Class07.docx

Class07.docx

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Class07

Lecture8前15分钟回首页

ThisisComputerProgrammingIfromtheUniversityofWashington,I’mMartinDickey.Andtodaywe’llcontinueourdiscussionoffunctionsinlectureA.AlthoughI’vetitledthelecture“functionparameters”.Parametersareonlyoneofmanyimportanttopicswe’lltalkabouttoday.Inadditiontoparameter,we’llbetalkingaboutreturnvalues,returntypes,returnstatement,localvariables,functionprototypesandheaderfiles.Thereistimesofnewterminologiesinthislectureaswell.Tilltheendofthelecture,I’lldoastandardexamplewithanaturalprogramtracedindetail.Andfollowingthat,IthinktherewillbetimeforacoupleofbonustopicsthatIthinkyoumayfindusefulwhenyousitdowntowriteyourprograms.Let’sgobacktotheexampleweuseextensivelylasttime.Afunctionwhichprintsabinalof2linesit’squiteasimplefunction.Oneoftheadvantagesoffunctions,oneofourprimarymotivations,istomakeprogrameasiertochangeincasethedesignerortheclientfindthattheyneedtodothat.Let’sconsideroneofthesechangesandseehowitmightaffectusandhowwemighthandleit.Supposetheclientnowdecides2rowsarenotenough,thereshouldbe5rowsofasteriskswhentheprogramstartsandstops.Butalltheothertimes,thereshouldbeonlytwo.Howcouldwesolvethisproblem?

Well,usingwhatweknownow,youcouldsitdownandwriteasecondfunction,maybeyoucallitprintBinalr2thatfunctioncouldprint5rowsofasterisks.Andthenbackinthemainprogramyouwouldcallitatthebeginningoftheprogram.Callitattheendandalltheotherspotyouwillstillhavecallstotheoriginalfunctions.Youcoulddothat,butcomputersciencetendencyarenewtoils（2:

12）fortryingtogeneralizetofindtheabstractionandcoversmorethanonecase.Couldwegeneralizehere?

Couldwesomehowtakeasinglefunction,printBinalLinesandmakeitdobothoftheoperationthatweneed?

Makeitprint5linessometimesandmakeit2linesothertimes.Inthatsense,that’swhatparametersareallabout.Andatagreatdealoflecturewe’llbedevotedtoshowinghowwecandothat.Someoftheconsequencesandsomeoftheprogrammingdetailsinvolved.Let’sstatetheproblemandslideitinadifferentway.Ouroriginalspecificationwastoprint2rowsofasterisks.We’dliketomodifythatsothatitcouldeitherprint2rowsor5rows.Amoregeneralwayofstatingthiswouldbe:

let’sprintnrows.Well,nissomequantitythatwedon’tknowaheadoftime,itmightbe2,itmightbe5,maybeitcouldbesomethingelse,OK.Atthemomentthefunctioniscalled,thatvaluewouldbeknownsothefunctionwouldknowwhattodo.Sonisn’tnumber,it’sinformationofthefunctionneedstoknowandthat’sacrucial（至关重要的）concept.Weneedawayofgettinginformationintothefunction.Let’ssupposethereisawaytodothat,Iwanttoshowyouthecodeforitrightnow,butlet’ssupposewecoulddothat.Howwouldthatchangeourmainprogram?

HereishowitlookswhenIcalledthenewversionofprintBianlLines.WhenIwanttoprint5lines,Iwouldputthatrequestedvalueinparenthesesfollowingthefunctionname.Nowpauseaminuteandtrytorememberwhatwasthereoriginally.WhatdidIusetohaveinsteadofaparenthesis5?

Ifyourememberorgobackandlook.Itusedtobeaparenthesiswithnothinginside,justemptyparenthesis,OK.Theparenthesisistheretoholdtheinformationthat’sbeingsenttothefunctionandwehaveanameforthatkindofinformation,it’scalledaparameter.Thisisterminologywe’llma（4:

39）useextensivelyintherestofthecourse.Theparameter,informationthatistobesenttothefunction.Howwouldwewritethisfunction?

I’mnotgoingtogiveyoufullcodenow.It’snotreallyrelevant（有关的）whatthefunctionbodyisdoingandwe’llseeotherexampleslateroninthislecture.Thecrucialthingisthebeginningofthefunction,howthefunctionknowswhetherinformationtolookfor.Andthisisdoneinawaythatitiswhetherparallel（这里是函数声明的意思）tothefunctioncall.WestillhavethefunctionnameprintBianlLines.Butnowinparenthesiswehavesomethingthatlookslikeadeclaration.Itsays“intn”.Thinkbacktowhatwastherebefore.Itwasnotanemptyparenthesisbefore,itwasamagickeyword“void”（魔：

程序中经常出现的）.I'vebrieflysaidthatthevoidmeantwerenotsendinginformationandtherearenoparameters.Nowthereisaparameter.Wewantthefunctiontoknowavalue.Sowhatwedoiswedeclare,whatwecallaformalparameter,we'llgiveitaname,we'llgiveitatype.ItisexactlythesameformatIusewhenIdeclarevariableintheprogramanditdoesdeclare"n".Itdeclareswhoselocationsandwhosenamesandwhosetypeisint.Thespecialaremagicthingsaboutthis"n"iswhenthefunctionstartstoexecute,itwillcontainan"n",thevaluesenttothefun