Chapter 8 File Processing

Transcription

1 Chapter 8 File Processing Outline 1 Introduction 2 The Data Hierarchy 3 Files and Streams 4 Creating a Sequential Access File 5 Reading Data from a Sequential Access File 6 Updating Sequential Access Files 7 Random Access Files 8 Creating a Random Access File 9 Writing Data Randomly to a Random Access File 10 Reading Data Sequentially from a Random Access File 11 Case Study: A Transaction Processing Program 12 Input/Output of Objects 1

2 Introduction Storage of data Arrays, variables are temporary Files are permanent Magnetic disk, optical disk, tapes Create, update, process files by programs C/C++ or other Sequential and random access Formatted and raw processing 2

3 The Data Hierarchy Data Hierarchy: Bit smallest data item Value of 0 or 1 Byte 8 bits Used to store a character (char) Decimal digits, letters, and special symbols Also Unicode for large character sets (wchar_t) 3

4 The Data Hierarchy From smallest to largest (continued) Field: group of characters with some meaning Your name Record: group of related fields struct or class in C++ In payroll system, could be name, SS#, address, wage Each field associated with same employee Record key: field used to uniquely identify record File: group of related records Payroll for entire company Sequential file: records stored by key Database: group of related files Payroll, accounts-receivable, inventory 4

5 The Data Hierarchy Data Hierarchy (continued): File group of related records Example: payroll file Database group of related files Sally Black Tom Blue Judy Green Iris Orange Randy Red File Judy Green Record Judy Field Byte (ASCII character J) 1 Bit 5

6 The Data Hierarchy Data files Record key Identifies a record to facilitate the retrieval of specific records from a file Sequential file Records typically sorted by key 6

7 Files and Streams C/C++ view file as sequence of bytes Ends with end-of-file marker n-1... end-of-file marker When file opened (C++) Object created, stream associated with it cin, cout, etc. created when <iostream> included Communication between program and file/device 7

8 Files and Streams When file opened (C) Returns pointer to a FILE structure Example file pointers: stdin - standard input (keyboard) stdout - standard output (screen) stderr - standard error (screen) FILE structure File descriptor Index into operating system array called the open file table File Control Block (FCB) Found in every array element, system uses it to administer the file 8

9 Files and Streams (C++) To perform file processing Include <iostream> and <fstream> Class templates basic_ifstream (input) basic_ofstream (output) basic_fstream (I/O) typedefs for specializations that allow char I/O ifstream (char input) ofstream (char output) fstream (char I/O) 9

10 Opening files Files and Streams (C++) Create objects from template Derive from stream classes Can use stream methods put, get, peek, etc. basic_ios basic_istream basic_ostream basic_ifstream basic_iostream basic_ofstream basic_fstream 10

11 Files and Streams (C) Read/Write functions in standard C library fgetc Reads one character from a file Takes a FILE pointer as an argument fgetc( stdin) equivalent to getchar() fputc Writes one character to a file Takes a FILE pointer and a character to write as an argument fputc('a', stdout) equivalent to putchar('a') fgets Reads a line from a file fputs Writes a line to a file fscanf / fprintf File processing equivalents of scanf and printf 11

12 Creating a Sequential-Access File (C++) C++ imposes no structure on file Concept of "record" must be implemented by programmer To open file, create objects Creates "line of communication" from object to file Classes ifstream (input only) ofstream (output only) fstream (I/O) Simplest ofstream outclientfile( filename"); ifstream inclientfile( filename"); Can use with file-open mode ofstream outclientfile( filename", fileopenmode ); ifstream inclientfile( filename", fileopenmode ); Several other ways 12

