C程序设计基础 英文版 课件 Chapter 10 LinkedList

Chapter10Cfiles10.1Streams10.2FileOperations10.3CharacterI/O10.4LineI/O10.5FormattedI/O10.6BlockI/O10.7FilePositioning10.1StreamsWhyfilesareneeded?Storinginafilewillpreserveyourdataeveniftheprogramterminates.youcaneasilyaccessthecontentsofthefileusingafewcommandsinC.Youcaneasilymoveyourdatafromonecomputertoanotherwithoutanychanges.10.1StreamsInC,thetermstreammeansanysourceofinputoranydestinationforoutput.Manysmallprogramsobtainalltheirinputfromonestream(thekeyboard)andwritealltheiroutputtoanotherstream(thescreen).Largerprogramsmayneedadditionalstreams.Streamsoftenrepresentfilesstoredonvariousmedia.However,theycouldjustaseasilybeassociatedwithdevicessuchasnetworkportsandprinters.4StreamsStandardStreams<stdio.h>

providesthreestandardstreams: FilePointer Stream DefaultMeaning

stdin

Standardinput Keyboard

stdout

Standardoutput Screen

stderr Standarderror Screen 5

WorkingwithfilesFilePointersAccessingastreamisdonethroughafilepointer,whichhastypeFILE

*.TheFILEtypeisdeclaredin<stdio.h>.Whenworkingwithfiles,youneedtodeclareapointeroftypefile.Thisdeclarationisneededforcommunicationbetweenthefileandtheprogram.

FILE*fp;6

TypesofFiles:TextfilesandBinaryfiles1.TextfilesTextfilesarethenormal

.txt

files.

In

textfile,the

text

andcharactersarestoredonecharacterperbyte.Forexample,theintegervalue12345occupy5bytesin

textfile.72.Binaryfiles

Insteadofstoringdatainplaintext,theystoreitinthebinaryform(0'sand1's).In

binaryfile,theintegervalue12345willoccupy4bytes.StreamsTextFilesversusBinaryFiles

Textfilesaredividedintolines.

Textfilesmaycontainaspecial“end-of-file”marker.Inabinaryfile,therearenoend-of-lineorend-of-filemarkers;allbytesaretreatedequally.Programsthatreadfromafileorwritetoafilemusttakeintoaccountwhetherit’stextorbinary.Whenwecan’tsayforsurewhetherafileistextorbinary,it’ssafertoassumethatit’sbinary.910.2FileOperationsFileOperationsInC,youcanperformfourmajoroperationsonfiles,eithertextorbinary:OpeningafileClosingafileReadingfromandwritinginformationtoafileMovingtoaspecificlocationinafile1010.2FileOperationsPrototypeforfopen: FILE*fopen(constchar*filename,constchar*mode);filenameisthenameofthefiletobeopened.Thisargumentmayincludeinformationaboutthefile’slocation,suchasadrivespecifierorpath.modeisa“modestring”thatspecifieswhatoperationsweintendtoperformonthefile.FileOperations

InWindows,becarefulwhenthefilenameinacalloffopenincludesthe\character.Thecallfopen("c:\homework\t1.txt","r") willfail,because\tistreatedasacharacterescape.Onewaytoavoidtheproblemistouse\\insteadof\: fopen("c:\\homework\\t1.txt","r")Analternativeistousethe/characterinsteadof\:

fopen("c:/homework/t1.txt","r")12FileOperations

fopenreturnsafilepointerthattheprogramcan(andusuallywill)saveinavariable: fp=fopen("t1.txt","r");//openst1.txtinthecurrentpathforreadingWhenitcan’topenafile,fopenreturnsanullpointer.13FileOperationsModesForopeningafile,fopenfunctionisusedwiththerequiredaccessmodes.Someofthecommonlyusedfileaccessmodesarementionedbelow.

14TextfileBinaryfileMeaningDuringInexistenceoffilerrbOpenforreading.Ifthefiledoesnotexist,fopen()returnsNULL.wwbOpenforwriting.Ifthefileexists,itscontentsareoverwritten.

Ifthefiledoesnotexist,itwillbecreated.aabOpenforappend.

Dataisaddedtotheendofthefile.Ifthefiledoesnotexist,itwillbecreated.r+rb+orr+bOpenforbothreadingandwriting.Ifthefiledoesnotexist,fopen()returnsNULL.w+wb+orw+bOpenforbothreadingandwriting.Ifthefileexists,itscontentsareoverwritten.

Ifthefiledoesnotexist,itwillbecreated.a+ab+ora+bOpenforbothreadingandappendingIfthefiledoesnotexist,itwillbecreated.FILE*fp1,*fp2;//Declarefilepointers

fp1=fopen("IN.TXT","r");//OpensatextfileIN.TXTinthecurrentpathforreadingfp2=fopen("OUT.DAT","wb");//CreatesanewbinaryfileOUT.DATinthecurrentpathforwritingIt’snotunusualtoseethecalloffopencombinedwiththedeclarationoffp: FILE*fp=fopen(“a.txt”,"r");FILE*fp;fp=fopen(“a.txt”,"r");TestNULL: if((fp=fopen(“a.txt”,"r"))!=NULL)…

