17.9. Writing Data Randomly to a Random-Access File

Figure 17.13 writes data to the file credit.dat and uses the combination of fstream functions seekp and write to store data at exact locations in the file. Function seekp sets the "put" file-position pointer to a specific position in the file, then write outputs the data. Note that line 19 includes the header file ClientData.h defined in Fig. 17.10, so the program can use ClientData objects.

Fig. 17.13. Writing to a random-access file.

 1   // Fig. 17.13: Fig17_13.cpp
 2   // Writing to a random-access file.
 3   #include <iostream>
 4   using std::cerr;
 5   using std::cin;
 6   using std::cout;
 7   using std::endl;
 8   using std::ios;
 9
10   #include <iomanip>
11   using std::setw;
12
13   #include <fstream>
14   using std::fstream;
15
16   #include <cstdlib>
17   using std::exit; // exit function prototype
18
19   #include "ClientData.h" // ClientData class definition
20
21   int main()
22   {
23      int accountNumber;
24      char lastName[ 15 ];
25      char firstName[ 10 ];
26      double balance;
27
28      fstream outCredit( "credit.dat", ios::in | ios::out | ios::binary );
29
30      // exit program if fstream cannot open file
31      if ( !outCredit )
32      {
33         cerr << "File could not be opened." << endl;
34         exit( 1 );
35      } // end if
36
37      cout << "Enter account number (1 to 100, 0 to end input)\n? ";
38
39      // require user to specify account number
40      ClientData client;
41      cin >> accountNumber;
42
43      // user enters information, which is copied into file
44      while ( accountNumber > 0 && accountNumber <= 100 )
45      {
46         // user enters last name, first name and balance
47         cout << "Enter lastname, firstname, balance\n? ";
48         cin >> setw( 15 ) >> lastName;
49         cin >> setw( 10 ) >> firstName;
50         cin >> balance;
51
52         // set record accountNumber, lastName, firstName and balance values
53         client.setAccountNumber( accountNumber );
54         client.setLastName( lastName );
55         client.setFirstName( firstName );
56         client.setBalance( balance );
57
58         // seek position in file of user-specified record   
59         outCredit.seekp( ( client.getAccountNumber() - 1 ) *
60            sizeof( ClientData ) );                          
61
62         // write user-specified information in file                  
63         outCredit.write( reinterpret_cast< const char * >( &client ),
64            sizeof( ClientData ) );                                   
65
66         // enable user to enter another account
67         cout << "Enter account number\n? ";
68         cin >> accountNumber;
69      } // end while
70
71      return 0;
72   } // end main

Enter account number (1 to 100, 0 to end input)
? 37
Enter lastname, firstname, balance
? Barker Doug 0.00
Enter account number
? 29
Enter lastname, firstname, balance
? Brown Nancy -24.54
Enter account number
? 96
Enter lastname, firstname, balance
? Stone Sam 34.98
Enter account number
? 88
Enter lastname, firstname, balance
? Smith Dave 258.34
Enter account number
? 33
Enter lastname, firstname, balance
? Dunn Stacey 314.33
Enter account number
? 0

Lines 59–60 position the "put" file-position pointer for object outCredit to the byte location calculated by

( client.getAccountNumber() - 1 ) * sizeof( ClientData )

Because the account number is between 1 and 100, 1 is subtracted from the account number when calculating the byte location of the record. Thus, for record 1, the file-position pointer is set to byte 0 of the file. Note that line 28 uses the fstream object outCredit to open the existing credit.dat file. The file is opened for input and output in binary mode by combining the file-open modes ios::in, ios::out and ios::binary. Multiple file-open modes are combined by separating each open mode from the next with the bitwise inclusive OR operator (|). Opening the existing credit.dat file in this manner ensures that this program can manipulate the records written to the file by the program of Fig. 17.12 , rather than creating the file from scratch. Chapter 19, Bits, Characters, CStrings and structs, discusses the bitwise inclusive OR operator in detail.