13 Creating a Sequential-Access File (C++) File-open modes Mode Description ios::app ios::ate ios::in ios::out ios::trunc ios::binary Write all output to the end of the file. Open a file for output and move to the end of the file (normally used to append data to a file). Data can be written anywhere in the file. Open a file for input. Open a file for output. Discard the file s contents if it exists (this is also the default action for ios::out) Open a file for binary (i.e., non-text) input or output. ofstream opened for output by default ofstream outclientfile( "clients.dat", ios::out ); ofstream outclientfile( "clients.dat"); 13

14 Creating a Sequential-Access File (C++) Operations Writing to file (just like cout) outclientfile << myvariable Closing file outclientfile.close() Automatically closed when destructor called 14

15 Creating a Sequential Access File (in C) C (like C++) imposes no file structure No notion of records in a file Programmer must provide file structure Creating a File FILE *myptr; Creates a FILE pointer called myptr myptr = fopen("myfile.dat", openmode); Function fopen returns a FILE pointer to file specified Takes two arguments file to open and file open mode If open fails, NULL returned fprintf Used to print to a file Like printf, except first argument is a FILE pointer (pointer to the file you want to print in) 15

16 Creating a Sequential Access File (in C) feof(file pointer) Returns true if end-of-file indicator (no more data to process) is set for the specified file fclose(file pointer) Closes specified file Performed automatically when program ends Good practice to close files explicitly Details Programs may process no files, one file, or many files Each file must have a unique name and should have its own pointer 16

17 Creating a Sequential Access File (in C) Table of file open modes: Mode Description r w a Open a file for reading. Create a file for writing. If the file already exists, discard the current contents. Append; open or create a file for writing at end of file. r+ Open a file for update (reading and writing). w+ Create a file for update. If the file already exists, discard the current contents. a+ Append; open or create a file for update; writing is done at the end of the file. 17

18 1 // Fig. 14.4: fig14_04.cpp 2 // Create a sequential file in C++ 3 #include <iostream> 4 5 using std::cout; 6 using std::cin; 7 using std::ios; 8 using std::cerr; 9 using std::endl; #include <fstream> Notice the header files 12 required for file I/O. 13 using std::ofstream; #include <cstdlib> // exit prototype int main() 18 { 19 // ofstream opens file 20 ofstream outclientfile( "clients.dat", ios::out ); // exit program if unable to create file 23 if (!outclientfile ) { 24 cerr << "File could not be opened" << endl; 25 exit( 1 ); } // end if ofstream object created and used to open file "clients.dat". If the file does not exist, it is created.! operator used to test if the file opened properly.

19 28 29 cout << "Enter the account, name, and balance." << endl 30 << "Enter end-of-file to end input.\n? "; int account; 33 char name[ 30 ]; 34 double balance; // read account, name and balance from cin, then place in file 37 while ( cin >> account >> name >> balance ) { 38 outclientfile << account << ' ' << name << ' ' << balance 39 << endl; 40 cout << "? "; } // end while return 0; // ofstream destructor closes file } // end main cin is implicitly converted to a pointer. When EOF is encountered, it returns 0 and the loop stops. Write data to file like a regular stream. File closed when destructor called for object. Can be explicitly closed with close().

20 /* Create a sequential file in C */ #include <stdio.h> int main() { int account; char name[ 30 ]; double balance; /* 1. Initialize variables and FILE pointer */ FILE *cfptr; /* cfptr = clients.dat file pointer */ /* 1.1 Link the pointer to a file */ if ( ( cfptr = fopen( "clients.dat", "w" ) ) == NULL ) printf( "File could not be opened\n" ); else { /* 2. Input data */ printf( "Enter the account, name, and balance.\n" ); printf( "Enter EOF to end input.\n" ); printf( "? " ); scanf( "%d%s%lf", &account, name, &balance );

21 /* 2.1 Write to file (fprintf) */ while (!feof( stdin ) ) { fprintf( cfptr, "%d %s %.2f\n", account, name, balance ); printf( "? " ); scanf( "%d%s%lf", &account, name, &balance ); } /* 3. Close file */ } fclose( cfptr ); } return 0; 21