FileOperationsClosingaFileThefclosefunctionallowsaprogramtocloseafilethatit’snolongerusing.Theargumenttofclosemustbeafilepointerobtainedfromacalloffopenorfreopen.Example:fclose(fp);

17Example:

ProgramtoOpenaFile,AndClosetheFile

#include<stdio.h>intmain(){

FILE*fp;//Declareafilepointer

fp=fopen(“a.txt",“r");//Openthefileusing“r"mode

if(fp==NULL)printf(“Error.\n");//CheckifthisfilePointerisnull

elseprintf("Thefileisnowopened.\n");

fclose(fp);//Closingthefileusingfclose()

return0;

}DetectingEnd-of-FileandErrorConditions

Cprovides

feof()

whichreturnsnon-zerovalueonlyifendoffilehasreached,otherwiseitreturns0.Syntax

intfeof(FILE*stream);while(!feof(fp)){//testiftheendoffilehasreached

…//ifnot,readorwritethefile}19Thecallferror()returnsanonzerovalueiftheerrorindicatorisset.syntaxintferror(FILE*stream);Example:ferror(fp);Clearerr()clearsboththeend-of-fileanderrorindicators.syntaxvoidclearerr(FILE*stream);clearerr(fp); /*clearseofanderrorindicatorsforfp*/10.3CharacterI/OOutputFunctions

fputcwritesacharactertoanarbitrarystream fputc(ch,fp);/*writeschtofp*/InputFunctions

fgetcreadsacharacterfromanarbitrarystream

ch=fgetc(fp);21#include<stdio.h>intmain(){FILE*fp;fp=fopen("a.txt","w");//openthe

file

fputc('x',fp);//write

single

character

”x”into

file

fclose(fp);//close

file

return0;}

Oneofthemostcommonusesoffgetcistoreadcharactersfromafile.Atypicalwhileloopforthatpurpose: while((ch=fgetc(fp))!=EOF){ … }

EOF

indicates"endoffile".

23#include<stdio.h>intmain(){FILE*fp;charc;fp=fopen("a.txt","r");//openthefilewhile((c=fgetc(fp))!=EOF){//Takinginputsinglecharacteratatimeprintf("%c",c);}fclose(fp);return0;}10.4LineI/OOutputFunctionsfputs

writesalineofcharactersintofile.int

fputs(const

char

*s,

FILE

*stream)

fputs("Hi!",fp);

/*writestofp*/

InputFunctions

fgetsreadsalineofcharactersfromfile.char*

fgets(char

*str,

int

n,

FILE

*stream)

charstr[100];fgets(str,99,fp);25#include<stdio.h>intmain(){FILE*fp;fp=fopen("a.txt","w");//openthefilefputs("hello",fp);//write”hello”into

file

fclose(fp);//close

file

return0;}#include<stdio.h>intmain(){FILE*fp;chartext[100];fp=fopen("a.txt","r");//openthefilefgets(text,99,fp);

//readfromfile

puts(text);fclose(fp);//close

file

return0;}10.5FormattedI/OThefprintf

functionwriteavariablenumberofdataitemstoanoutputstream,usingaformatstringtocontroltheappearanceoftheoutput.Theprototypesforthefunctionsendwiththe...symbol(anellipsis),whichindicatesavariablenumberofadditionalarguments: intfprintf(FILE*restrictstream,constchar*restrictformat,...);

fprintf(fp,“%d\n",a);

/*writestofp*/FormattedI/O

fscanf

readdataitemsfromaninputstream,usingaformatstringtoindicatethelayoutoftheinput. fscanf(fp,"%d%d",&a,&b);

/*readsfromfp*/

2910.6BlockI/OThefreadandfwritefunctionsallowaprogramtoreadandwritelargeblocksofdatainasinglestep.freadandfwriteareusedprimarilywithbinarystreams30BlockI/Ofwriteisdesignedtocopyanarrayfrommemorytoastream.size_tfwrite(constvoid*buffer,size_tsize,size_tcount,FILE*stream);Argumentsinacalloffwrite:AddressofarraySizeofeacharrayelement(inbytes)NumberofelementstowriteFilepointerAcalloffwritethatwritesastructurevariablestoafile:

fwrite(&s,sizeof(s),1,fp);

BlockI/Ofreadwillreaddatafromthegiven

stream

intothearraypointedto.size_tfread(void*buffer,size_tsize,size_tcount,FILE*stream);Parametersbuffer

−Thisisthepointertoablockofmemorysize

−Thisisthesizeinbytesofeachelementtoberead.count

−Thisisthenumberofelements,eachonewithasizeof

size

bytes.stream

−ThisisthepointertoaFILEobjectthatspecifiesaninputstream.Acalloffreadthatreadsthecontentsofafileintothearray:

fread(array,sizeof(int),100,fp);3210.7FilePositioningEverystreamhasanassociatedfileposition.

Whenafileisopened,thefilepositionissetatthebeginningofthefile.In“append”mode,theinitialfilepositionmaybeatthebeginningorend,dependingontheimplementation.Whenareadorwriteoperationisperformed,thefilepositionadvancesautomatically,providingsequentialaccesstodata.33FilePositioningAlthoughsequentialaccessisfineformanyapplications,someprogramsneedtheabilitytojumparoundwithinafile.Ifafilecontainsaseriesofrecords,wemightwantt