File_Processing（英文版）

1、Programming in CLecture 8: C File ProcessingOutlinenData and Data FilesnFiles and StreamsnSequential Access FilesnRandom Access Files2Data and Data FilesnData Files can be created, updated, and processed by C programs nused for permanent storage(持久存儲) of large amounts of datanStorage of data in vari

2、ables and arrays is only temporary(臨時(shí)的)nAll such data are lost when the program terminates3The Data Hierarchy - 1nBit smallest data itemnValue of 0 or 1nByte 8 bits nUsed to store a character: Decimal digits, letters, and special symbolsnField group of characters conveying meaning nExample: your nam

3、e4The Data Hierarchy - 2nRecord group of related fieldsnRepresented by a struct (or a class in C+)nExample: In a payroll system, a record for a particular employee that contained his/her identification number (possible record key), name, address, etc.nFile group of related recordsnExample: payroll f

4、ilenDatabase group of related files5The Data Hierarchy - 36The Data Hierarchy - 4nExample (Fig. 11.1):1Bit01001010ByteJudyFieldJudyGreenRecordSallyBlackJudyBlueIrisOrangeRandyRedFile7OutlinenData and Data FilesnFiles and StreamsnSequential Access FilesnRandom Access Files8Files and StreamsnC views e

5、ach file as a sequence of bytesnFile ends with the end-of-file (EOF) markernOr, file ends at a specified byte9EOF and feof() are defined in .Example: while (c = getchar() != EOF) if (!feof(stdin) EOF Key CombinationsnDifferent for different OS:10Computer systemKey combinationUNIX systems dWindows zM

6、acintosh dVAX(VMS) zFiles and StreamsnA Stream is created when a file is openednProvide communication channel(通信渠道) between files and programsnOpening a file returns a pointer to a FILE structurenExample file pointers:nstdin - standard input (keyboard)nstdout - standard output (screen)nstderr - stan

7、dard error (screen)11FILE structuren*The programmer need not know the specifics of the FILE structure to use files.nFile descriptor(文件描述符)nIndex into operating system (OS) array called the open file tablenFile Control Block (FCB)nFound in every array element, system uses it to administer the file(Re

8、fer to Fig. 11.5 on page 437)12OutlinenData and Data FilesnFiles and StreamsnSequential Access FilesnRandom Access Files13Read/Write Functions - 1#include nint fgetc(FILE *stream)nRead one character from a filenTake a FILE pointer as an argumentnfgetc(stdin) equivalent to getchar()nint fputc(int c,

9、FILE *stream)nWrite one character to a filenTake a FILE pointer and a character to be written as argumentsnfputc(a, stdout) equivalent to putchar(a)14Read/Write functions - 2nchar* fgets(char *s, int n, FILE *stream)nReads a line from a filenint fputs(char *s, FILE *stream)nWrites a line to a filenf

10、scanf / fprintfnint fscanf(FILE *stream, const char * format, )nint fprintf(FILE *stream, const char * format, )nFile processing equivalents of scanf and printf1516 /* Framework for Create a sequential file, see Fig. 11.3 on P433 for details */ #include int main() FILE *cfPtr; /* cfPtr = clients.dat

11、 file pointer */if ( ( cfPtr = fopen( clients.dat, w ) ) = NULL )printf( File could not be opened.n );else scanf( %d%s%lf, &account, name, &balance ); while ( !feof( stdin ) ) fprintf( cfPtr, %d %s %.2fn, account, name, balance );printf( ? );scanf( %d%s%lf, &account, name, &balance )

12、;fclose( cfPtr );return 0; Creating a Sequential Access FilenFILE *cfPtr;nDeclares a FILE pointer called cfPtrncfPtr = fopen(clients.dat, w);nFunction fopen returns a FILE pointer to file specifiednTakes two arguments the file to be opened and the open modenIf open fails, NULL is returned17Creating

13、a Sequential Access FilenfprintfnUsed to print to a filenLike printf, except first argument is a FILE pointer (pointer to the file you want to print )nfeof(FILE pointer)nReturns true if end-of-file (EOF) indicator (no more data to process) is set for the specified filenfclose(FILE pointer)nCloses sp

14、ecified filenPerformed automatically when program endsnGood practice: close files explicitly18File Open Modes19Computer systemKey combinationrOpen a file for reading. wCreate a file for writing. If the file already exists, discard the current contents. aAppend; open or create a file for writing at e

15、nd of file. r+Open a file for update (reading and writing); the file must exists.w+Create an empty file for both reading and writing. If the file already exists, discard the current contents. a+Open a file for reading and appending; writing is done at the end of the file. rbOpen a file for reading i

16、n binary mode. wbCreate a file for writing in binary mode. If the file already exists, discard the current contents. abAppend; open or create a file for writing at end of file in binary mode. rb+ (or r+b)Open a file for update (reading and writing) in binary mode. wb+ (or w+b)Create a file for updat

17、e (reading and writing) in binary mode. If the file already exists, discard the current contents. ab+ (or a+b)Read and Append; open or create a file for update in binary mode; writing is done at the end of the file. Sequential Access FilesnAlso called text file (文本文件)nEach byte stores an ASCII code,

18、 representing a characternFormat of data in a text file is not identical with its format stored in memory.nE.g. 7,-14 are ints occupying 4 bytes each internally;but they are of different size(1 and 3 bytes) as text in a file.2021/* Fig. 11.7: reading and printing a sequential file*/#include int main

19、() FILE *cfPtr; /* cfPtr = clients.dat file pointer */if ( ( cfPtr = fopen( clients.dat, r ) ) = NULL )printf( File could not be openedn );else printf( %-10s%-13s%sn, Account, Name, Balance );fscanf( cfPtr, %d%s%lf, &account, name, &balance );while ( !feof( cfPtr ) ) printf( %-10d%-13s%7.2fn

20、, account, name, balance );fscanf( cfPtr, %d%s%lf, &account, name, &balance ); fclose( cfPtr ); return 0;Reading Data from a Sequential Access FilenCreate a FILE pointer, link it to the file to readncfPtr = fopen( “clients.dat, r );nUse fscanf to read from the filenLike scanf, except first a

21、rgument is a FILE pointernfscanf( cfPtr, %d%s%f, &accounnt, name, &balance );nData read from beginning to end22Reading Data from a Sequential Access FilenFile position pointernIndicates number of next byte to be read / writtennNot really a pointer, but an integer value (specifies byte locati

22、on)nAlso called byte offsetnrewind( cfPtr )nRepositions file position pointer to beginning of file (byte 0)23Reading Data from a Sequential Access FilenFields can vary in sizenDifferent representation in files and screen than internal representationn1, 34, -890 are all ints, but have different sizes

23、 in filenCannot be modified without the risk of destroying other data(修改文件內(nèi)容時(shí)有可能破壞不該破壞的數(shù)據(jù))24Reading Data from a Sequential Access File25300 White 0.00 400 Jones 32.87 (old data in file)If we want to change “White” to “Worthington”,300 White 0.00 400 Jones 32.87300 Worthington 0.00ones 32.87300 Worth

24、ington 0.00Data gets overwrittenOutlinenData and Data FilesnFiles and StreamsnSequential Access FilesnRandom Access Files26Random Access FilesnAlso called binary files(二進(jìn)制文件)nFormat of data in a binary file is identical(同樣的) with its format stored in memory.nbyte doesnt necessarily represent charact

25、er; groups of bytes might represent other types of data, such as integers and floating-point numbersnRecords in binary files have identical length.27Random Access FilesnExample:28001100010011001000110011001101000011010100000000000000000011000000111001123451 2 3 4 5ASCII codes of 5 charactersAn integ

26、erRandom Access FilesnAccess individual records without searching through other recordsnInstant access to records in a file(對文件記錄的隨機(jī)訪問)nData can be inserted without destroying other datanData previously stored can be updated or deleted without overwriting29Random Access FilesnC view of a random-acce

27、ss file:30100Bytes100Bytes100Bytes100Bytes100Bytes100Bytes0100200300400500ByteOffsets31/* Fig. 11.11: Creating a randomly accessed file sequentially , with empty structs. */#include struct clientData int acctNum; char lastName 15 ; char firstName 10 ; double balance;int main() int i;struct clientDat

28、a blankClient = 0, , , 0.0 ;FILE *cfPtr; if ( ( cfPtr = fopen( credit.dat, w ) ) = NULL )printf( File could not be opened.n );else for ( i = 1; i = 100; i+ )fwrite( &blankClient, sizeof( struct clientData ), 1, cfPtr );fclose( cfPtr );return 0;Unformatted File I/O Functionsnfwrite - Transfer byt

29、es from a location in memory to a filensize_t fwrite(const void * buffer, size_t size, size_t nmemb, FILE * fp);nfread - Transfer bytes from a file to a location in memorynsize_t fread(void * buffer, size_t size, size_t nmemb, FILE * fp);32Unformatted File I/O FunctionsnExample:nfwrite( &number,

30、 sizeof( int ), 1, myPtr ); nNumber - an integer variablen&number - location to transfer bytes fromnsizeof( int ) - number of bytes to transfern1 - for arrays, number of elements to transfernIn this case, one element of an array, i.e. one number is being transferrednmyPtr - File to transfer tonf

31、read similar 33Unformatted File I/O Functionsnfwrite( &myObject, sizeof (struct myStruct), 1, myPtr );nTo write a data block with designated size to a filensizeof return the size in bytes of the object in parenthesesnTo write several array elementsnPointer to array as first argumentnNumber of el

32、ements to write as third argument3435/* Fig. 11.12: Writing to a random access file*/.int main() FILE *cfPtr;struct clientData client = 0, , , 0.0 ;if ( ( cfPtr = fopen( credit.dat, r+ ) ) = NULL )printf( File could not be opened.n );else while ( client.acctNum != 0 ) printf( Enter lastname, firstna

33、me, balancen? );fscanf( stdin, %s%s%lf, client.lastName, client.firstName, &client.balance );fseek( cfPtr, ( client.acctNum - 1 ) * sizeof( struct clientData ), SEEK_SET );fwrite( &client, sizeof( struct clientData ), 1, cfPtr );printf( Enter account numbern? );scanf( %d, &client.acctNum

34、 );fclose( cfPtr ); return 0;Writing Data Randomly to a Random Access Filenint fseek( FILE *stream, long int offset, int whence);nSet the file position pointer to a specific positionnstream - pointer to filenoffset - file position pointer (0 is first location)nwhence - specifies where in file we are

35、 reading fromnSEEK_SET - seek starts at beginning of filenSEEK_CUR - seek starts at current location in filenSEEK_END - seek starts at end of filenSucceed: return 036Writing Data Randomly to a Random Access File37/*Fig. 11.15 Reading a random access file sequentially */ int main() FILE *cfPtr;struct

36、 clientData client;if ( ( cfPtr = fopen( credit.dat, r ) ) = NULL )printf( File could not be opened.n );else printf( %-6s%-16s%-11s%10sn, Acct, Last Name, First Name, Balance );while ( fread( &client, sizeof( struct clientData ), 1, cfPtr ) ) if ( client.acctNum != 0 )printf( %-6d%-16s%-11s%10.2fn, client.acctNum, client.lastName, client.firstName, client.balance );fclose( cfPtr ); return 0; Reading Data Sequentially from a Random Access Filenint fread(