22 Program Output Enter the account, name, and balance. Enter EOF to end input.? 100 Jones 24.98? 200 Doe ? 300 White 0.00? 400 Stone ? 500 Rich ? 22

23 Reading Data from a Sequential-Access File (C++) Reading files ifstream inclientfile( "filename", ios::in );!inclientfile tests if file was opened properly Stops when EOF found while (!inclientfile.eof()) or while ( inclientfile >> account >> name >> balance ) 23

24 1 // Fig. 14.7: fig14_07.cpp 2 // Reading and printing a sequential file in C++. 3 #include <iostream> 4 5 using std::cout; 6 using std::cin; 7 using std::ios; 8 using std::cerr; 9 using std::endl; 10 using std::left; fig14_07.cpp (1 of 3) 11 using std::right; 12 using std::fixed; 13 using std::showpoint; #include <fstream> using std::ifstream; #include <iomanip> using std::setw; 22 using std::setprecision; #include <cstdlib> // exit prototype void outputline( int, const char * const, double );

25 28 int main() 29 { 30 // ifstream constructor opens the file 31 ifstream inclientfile( "clients.dat", ios::in ); // exit program if ifstream could not open file 34 if (!inclientfile ) { 35 cerr << "File could not be opened" << endl; 36 exit( 1 ); 37 fig14_07.cpp (2 of 3) 38 } // end if int account; 41 char name[ 30 ]; 42 double balance; cout << left << setw( 10 ) << "Account" << setw( 13 ) 45 << "Name" << "Balance" << endl << Read fixed from << showpoint; file until EOF 46 found. 47 // display each record in file 48 while ( inclientfile >> account >> name >> balance ) 49 outputline( account, name, balance ); return 0; // ifstream destructor closes the file } // end main Open and test file for input.

26 54 55 // display single record from file 56 void outputline( int account, const char * const name, 57 double balance ) 58 { 59 cout << left << setw( 10 ) << account << setw( 13 ) << name 60 << setw( 7 ) << setprecision( 2 ) << right << balance fig14_07.cpp (3 of 3) 61 << endl; 62 fig14_07.cpp output (1 of 1) 63 } // end function outputline Account Name 100 Jones Doe White Stone Rich Balance

27 Reading Data from a Sequential-Access File (C++) File position pointers Number of next byte to read/write Functions to reposition pointer seekg (seek get for istream class) seekp (seek put for ostream class) Classes have "get" and "put" pointers seekg and seekp take offset and direction Offset: number of bytes relative to direction Direction (ios::beg default) ios::beg - relative to beginning of stream ios::cur - relative to current position ios::end - relative to end 27

28 Reading Data from a Sequential-Access File (C++) Examples fileobject.seekg(0) Goes to front of file (location 0) because ios::beg is default fileobject.seekg(n) Goes to nth byte from beginning fileobject.seekg(n, ios::cur) Goes n bytes forward fileobject.seekg(y, ios::end) Goes y bytes back from end fileobject.seekg(0, ios::cur) Goes to last byte seekp similar 28

29 Reading Data from a Sequential-Access File (C++) To find pointer location tellg and tellp location = fileobject.tellg() Upcoming example Credit manager program List accounts with zero balance, credit, and debit 29

30 1 // Fig. 14.8: fig14_08.cpp 2 // Credit-inquiry program. 3 #include <iostream> 4 5 using std::cout; 6 using std::cin; 7 using std::ios; 8 using std::cerr; 9 using std::endl; 10 using std::fixed; 11 using std::showpoint; 12 using std::left; 13 using std::right; #include <fstream> using std::ifstream; #include <iomanip> using std::setw; 22 using std::setprecision; #include <cstdlib> 25

