtheentireroombeforeIfinallygotbacktomyselfifIdidn抰makethatshortcut.Okay.

Soanotherillustrationofwhatisreallyaverysimpleidea?

thatifyou抮eforcedtolookatalloftheelementsthatyouaresearchingamong,thenthat抯badinandofitselfifyoucoulddobetter.Sowe抣lseeifwecandobetter,buthereisaratherlargerdataset?

286areacodes?

thethree-digitnumbersthatcomebeforetherestofthesevendigitsinyourphonenumbers.

Butwe抮etryingtofind650,whichisthiscounty抯areacode.Sohere抯alargerarray.Whatwouldhappenifweweresearchinginthis?

Well,itdepends.Ifwehaveafastcomputer,itmaynotmatter.Theseare?

there抯onlyabout300numbershere,anddoingsomething300timesissomethingthatcomputersarereasonablygoodat.

ButIthoughtI抎puttogetherthissortofcompellingexampleofwhythiscanbecomeabad