File_Processing（英文版）.ppt

ProgramminginC Lecture8 CFileProcessing Outline DataandDataFilesFilesandStreamsSequentialAccessFilesRandomAccessFiles DataandDataFiles DataFilescanbecreated updated andprocessedbyCprogramsusedforpermanentstorage 持久存储 oflargeamountsofdataStorageofdatainvariablesandarraysisonlytemporary 临时的 Allsuchdataarelostwhentheprogramterminates TheDataHierarchy 1 Bit smallestdataitemValueof0or1Byte 8bitsUsedtostoreacharacter Decimaldigits letters andspecialsymbolsField groupofcharactersconveyingmeaningExample yourname TheDataHierarchy 2 Record groupofrelatedfieldsRepresentedbyastruct oraclassinC Example Inapayrollsystem arecordforaparticularemployeethatcontainedhis heridentificationnumber possiblerecordkey name address etc File groupofrelatedrecordsExample payrollfileDatabase groupofrelatedfiles TheDataHierarchy 3 TheDataHierarchy 4 Example Fig 11 1 Outline DataandDataFilesFilesandStreamsSequentialAccessFilesRandomAccessFiles FilesandStreams CviewseachfileasasequenceofbytesFileendswiththeend of file EOF markerOr fileendsataspecifiedbyte EOFandfeof aredefinedin Example while c getchar EOF if feof stdin EOFKeyCombinations DifferentfordifferentOS FilesandStreams AStreamiscreatedwhenafileisopenedProvidecommunicationchannel 通信渠道 betweenfilesandprogramsOpeningafilereturnsapointertoaFILEstructureExamplefilepointers stdin standardinput keyboard stdout standardoutput screen stderr standarderror screen FILEstructure TheprogrammerneednotknowthespecificsoftheFILEstructuretousefiles Filedescriptor 文件描述符 Indexintooperatingsystem OS arraycalledtheopenfiletableFileControlBlock FCB Foundineveryarrayelement systemusesittoadministerthefile RefertoFig 11 5onpage437 Outline DataandDataFilesFilesandStreamsSequentialAccessFilesRandomAccessFiles Read WriteFunctions 1 includeintfgetc FILE stream ReadonecharacterfromafileTakeaFILEpointerasanargumentfgetc stdin equivalenttogetchar intfputc intc FILE stream WriteonecharactertoafileTakeaFILEpointerandacharactertobewrittenasargumentsfputc a stdout equivalenttoputchar a Read Writefunctions 2 char fgets char s intn FILE stream Readsalinefromafileintfputs char s FILE stream Writesalinetoafilefscanf fprintfintfscanf FILE stream constchar format intfprintf FILE stream constchar format Fileprocessingequivalentsofscanfandprintf FrameworkforCreateasequentialfile seeFig 11 3onP433fordetails includeintmain FILE cfPtr cfPtr clients datfilepointer if cfPtr fopen clients dat w NULL printf Filecouldnotbeopened n else scanf d s lf CreatingaSequentialAccessFile FILE cfPtr DeclaresaFILEpointercalledcfPtrcfPtr fopen clients dat w FunctionfopenreturnsaFILEpointertofilespecifiedTakestwoarguments thefiletobeopenedandtheopenmodeIfopenfails NULLisreturned CreatingaSequentialAccessFile fprintfUsedtoprinttoafileLikeprintf exceptfirstargumentisaFILEpointer pointertothefileyouwanttoprint feof FILEpointer Returnstrueifend of file EOF indicator nomoredatatoprocess issetforthespecifiedfilefclose FILEpointer ClosesspecifiedfilePerformedautomaticallywhenprogramendsGoodpractice closefilesexplicitly FileOpenModes SequentialAccessFiles Alsocalledtextfile 文本文件 EachbytestoresanASCIIcode representingacharacterFormatofdatainatextfileisnotidenticalwithitsformatstoredinmemory E g 7 14areintsoccupying4byteseachinternally buttheyareofdifferentsize 1and3bytes astextinafile Fig 11 7 readingandprintingasequentialfile includeintmain FILE cfPtr cfPtr clients datfilepointer if cfPtr fopen clients dat r NULL printf Filecouldnotbeopened n else printf 10s 13s s n Account Name Balance fscanf cfPtr d s lf ReadingDatafromaSequentialAccessFile