31 26 enum RequestType { ZERO_BALANCE = 1, CREDIT_BALANCE, 27 DEBIT_BALANCE, END }; 28 int getrequest(); 29 bool shoulddisplay( int, double ); 30 void outputline( int, const char * const, double ); int main() 33 { 34 // ifstream constructor opens the file 35 ifstream inclientfile( "clients.dat", ios::in ); fig14_08.cpp (2 of 6) // exit program if ifstream could not open file 38 if (!inclientfile ) { 39 cerr << "File could not be opened" << endl; 40 exit( 1 ); } // end if int request; 45 int account; 46 char name[ 30 ]; 47 double balance; // get user's request (e.g., zero, credit or debit balance) 50 request = getrequest(); 51

32 52 // process user's request 53 while ( request!= END ) { switch ( request ) { case ZERO_BALANCE: 58 cout << "\naccounts with zero balances:\n"; 59 break; 60 fig14_08.cpp (3 of 6) 61 case CREDIT_BALANCE: 62 cout << "\naccounts with credit balances:\n"; 63 break; case DEBIT_BALANCE: 66 cout << "\naccounts with debit balances:\n"; 67 break; } // end switch 70

33 71 // read account, name and balance from file 72 inclientfile >> account >> name >> balance; // display file contents (until eof) 75 while (!inclientfile.eof() ) { // display record 78 if ( shoulddisplay( request, balance ) ) 79 outputline( account, name, balance ); // read account, name and balance from file 82 inclientfile >> account >> name >> balance; 83 Use clear to reset eof. Use 84 } // end inner while seekg to set file position 85 pointer to beginning of file. 86 inclientfile.clear(); // reset eof for next input 87 inclientfile.seekg( 0 ); // move to beginning of file 88 request = getrequest(); // get additional request from user } // end outer while cout << "End of run." << endl; return 0; // ifstream destructor closes the file } // end main

34 97 98 // obtain request from user 99 int getrequest() 100 { 101 int request; // display request options 104 cout << "\nenter request" << endl 105 << " 1 - List accounts with zero balances" << endl 106 << " 2 - List accounts with credit balances" << endl 107 << " 3 - List accounts with debit balances" << endl 108 << " 4 - End of run" << fixed << showpoint; // input user request 111 do { 112 cout << "\n? "; 113 cin >> request; } while ( request < ZERO_BALANCE && request > END ); return request; } // end function getrequest 120

35 121 // determine whether to display given record 122 bool shoulddisplay( int type, double balance ) 123 { 124 // determine whether to display credit balances 125 if ( type == CREDIT_BALANCE && balance < 0 ) 126 return true; // determine whether to display debit balances 129 if ( type == DEBIT_BALANCE && balance > 0 ) 130 return true; fig14_08.cpp (6 of 6) // determine whether to display zero balances 133 if ( type == ZERO_BALANCE && balance == 0 ) 134 return true; return false; } // end function shoulddisplay // display single record from file 141 void outputline( int account, const char * const name, 142 double balance ) 143 { 144 cout << left << setw( 10 ) << account << setw( 13 ) << name 145 << setw( 7 ) << setprecision( 2 ) << right << balance 146 << endl; 147

36 Enter request 1 - List accounts with zero balances 2 - List accounts with credit balances 3 - List accounts with debit balances 4 - End of run? 1 Accounts with zero balances: fig14_08.cpp output (1 of 2) 300 White 0.00 Enter request 1 - List accounts with zero balances 2 - List accounts with credit balances 3 - List accounts with debit balances 4 - End of run? 2 Accounts with credit balances: 400 Stone

37 Enter request 1 - List accounts with zero balances 2 - List accounts with credit balances 3 - List accounts with debit balances 4 - End of run? 3 Accounts with debit balances: 100 Jones Doe Rich Enter request 1 - List accounts with zero balances 2 - List accounts with credit balances 3 - List accounts with debit balances 4 - End of run? 4 End of run.

