《硬件趣学Python编程》 -16 file

Lecture16

FileOperationsCProgrammingLanguageOutlineTypesofFilesFileOperationscreate/openfileread/writefileclosefileRandomAccessFileErrorHandlingWindowsFileSystemFileisasetofdatastoredontheexternalstoragewiththeformatofbinarynumbers.Fileisidentifiedbyauniquenamewithinadirectory.Fileoperationscreate/modify/delete/copyread/writeTypesofFilesclassifybyreadingformattextfile(ASCIIfile)readthedataoffile,andtranslateittocharactersbyASCIIencodingset,orotherencodingsets.binaryfilereadthedataoffileasthebinarydata.TypesofFilesclassifybydataformatsequenceaccessfilethedataoffilearestoredtogetherwithnowaytobreakthem.randomaccessfilethedataoffilearestoredwithsomespecificformatsothatyoucanfindsomedatabyindexorotherways.TypesofFilesclassifybymediaformatdevicefilenottherealfiles,justregardthedevicesasthefiletosimplifytheoperations.commonfilerealfileswithdataonthedisks.TwoConceptsStream:torepresentthedatasequenceflowtoandfromafileFilebuffer:areusedtoimprovetheefficiencyofthefileoperations.Datatoandfromfilewillbestoredonthefilebuffertemporallyforabetterperformancepurpose.ProcessofFileOperationscreate/openfileread/writefileclosefileAllaboveoperationsarecarriedoutbydifferentlibraryfunctions.FilePointerFilepointerisapointerpointstoafile.Alloperationsoffilewilluseit.Syntax:

FILE*Pointer_Name;

//needstdio.hOpen/CreateaFileusefunctionfopentoopen/createafile.Syntax:

FILE*fopen(File_Name,Option)Optionistosetthewayyouwilloperatethefile.

FILE*fp;fp=fopen(“a.txt”,“r”);Open/CreateaFileIfopenoperationfails,fopenreturnsNULLFILE*fp;fp=fopen(“a.txt”,“r”);if(fp==NULL){printf(“error:cannotopenfile!”);

……}Open/CreateaFileFile_Namecanbeundertheabsolutepath,orthecurrentpath.absolutepath:wholedirectorynameandfilename,suchas“D:\\test\\a.txt”currentpath:ifthefileisatthesamedirectoryoftheprogram,justusethefilename,suchas“a.txt”Open/CreateaFilefopenoptions“r” readonlymode“w” writeonlymode“a” appendonlymode“+” read/writeplus“t” textfileformatoperation“b” binaryfileformatoperationOpen/CreateaFileBydefaultfopenoptionswillregardfileasthetextfile.Ifyouwanttoopenfileasbinaryformat,combineread/writemodeandformatmodetogether.“r”or“rt” readtextfile“rb” readbinaryfileOpen/CreateaFileSomeRules“r”mode:ifthefiledosenotexist,fopenwillreturnNULL“w”mode:iffiledosenotexist,createthefileandopenitforwriting.Ifthefileexists,openandemptythefileforwriting.“a”mode:filemustexists,openthefileandappenddatatotheendofthefile.CloseaFilefunctionfcloseisusedtocloseafileSyntax:

intfclose(File_Pointer);Iffileisclosedsuccessfully,fclosereturns0,ifthereisaerror,itreturnothervalues.Example1CloseaFileCloseaopenedfileisveryimportant.Allthedatainthefilebufferwillbestoredproperlyduringtheclosing.Ifyoudonotclosefile,somedatamaybelost.Justclosethefileyouhaveopened,donottrytocloseotherfiles.SequenceAccessFileRead/writefilefromthestartpointofthefilebythesequenceorder.ThereisaFLAGtoindicatethecurrentread/writelocation.Therearelibraryfunctionstoread/writedataascharacter,string,orformattedblock.ReadCharactersFunction:

int

fgetc(file_pointer)；Itreturnsthenextcharacterofstreamasanunsignedchar(convertedtoanint).ReadCharactersAftercallingfgetc,theread/writeFLAGwillautomaticallymovetothenextcharacter,soyoucankeepreadingthenextone.Attheendofatextfile,fgetcwillreturnaconstantEOF.Forbinaryfile,youneedtousefunctionfeof(fp)tochecktheendoffile.Returnnon-zerovaluemeansithasreachedtotheendofthefile.Example2WriteCharactersFunction:

intfputc(value_to_write，file_pointer)；Ifwritingfileissuccessful,fputcwillreturnthewrittencharacter.OritwillreturnEOF.Example3,4,5ReadStringsFunction:

char*fgets(char_array,n,file_pointer)；fgetsreadn-1charactersandputthemtochar_array,andthenadd‘\0’tothenextlocationautomatically.Itreturnstheaddressofthechar_array.ReadStringsThereadingwillstopifanewlineisencountered;thenewlineisincludedinthearray,whichisterminatedby’\0’.ThereadingwillstopifmeetingEOF.returnsNULLifendoffileorerroroccurs.Example6WriteStringsFunction:

intfputs(char_array,file_pointer)；fputswritethestringstoredinchar_arraytothefile,‘\0’willnottobewritten.Itreturnsnon-negative,orEOFforanerror..Example7Read/WriteFormattedDataFunctions:intfscanf(file_pointer,format,values)； intfprintf(file_pointer,format,values)；Similarasreading/writingdatafrom/tokeyboard/screen.fscanfcanonlyreaddataasthesameformatthatfprintfhaswritten.Returnvalueisthenumbersofthedata.Example8Read/WriteDataBlockRead/writeablockdata,suchasarrays,structures.Functions:intfread(buffer,size,n,file_pointer)； intfwrite(buffer,size,n,file_pointer)；

Read/writesizebytesdataforntimesfromfiletobuffer,orfrombuffertofile.Read/WriteDataBlockThereturnvalueofthesetwoargumentsistheactualnumbersofdatathathavebeenreadorwrite.Theactualnumbersofdatamaylessthanrequirednumbersbecauseofreachingtotheendoffile.Example9RandomAccessFileYoucanread/writedataataspecificlocationfromafile.Beforedoingthat,youneedtomoveread/writeFLAGtothepoint.Funtion:intfseek(file_pointer,offset,init_point);MoveFLAGoffset(alongtypenumber)bytesfromtheinit_pointposition.Itreturnsannon-zerovalueonerrors.RandomAccessFileinit_point

isainteger,butnormallyweuseconstanti