0321537904_ppt73679-0321537904_pptSavitch_ch_06

Copyright2008PearsonAddison-Wesley.Allrightsreserved.,Chapter6,I/OStreamsasanIntroductiontoObjectsandClasses,Slide6-3,Overview,6.1StreamsandBasicFileI/O6.2ToolsforStreamI/O6.3CharacterI/O,Copyright2008PearsonAddison-Wesley.Allrightsreserved.,6.1,StreamsandBasicFileI/O,Slide6-5,I/OStreams,I/OreferstoprograminputandoutputInputisdeliveredtoyourprogramviaastreamobjectInputcanbefromThekeyboardAfileOutputisdeliveredtotheoutputdeviceviaastreamobjectOutputcanbetoThescreenAfile,Slide6-6,Objects,ObjectsarespecialvariablesthatHavetheirownspecial-purposefunctionsSetC+apartfromearlierprogramminglanguages,Slide6-7,StreamsandBasicFileI/O,FilesforI/OarethesametypeoffilesusedtostoreprogramsAstreamisaflowofdata.Inputstream:DataflowsintotheprogramIfinputstreamflowsfromkeyboard,theprogramwillacceptdatafromthekeyboardIfinputstreamflowsfromafile,theprogramwillacceptdatafromthefileOutputstream:DataflowsoutoftheprogramTothescreenToafile,Slide6-8,cinAndcoutStreams,cinInputstreamconnectedtothekeyboardcoutOutputstreamconnectedtothescreencinandcoutdefinedintheiostreamlibraryUseincludedirective:#includeYoucandeclareyourownstreamstousewithfiles.,Slide6-9,WhyUseFiles?,Filesallowyoutostoredatapermanently!DataoutputtoafilelastsaftertheprogramendsAninputfilecanbeusedoverandoverNotypingofdataagainandagainfortestingCreateadatafileorreadanoutputfileatyourconvenienceFilesallowyoutodealwithlargerdatasets,Slide6-10,FileI/O,ReadingfromafileTakinginputfromafileDonefrombeginningtotheend(fornow)Nobackinguptoreadsomethingagain(OKtostartover)JustasdonefromthekeyboardWritingtoafileSendingoutputtoafileDonefrombeginningtoend(fornow)Nobackinguptowritesomethingagain(OKtostartover)Justasdonetothescreen,Slide6-11,StreamVariables,Likeothervariables,astreamvariableMustbedeclaredbeforeitcanbeusedMustbeinitializedbeforeitcontainsvaliddataInitializingastreammeansconnectingittoafileThevalueofthestreamvariablecanbethoughtofasthefileitisconnectedtoCanhaveitsvaluechangedChangingastreamvaluemeansdisconnectingfromonefileandconnectingtoanother,Slide6-12,StreamsandAssignment,AstreamisaspecialkindofvariablecalledanobjectObjectscanusespecialfunctionstocompletetasksStreamsusespecialfunctionsinsteadoftheassignmentoperatortochangevalues,Slide6-13,DeclaringAnInput-fileStreamVariable,Input-filestreamsareoftypeifstreamTypeifstreamisdefinedinthefstreamlibraryYoumustusetheincludeandusingdirectives#includeusingnamespacestd;Declareaninput-filestreamvariableusingifstreamin_stream;,Slide6-14,DeclaringAnOutput-fileStreamVariable,Ouput-filestreamsofaretypeofstreamTypeofstreamisdefinedinthefstreamlibraryYoumustusetheseincludeandusingdirectives#includeusingnamespacestd;Declareaninput-filestreamvariableusingofstreamout_stream;,Slide6-15,Onceastreamvariableisdeclared,connectittoafileConnectingastreamtoafileisopeningthefileUsetheopenfunctionofthestreamobjectin_stream.open(infile.dat);,ConnectingToAFile,Slide6-16,UsingTheInputStream,Onceconnectedtoafile,theinput-streamvariablecanbeusedtoproduceinputjustasyouwouldusecinwiththeextractionoperatorExample:intone_number,another_number;in_streamone_numberanother_number;,Slide6-17,UsingTheOutputStream,Anoutput-streamworkssimilarlytotheinput-streamofstreamout_stream;out_stream.open(outfile.dat);out_streamonenumber=one_numberanothernumber=another_number;,Slide6-18,ExternalFileNames,AnExternalFileNameIsthenameforafilethattheoperatingsystemusesinfile.datandoutfile.datusedinthepreviousexamplesIsthereal,on-the-disk,nameforafileNeedstomatchthenamingconventionsonyoursystemUsuallyonlyusedinthestreamsopenstatementOnceopen,referredtousingthenameofthestreamconnectedtoit.,Afterusingafile,itshouldbeclosedThisdisconnectsthestreamfromthefileClosefilestoreducethechanceofafilebeingcorruptediftheprogramterminatesabnormallyItisimportanttocloseanoutputfileifyourprogramlaterneedstoreadinputfromtheoutputfileThesystemwillautomaticallyclosefilesifyouforgetaslongasyourprogramendsnormally,Slide6-19,Display6.1,ClosingaFile,Slide6-20,Objects,Anobjectisavariablethathasfunctionsanddataassociatedwithitin_streamandout_streameachhaveafunctionnamedopenassociatedwiththemin_streamandout_streamusedifferentversionsofafunctionnamedopenOneversionofopenisforinputfilesAdifferentversionofopenisforoutputfiles,Slide6-21,MemberFunctions,AmemberfunctionisafunctionassociatedwithanobjectTheopenfunctionisamemberfunctionofin_streaminthepreviousexamplesAdifferentopenfunctionisamemberfunctionofout_streaminthepreviousexamples,Slide6-22,ObjectsandMemberFunctionNames,ObjectsofdifferenttypeshavedifferentmemberfunctionsSomeofthesememberfunctionsmighthavethesamenameDifferentobjectsofthesametypehavethesamememberfunctions,Slide6-23,Classes,Atypewhosevariablesareobjects,isaclassifstreamisthetypeofthein_streamvariable(object)ifstreamisaclassTheclassofanobjectdeterminesitsmemberfunctionsExample:ifstreamin_stream1,in_stream2;in_stream1.openandin_stream2.openarethesamefunctionbutmighthavedifferentarguments,Slide6-24,ClassMemberFunctions,MemberfunctionsofanobjectarethememberfunctionsofitsclassTheclassdeterminesthememberfunctionsoftheobjectTheclassifstreamhasanopenfunctionEveryvariable(object)declaredoftypeifstreamhasthatopenfunction,Slide6-25,CallingaMemberFunction,CallingamemberfunctionrequiresspecifyingtheobjectcontainingthefunctionThecallingobjectisseparatedfromthememberfunctionbythedotoperatorExample:in_stream.open(infile.dat);,Slide6-26,MemberFunctionCallingSyntax,Syntaxforcallingamemberfunction:Calling_object.Member_Function_Name(Argument_list);,Slide6-27,ErrorsOnOpeningFiles,OpeningafilecouldfailforseveralreasonsCommonreasonsforopentofailincludeThefilemightnotexistThenamemightbetypedincorrectlyMaybenoerrormessageifthecalltoopenfailsProgramexecutioncontinues!,Slide6-28,CatchingStreamErrors,Memberfunctionfail,canbeusedtotestthesuccessofastreamoperationfailreturnsabooleantype(trueorfalse)failreturnstrueifthestreamoperationfailed,Slide6-29,HaltingExecution,Whenastreamopenfunctionfails,itisgenerallybesttostoptheprogramThefunctionexit,haltsaprogramexitreturnsitsargumenttotheoperatingsystemexitcausesprogramexecutiontostopexitisNOTamemberfunctionExitrequirestheincludeandusingdirectives#includeusingnamespacestd;,Immediatelyfollowingthecalltoopen,checkthattheoperationwassuccessful:in_stream.open(stuff.dat);if(in_stream.fail()coutthe_number;Theinputfilemustcontainexactlythedataexpected,OutputexamplessofarcreatenewfilesIftheoutputfilealreadycontainsdata,thatdataislostToappendnewoutputtotheendanexistingfileusetheconstantios:appdefinedintheiostreamlibrary:outStream.open(important.txt,ios:app);Ifthefiledoesnotexist,anewfilewillbecreated,Slide6-32,Display6.3,AppendingData(optional),Slide6-33,FileNamesasInput(optional),ProgramuserscanenterthenameofafiletouseforinputorforoutputProgrammustuseavariablethatcanholdmultiplecharactersAsequenceofcharactersiscalledastringDeclaringavariabletoholdastringofcharacters:charfile_name16;file_nameisthenameofavariableBracketsenclosethemaximumnumberofcharacters+1Thevariablefile_namecontainsupto15characters,charfile_name16;coutfile_name;ifstreamin_stream;in_stream.open(file_name);if(in_stream.fail()coutInputfileopeningfailed.n;exit(1);,Slide6-34,Display6.4(1),Display6.4(2),UsingACharacterString,Slide6-35,Section6.1Conclusion,CanyouWriteaprogramthatusesastreamcalledfinwhichwillbeconnectedtoaninputfileandastreamcalledfoutwhichwillbeconnectedtoanoutputfile?Howdoyoudeclarefinandfout?Whatincludedirective,ifany,doyouneetoplaceinyourprogramfile?Nameatleastthreememberfunctionsofaniostreamobjectandgiveexamplesofusageofeach?,Copyright2008PearsonAddison-Wesley.Allrightsreserved.,6.2,ToolsforStreamsI/O,Slide6-37,ToolsforStreamI/O,TocontroltheformatoftheprogramsoutputWeusecommandsthatdeterminesuchdetailsas:ThespacesbetweenitemsThenumberofdigitsafteradecimalpointThenumericstyle:scientificnotationforfixedpointShowingdigitsafteradecimalpointeveniftheyarezeroesShowingplussignsinfrontofpositivenumbersLeftorrightjustifyingnumbersinagivenspace,Slide6-38,FormattingOutputtoFiles,Formatoutputtothescreenwith:cout.setf(ios:fixed);cout.setf(ios:showpoint);cout.precision(2);Formatoutputtoafileusingtheout-filestreamnamedout_streamwith:out_stream.setf(ios:fixed);out_stream.setf(ios:showpoint);out_stream.precision(2);,Slide6-39,out_stream.precision(2);,precisionisamemberfunctionofoutputstreamsAfterout_stream.precision(2);Outputofnumberswithdecimalpointswillshowatotalof2significantdigits23.2.2e72.26.9e-10.00069ORwillshow2digitsafterthedecimalpoint23.562.26e72.210.690.69e-4Callstoprecisionapplyonlytothestreamnamedinthecall,Slide6-40,setf(ios:fixed);,setfisamemberfunctionofoutputstreamssetfisanabbreviationforsetflagsAflagisaninstructiontodooneoftwooptionsios:fixedisaflagAfterout_stream.setf(ios:fixed);AllfurtheroutputoffloatingpointnumbersWillbewritteninfixed-pointnotation,thewaywenormallyexpecttoseenumbersCallstosetfapplyonlytothestreamnamedinthecall,Afterout_stream.setf(ios:showpoint);OutputoffloatingpointnumbersWillshowthedecimalpointevenifalldigitsafterthedecimalpointarezeroes,Slide6-41,Display6.5,setf(ios:showpoint);,Slide6-42,(ios:right),(ios:left),CreatingSpaceinOutput,ThewidthfunctionspecifiesthenumberofspacesforthenextitemAppliesonlytothenextitemofoutputExample:Toprintthedigit7infourspacesuseout_stream.width(4);out_stream7endl;Threeofthespaceswillbeblank,Slide6-43,NotEnoughWidth?,Whatiftheargumentforwidthistoosmall?Suchasspecifyingcout.width(3);whenthevaluetoprintis3456.45TheentireitemisalwaysoutputIftoofewspacesarespecified,asmanymorespacesasneededareused,Slide6-44,UnsettingFlags,Anyflagthatisset,maybeunsetUsetheunsetffunctionExample:cout.unsetf(ios:showpos);causestheprogramtostopprintingplussignsonpositivenumbers,Slide6-45,Manipulators,AmanipulatorisafunctioncalledinanontraditionalwayManipulatorsinturncallmemberfunctionsManipulatorsmayormaynothaveargumentsUsedaftertheinsertionoperator()asifthemanipulatorfunctioncallisanoutputitem,Slide6-46,TwoSpaces,FourSpaces,ThesetwManipulator,setwdoesthesametaskasthememberfunctionwidthsetwcallsthewidthfunctiontosetspacesforoutputExample:coutStartsetw(4)10setw(4)setw(6)30;produces:Start102030,Slide6-47,ThesetprecisionManipulator,setprecisiondoesthesametaskasthememberfunctionprecisionExample:cout.setf(ios:fixed);cout.setf(ios:showpoint);cout$setprecision(2)10.3endl$20.5next)Readsavaluefromin_streamandstoresitinnextTrueifavaluecanbereadandstoredinnextFalseifthereisnotavaluetoberead(theendofthefile),Slide6-51,EndofFileExample,Tocalculatetheaverageofthenumbersinafiledoublenext,sum=0;intcount=0;while(in_streamnext)sum=sum+next;count+;doubleaverage=sum/count;,Slide6-52,StreamArgumentsandNamespaces,UsingdirectiveshavebeenlocaltofunctiondefinitionsintheexamplessofarWhenparametertypenamesareinanamespaceAusingdirectivemustbeoutsidethefunctionsoC+willunderstandtheparametertypenamessuchasifstreamEasysolutionistoplacetheusingdirectiveatthebeginningofthefileManyexpertsdonotapproveasthisdoesnotallowusingmultiplenamespaceswithnamesincommon,TheprograminDisplay6.6Takesinputfromrawdata.datWritesoutputtothescreenandtoneat.datFormattinginstructionsareusedtocreateaneaterlayoutNumbersarewrittenoneperlineinafieldwidthof12Eachnumberiswrittenwith5digitsafterthedecimalpointEachnumberiswrittenwithaplusorminussignUsesfunctionmake_neatthathasformalparametersfortheinput-filestreamandoutput-filestream,Slide6-53,Display6.6(1),Display6.6(2),Display6.6(3),ProgramExample,Slide6-54,Section6.2Conclusion,CanyouShowtheoutputproducedwhenthefollowinglineisexecuted?cout*setw(3)12345)whichperformssomeautomaticworkDoesnotskipblanks,Slide6-59,Usingget,Theselinesusegettoreadacharacterandstoreitinthevariablenext_symbolcharnext_symbol;cin.get(next_symbol);AnycharacterwillbereadwiththesestatementsBlankspacestoo!ntoo!(Thenewlinecharacter),Slide6-60,getSyntax,input_stream.get(char_variable);Examples:charnext_symbol;cin.get(next_symbol);ifstreamin_stream;in_stream.open(infile.dat);in_stream.get(next_symbol);,Slide6-61,MoreAboutget,Giventhiscode:charc1,c2,c3;cin.get(c1);cin.get(c2);cin.get(c3);andthisinput:ABCDc1=Ac2=Bc3=ncinc1c2c3;wouldplaceCinc3(theoperatorskipsthenewlinecharacter),Slide6-62,TheEndofTheLine,ToreadandechoalineofinputLookfornattheendoftheinputline:coutEnteralineofinputandIwillechoit.n;charsymbol;docin.get(symbol);coutandcin.get,Besuretodealwiththenthatendseachinputlineifusingcinandcin.getcinreadsuptothenThenremainsintheinputstreamUsingcin.getnextwillreadthenThenew_linefunctionfromDisplay6.7canbeusedtoclearthen,Slide6-73,TheDialogue:Enteranumber:21Nowenteraletter:A,TheResult:number=21symbol=n,nExample,TheCode:coutnumber;coutsymbol;,Slide6-75,AnothernFix,coutnumber;new_line();/FromDisplay6.7coutNowenteraletter:n;charsymbol;cin.get(symbol);,Slide6-76,DetectingtheEndofaFile,MemberfunctioneofdetectstheendofafileMemberfunctionofeveryinput-filestreameofstandsforendoffileeofreturnsabooleanvalueTruewhentheendofthefilehasbeenreachedFalsewhenthereismoredatatoreadNormallyusedtodeterminewhenweareNOTattheendofthefileExample:if(!in_stream.eof(),Slide6-77,Usingeof,Thisloopreadseachcharacter,andwritesittothescreenin_stream.get(next);while(!in_stream.eof()coutnext)while(!in_stream.eof()Whichshouldbeused?Ingeneral,useeofwheninputistreatedastextandusingamemberfunctiongettoreadinputIngeneral,usetheextractionoperatormethodwhenprocessingnumericdata,TheprogramofDisplay6.8Readseverycharacteroffilecad.datandcopiesittofilecplusad.datexceptthateveryCischangedtoC+incplusad.datPreserveslinebreaksincad.datgetisusedforinputastheextractionoperatorwouldskiplinebreaksgetisusedtopreservespacesaswellUseseoftotestforendoffile,Slide6-80,Display