38 Reading Data from a Sequential Access File (in C) Reading a sequential access file Create a FILE pointer, link it to the file to read myptr = fopen( "myfile.dat", "r" ); Use fscanf to read from the file Like scanf, except first argument is a FILE pointer fscanf( myptr, "%d%s%f", &myint, &mystring, &myfloat ); Data read from beginning to end File position pointer Indicates number of next byte to be read / written Not really a pointer, but an integer value (specifies byte location) Also called byte offset rewind( myptr ) Repositions file position pointer to beginning of file (byte 0) 38

39 1 /* 2 Reading and printing a sequential file in C */ 3 #include <stdio.h> 4 5 int main() 6 { 7 int account; 8 char name[ 30 ]; 9 double balance; 10 FILE *cfptr; /* cfptr = clients.dat file pointer */ if ( ( cfptr = fopen( "clients.dat", "r" ) ) == NULL ) 13 printf( "File could not be opened\n" ); 14 else { 15 printf( "%-10s%-13s%s\n", "Account", "Name", "Balance" ); 16 fscanf( cfptr, "%d%s%lf", &account, name, &balance ); while (!feof( cfptr ) ) { 19 printf( "%-10d%-13s%7.2f\n", account, name, balance ); 20 fscanf( cfptr, "%d%s%lf", &account, name, &balance ); 21 } fclose( cfptr ); 24 } return 0; 27}

40 Program Output Account Name Balance 100 Jones Doe White Stone Rich

41 /* Credit inquiry program in C */ #include <stdio.h> int main() { int request, account; double balance; char name[ 30 ]; FILE *cfptr; if ( ( cfptr = fopen( "clients.dat", "r" ) ) == NULL ) printf( "File could not be opened\n" ); else { printf( "Enter request\n" " 1 - List accounts with zero balances\n" " 2 - List accounts with credit balances\n" " 3 - List accounts with debit balances\n" " 4 - End of run\n? " ); scanf( "%d", &request ); while ( request!= 4 ) { fscanf( cfptr, "%d%s%lf", &account, name, &balance );

42 switch ( request ) { case 1: printf( "\naccounts with zero " "balances:\n" ); while (!feof( cfptr ) ) { if ( balance == 0 ) printf( "%-10d%-13s%7.2f\n", account, name, balance ); } fscanf( cfptr, "%d%s%lf", &account, name, &balance ); break; 42

43 case 2: printf( "\naccounts with credit " "balances:\n" ); while (!feof( cfptr ) ) { if ( balance < 0 ) printf( "%-10d%-13s%7.2f\n", account, name, balance ); } fscanf( cfptr, "%d%s%lf", &account, name, &balance ); break; 43

44 case 3: printf( "\naccounts with debit " "balances:\n" ); while (!feof( cfptr ) ) { if ( balance > 0 ) printf( "%-10d%-13s%7.2f\n", account, name, balance ); } fscanf( cfptr, "%d%s%lf", &account, name, &balance ); } break; 44

45 } rewind( cfptr ); printf( "\n? " ); scanf( "%d", &request ); } printf( "End of run.\n" ); fclose( cfptr ); } return 0;

46 Enter request 1 - List accounts with zero balances 2 - List accounts with credit balances 3 - List accounts with debit balances 4 - End of run? 1 Program Output Accounts with zero balances: 300 White 0.00? 2 Accounts with credit balances: 400 Stone ? 3 Accounts with debit balances: 100 Jones Doe Rich ? 4 End of run.

47 Updating Sequential Access File: Problem Sequential access file Cannot be modified without the risk of destroying other data Fields can vary in size Formatted text different from internal representation Different representation in files and screen than internal representation 1, 34, -890 are allints, but have different sizes on disk 47

48 Updating Sequential Access File: Problem Updating sequential files Risk overwriting other data Problem can be avoided, but awkward 300 White Jones (old data in file) If we want to change White's name to Worthington, 300 Worthington White Jones Data gets overwritten 300 Worthington 0.00ones