Programming MethodologyLecture15Word文档下载推荐.docx

Programming MethodologyLecture15Word文档下载推荐.docx

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thisistheturningpointofthequarter.Thisislikethepivotofthequarter.Aftertoday,it抯alldownhill,becausewe抳egonethroughjustasmanydaysaswehaveleft.Fairlyexciting.Asamatteroffact,theyactuallyhavefewerdaysleft,becausewehavelikethelastclassisnothappeningandstuff.

Butafewannouncementsbeforewedelveintothingstoday.Thehandoutsfromlasttime,ifyoudidn抰getthehandoutsfromlasttime,namely,especiallythepracticemidtermandsolutionstothepracticemidtermaswellasassignmentNo.4,ifyoudidn抰getthose,they抮eavailableinthebacktoday.Ifyoualreadygotthem,youdon抰needtopickupadditionalcopies.There抯noadditionalhandoutsfortoday,buttherearejustcopiesoftheonesfromlastweek.

So,again,withthepracticemidterm,Iwouldveryhighlyencourageyoutotakeitand,ifyouwant,also,forthefulleffect,actuallytimeyourselfdoingitsoyougetsomenotionofwhat抯takingyoulongerorwhat抯takinglesstime.Thatwillgiveyouadiagnosticofthekindofthingsyouneedtobrushupon,so?

andit抣lalsogiveyouachancetoseewhatwe抮ereallykindofexpectingforthemidterm,andtherewasabunchofstuffontherewhereitexplainsthatthemidtermisopenbookandopennote,butclosedcomputer,andalltheguidelinesforthemidtermareallexplainedonthefirstpageofthathandout.

Sothemidtermiscomingup.It抯nextweek,justafewdaysaway.Tuesday,October30th,7:

00p.m.to8:

30p.m.inKresgeAuditorium.Ifyoudon抰knowwhereKresgeAuditoriumis,findoutwhereKresgeAuditoriumis.It抯yourfriend,it抯big,it抯bad,it抯Kresge.It抯justahugeauditorium.Ifyou抳eneverbeentherebefore,it抯kindofcavernous.Yougointhereandyougo,myGod,Ididn抰knowtherewasanauditoriumthisbiginStanford,butthereis,andit抯oursforthatshortperiodoftime.

Ifyouhaveamidtermconflictandyoualreadysentmeemail?

ifyouhaven抰alreadysentmeemail,youcangoaheadandtry,butIcan抰accommodateanymorerequests.Icanhopefullyjusttellyouwhenthealternatemidtermis,butyouwillgetanemailfrommethisweekendlettingyouknowwhenandwherethealternatemidtermis,soreadyouremailthisweekend.It抣lprobablycomesometimeintheweehoursofthenighteitheronSaturdayorSunday,whichwouldbetheweekend,strangelyenough.Sowatchyouremailforthealternatemidterm.

Ifyou抮eanSAPDstudent,SAPDstudentshavetwooptions.Ifyou抮ealocalSAPDstudent,youcancomeinfortheexam,sojustcomeondown,7:

30p.m.,KresgeAuditorium,October30th.Takeitalongwitheveryoneelse,introduceyourself,comeby,sayhi.IalwaysliketomeettheSAPDstudents.

Ifyou抮eremote,becauseIknowsomeofyouaresaying,notinthestateofCalifornia,anditwouldkindofbeastretchtoaskyoutoflyherejusttotakethemidtermexam,youmaytaketheexamremotelyatyoursite.Ifyouplanontakingtheexamremotelyatyoursite,youneedtosendmeemail,andinthatemailyouneedtoletmeknowyournameaswellasyoursiteadministrator.Findoutwhoyoursiteadministratorisifyoudonotknow,becauseyoursiteadministratorwillbeadministeringtheexamtoyou,andIneedtoknowtheiremailaddress.

Sosendmeyournameandemail,whichI抣lgetwhenyousendmeemail,butyoursiteadministrator抯nameandemailsoIcanemailthemtheexamandtheycanadministerittoyou.Andifyou抮ealocalstudent,goaheadandsendmeemail,andjustsayyou抮ecominginlocally,justsoIknowthatyou抮ecominginlocallyandthatwayIcankeeptrackofeveryone.

Butextremelyimportantformoststudents,ifyou抮earemotestudentandyoudon抰sendmeemail,soIhavenowayofcontactingyoursiteadministrator,Ican抰giveyouthemidterm,andthat抯realbadtimesforjustmorereasonsthanyoucanshakeastickat,butyou抮ecertainlywelcometotry.

Andlastbutnotleast,sameannouncementaslasttime.Thereisasyllabuschange,averyminorsyllabuschange,basedonjustmovingthediscussionofarraysupbyoneday.Soifyou抮esortofreadingalonginthebookandyouwannaknowwhattoreadforMonday,Chapter11onarraysiswhatwe抣lbecoveringonMonday,andifyou抮enotfollowingalonginthebook,youshouldbefollowingalonginthebook,butnowyouknow.

Allright,sowiththatsaid,Iwannadojustatinylittlebitofwrapuponwhatwetalkedabout,memoryandthiswholenotionofpointerslasttime,andgointoournextgreattopic.Allright?

Sotimeforournextgreattopic.