CreateaFILEpointer linkittothefiletoreadcfPtr fopen clients dat r UsefscanftoreadfromthefileLikescanf exceptfirstargumentisaFILEpointerfscanf cfPtr d s f Datareadfrombeginningtoend ReadingDatafromaSequentialAccessFile FilepositionpointerIndicatesnumberofnextbytetoberead writtenNotreallyapointer butanintegervalue specifiesbytelocation Alsocalledbyteoffsetrewind cfPtr Repositionsfilepositionpointertobeginningoffile byte0 ReadingDatafromaSequentialAccessFile FieldscanvaryinsizeDifferentrepresentationinfilesandscreenthaninternalrepresentation1 34 890areallints buthavedifferentsizesinfileCannotbemodifiedwithouttheriskofdestroyingotherdata 修改文件内容时有可能破坏不该破坏的数据 ReadingDatafromaSequentialAccessFile Outline DataandDataFilesFilesandStreamsSequentialAccessFilesRandomAccessFiles RandomAccessFiles Alsocalledbinaryfiles 二进制文件 Formatofdatainabinaryfileisidentical 同样的 withitsformatstoredinmemory bytedoesn tnecessarilyrepresentcharacter groupsofbytesmightrepresentothertypesofdata suchasintegersandfloating pointnumbersRecordsinbinaryfileshaveidenticallength RandomAccessFiles Example 12345 ASCIIcodesof5characters Aninteger RandomAccessFiles AccessindividualrecordswithoutsearchingthroughotherrecordsInstantaccesstorecordsinafile 对文件记录的随机访问 DatacanbeinsertedwithoutdestroyingotherdataDatapreviouslystoredcanbeupdatedordeletedwithoutoverwriting RandomAccessFiles C viewofarandom accessfile Fig 11 11 Creatingarandomlyaccessedfilesequentially withemptystructs includestructclientData intacctNum charlastName 15 charfirstName 10 doublebalance intmain inti structclientDatablankClient 0 0 0 FILE cfPtr if cfPtr fopen credit dat w NULL printf Filecouldnotbeopened n else for i 1 i 100 i fwrite UnformattedFileI OFunctions fwrite Transferbytesfromalocationinmemorytoafilesize tfwrite constvoid buffer size tsize size tnmemb FILE fp fread Transferbytesfromafiletoalocationinmemorysize tfread void buffer size tsize size tnmemb FILE fp UnformattedFileI OFunctions Example fwrite Number anintegervariable number locationtotransferbytesfromsizeof int numberofbytestotransfer1 forarrays numberofelementstotransferInthiscase oneelement ofanarray i e onenumberisbeingtransferredmyPtr Filetotransfertofreadsimilar UnformattedFileI OFunctions fwrite Towriteadatablockwithdesignatedsizetoafilesizeof returnthesizeinbytesoftheobjectinparenthesesTowriteseveralarrayelementsPointertoarrayasfirstargumentNumberofelementstowriteasthirdargument Fig 11 12 Writingtoarandomaccessfile intmain FILE cfPtr structclientDataclient 0 0 0 if cfPtr fopen credit dat r NULL printf Filecouldnotbeopened n else while client acctNum 0 printf Enterlastname firstname balance n fscanf stdin s s lf client lastName client firstName WritingDataRandomlytoaRandomAccessFile intfseek FILE stream longintoffset intwhence Setthefilepositionpointertoaspecificpositionstream pointertofileoffset filepositionpointer 0isfirstlocation whence specifieswhereinfilewearereadingfromSEEK SET seekstartsatbeginningoffileSEEK CUR seekstartsatcurrentlocationinfileSEEK END seekstartsatendoffileSucceed return0 WritingDataRandomlytoaRandomAccessFile Fig 11 15Readingarandomaccessfilesequentially intmain FILE cfPtr structclientDataclient if cfPtr fopen credit dat r NULL printf Filecouldnotbeopened n else printf 6s 16s 11s 10s n Acct LastName FirstName Balance while fread ReadingDataSequentiallyfromaRandomAccessFile intfread ReadsaspecifiednumberofbytesfromafileintomemoryCanreadseveralfixed sizearrayelementsProvidepointertoarrayIndicatenumberofelementstoreadNumberspecifiedinthe3rdargumentreturnthenumberofitemssuccessfullyr