Wellweneedtodoalittlebitofpointersfirst.Sothefirstexampleissomethingthatwetalkedaboutverybrieflylasttime,butIwannamakesureeveryone抯sortofonboardwiththis.SoifIhaveourfriend,thepoint,thatwetalkedaboutlasttime,whichisjustaclassthatstorestwovalues,anxandaythatwepassintotheconstructor,whenwedosomethinglikethis?

I抣ldrawitsortofoveronthisboard.

NowI抦gonnadrawitina?

actually,I抣ldrawitoverhere,andI抣ldrawitinawaythatwedon抰needtoworryaboutallthememoryaddressesandoverheadandallthatstuff,becausethatsometimesmakesthediagramsalittlebitugly,andsoI抣lshowyoutheverysimple,stylizedversionofkeepingtrackofthestackandtheheap.

Sooverhereistheheap,overhereisthestack,andwhatwe抮egonnadoissay,hey,whenwedeclarepointponebeforewehavedonethisnew,whatwegetispone,whichisbasicallyjustsomeboxonthestack.Whatdoesitholdrightnow?

Well,untilwedothisline,itholdsnothing.Itholdsnothingthatweknowabout,okay?

Whenwecallthisnew,itsays,hey,heap,getmesomenewmemoryforapoint,andsotheheapkindofgoesoverandgoes,boink,here抯somememoryforapoint,anditcallstheconstructorandsays,hey,constructor,putinthevaluesoneone.Soifsomewhereontheheapwehavethevaluesoneandoneforourlittleprivatexandourprivateysomewhereontheheap,andthevaluethatnewgivesback,thatweassignedtopone,istheaddressofthisthing.

Remember,wedon抰reallycareaboutwhereinmemorythisthing抯actuallystored,andsosometimes,whatweliketodoisjustkeeptrackofaddressesbypointer.Soweknowthatthisisjustsomememoryaddressoverhere,wherethisparticularlyobjectisactuallystoredontheheap.Okay?

Sonow,ifwedopointptwo?

actually,letmenotlet?

putthesemicolononyet.Ifwedopointptwo,whatwegetis,onthestack,wegetsomenewspacesetasideforptwoand,again,itsbeginningvalueisunknown.

Nowwehaveacoupleoptions.Onethingwecoulddoissaynewpoint,andgiveitsomexylocation,inwhichcasewe抣lgetanewpointontheheapandptwowillpointtothatnewpoint.Wecoulddosomethingfunkylikesay,hey,ptwoequalspone.Wellallthat抯happenedthereissaying,takethevaluethat抯inpone,whichisapointerovertothislocationontheheap,andsetptwoequaltothat.SoifthishappenstobeamemorylocationAAAE,thisguygetsAAAE,whichmeans,inoursortofstylisticrepresentation,it抯pointingtothesameplace.Okay?

Arewesortofonboardwiththat?

Ifyou抮eonboardwiththat,nodyourhead.Excellente.

Allright.Sonowifwedoptwodotmove,andwetellittomovebyanamountlikethreecommafour,itsays,well,inordertocallthemovemethod,Ineedtocallthemovemethodusingtheobjectptwo.Wheredoestheobjectptwolive?

Itlooksitupthroughthepointerandsays,here,thisiswhereyoulive.Callthemovemethodwiththisparticularobject.Soapathtothispointertothislocationinmemory,anditsays,movethreefour,andasyousawwithmovelasttime,allmovedoesisaddthosevaluestotherespectivexandy.Sopxbecomesfour,pybecomesfive,andnowthatmovemethodisdone.And,interestinglyenough,whatI抳edoneischangenotonlyptwo,butalsopone,becausetheywerepointingtothesameplaceinmemory.Theywerethesameactuallypoint,andthat抯theimportantthingtokeepinmind.

IfIsetanobjectequaltoanotherobjectlikethis,theyarethesameactualobject.There抯onlyoneofthoseobjectsthatlivessomewhere,andIreallyhavetwothingsthatarepointing,orwhatwereallyliketorefertoasreferring,tothatobject.Sooftentimeswerefertothesethingsasreferences,okay?

NowwhathappensifIcomealongandsay,pointpthree.Igetpthreeoverhereonthestack,right?

Andagain?

I抣lwritethepthreeoverheresoasnottointerferewiththelines,andI抣ldrawthislinealittlebitaroundpthreesoitkindofgoeslikethat.Itsays,pthree,Idon抰wannainterferewithyou,I抦justgonnazigzagaround.

Pthree,whatvaluedoesitstartwith?

Idon抰know.It抯notpointingtoanobjectyet,becauseithasn抰beeninitializedtopointtoanobject.Ithasn抰beeninitializedtopointtoanewobject.AndsoifIcomealongandsay,hey,youknowwhatpthree?

Ijustwannamoveyou.Movetofourcommafive,becausethat抯whereIheartheparty抯at.That抯whereponeandptwoare.Canyoumoveyourselftofourcommafive?

WhathappenswhenIdothis?

Badtimes,that抯whathappens.WhatIget?

wellfirstofall,Igetasocial,becauseawholebunchofpeoplesaidit.ButwhatIgetisthisthing.Whoknowswhereit抯pointing?

Asamatterofafact,thispointercould,oftentimes,besomethingwerefertoasanull,whichmeansit抯notpointingtoanyparticularobject,inwhichcase,whenItrytodothis,Igetwhat抯calledanulldreference.Anyonehappentoseesomethingcalledanull