Reading Data from a Sequential File
Files store data so it may be retrieved for processing when needed. The previous section demonstrated how to create a file for sequential access. In this section, we discuss how to read data sequentially from a file.
Figure 17.7 reads records from the clients.dat file that we created using the program of Fig. 17.4 and displays the contents of these records. Creating an ifstream object opens a file for input. The ifstream constructor can receive the filename and the file open mode as arguments. Line 31 creates an ifstream object called inClientFile and associates it with the clients.dat file. The arguments in parentheses are passed to the ifstream constructor function, which opens the file and establishes a "line of communication" with the file.
Figure 17.7. Reading and printing a sequential file.
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1 // Fig. 17.7: Fig17_07.cpp 2 // Reading and printing a sequential file. 3 #include 4 using std::cerr; 5 using std::cout; 6 using std::endl; 7 using std::fixed; 8 using std::ios; 9 using std::left; 10 using std::right; 11 using std::showpoint; 12 13 #include // file stream 14 using std::ifstream; // input file stream 15 16 #include 17 using std::setw; 18 using std::setprecision; 19 20 #include 21 using std::string; 22 23 #include 24 using std::exit; // exit function prototype 25 26 void outputLine( int, const string, double ); // prototype 27 28 int main() 29 { 30 // ifstream constructor opens the file 31 ifstream inClientFile( "clients.dat", ios::in ); 32 33 // exit program if ifstream could not open file 34 if ( !inClientFile ) 35 { 36 cerr << "File could not be opened" << endl; 37 exit( 1 ); 38 } // end if 39 40 int account; 41 char name[ 30 ]; 42 double balance; 43 44 cout << left << setw( 10 ) << "Account" << setw( 13 ) 45 << "Name" << "Balance" << endl << fixed << showpoint; 46 47 // display each record in file 48 while ( inClientFile >> account >> name >> balance ) 49 outputLine( account, name, balance ); 50 51 return 0; // ifstream destructor closes the file 52 } // end main 53 54 // display single record from file 55 void outputLine( int account, const string name, double balance ) 56 { 57 cout << left << setw( 10 ) << account << setw( 13 ) << name 58 << setw( 7 ) << setprecision( 2 ) << right << balance << endl; 59 } // end function outputLine
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Good Programming Practice 17.1
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Open a file for input only (using ios::in) if the file's contents should not be modified. This prevents unintentional modification of the file's contents and is an example of the principle of least privilege. |
Objects of class ifstream are opened for input by default. We could have used the statement
ifstream inClientFile( "clients.dat" );
to open clients.dat for input. Just as with an ofstream object, an ifstream object can be created without opening a specific file, because a file can be attached to it later.
The program uses the condition !inClientFile to determine whether the file was opened successfully before attempting to retrieve data from the file. Line 48 reads a set of data (i.e., a record) from the file. After the preceding line is executed the first time, account has the value 100, name has the value "Jones" and balance has the value 24.98. Each time line 48 executes, it reads another record from the file into the variables account, name and balance. Line 49 displays the records, using function outputLine (lines 5559), which uses parameterized stream manipulators to format the data for display. When the end of file has been reached, the implicit call to operator void * in the while condition returns the null pointer (which converts to the bool value false), the ifstream destructor function closes the file and the program terminates.
To retrieve data sequentially from a file, programs normally start reading from the beginning of the file and read all the data consecutively until the desired data is found. It might be necessary to process the file sequentially several times (from the beginning of the file) during the execution of a program. Both istream and ostream provide member functions for repositioning the file-position pointer (the byte number of the next byte in the file to be read or written). These member functions are seekg ("seek get") for istream and seekp ("seek put") for ostream. Each istream object has a "get pointer," which indicates the byte number in the file from which the next input is to occur, and each ostream object has a "put pointer," which indicates the byte number in the file at which the next output should be placed. The statement
inClientFile.seekg( 0 );
repositions the file-position pointer to the beginning of the file (location 0) attached to inClientFile. The argument to seekg normally is a long integer. A second argument can be specified to indicate the seek direction. The seek direction can be ios::beg (the default) for positioning relative to the beginning of a stream, ios::cur for positioning relative to the current position in a stream or ios::end for positioning relative to the end of a stream. The file-position pointer is an integer value that specifies the location in the file as a number of bytes from the file's starting location (this is also referred to as the offset from the beginning of the file). Some examples of positioning the "get" file-position pointer are
// position to the nth byte of fileObject (assumes ios::beg) fileObject.seekg( n ); // position n bytes forward in fileObject fileObject.seekg( n, ios::cur ); // position n bytes back from end of fileObject fileObject.seekg( n, ios::end ); // position at end of fileObject fileObject.seekg( 0, ios::end );
The same operations can be performed using ostream member function seekp. Member functions tellg and tellp are provided to return the current locations of the "get" and "put" pointers, respectively. The following statement assigns the "get" file-position pointer value to variable location of type long:
location = fileObject.tellg();
Figure 17.8 enables a credit manager to display the account information for those customers with zero balances (i.e., customers who do not owe the company any money), credit (negative) balances (i.e., customers to whom the company owes money), and debit (positive) balances (i.e., customers who owe the company money for goods and services received in the past). The program displays a menu and allows the credit manager to enter one of three options to obtain credit information. Option 1 produces a list of accounts with zero balances. Option 2 produces a list of accounts with credit balances. Option 3 produces a list of accounts with debit balances. Option 4 terminates program execution. Entering an invalid option displays the prompt to enter another choice.
Figure 17.8. Credit inquiry program.
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1 // Fig. 17.8: Fig17_08.cpp 2 // Credit inquiry program. 3 #include 4 using std::cerr; 5 using std::cin; 6 using std::cout; 7 using std::endl; 8 using std::fixed; 9 using std::ios; 10 using std::left; 11 using std::right; 12 using std::showpoint; 13 14 #include 15 using std::ifstream; 16 17 #include 18 using std::setw; 19 using std::setprecision; 20 21 #include 22 using std::string; 23 24 #include 25 using std::exit; // exit function prototype 26 27 enum RequestType { ZERO_BALANCE = 1, CREDIT_BALANCE, DEBIT_BALANCE, END }; 28 int getRequest(); 29 bool shouldDisplay( int, double ); 30 void outputLine( int, const string, double ); 31 32 int main() 33 { 34 // ifstream constructor opens the file 35 ifstream inClientFile( "clients.dat", ios::in ); 36 37 // exit program if ifstream could not open file 38 if ( !inClientFile ) 39 { 40 cerr << "File could not be opened" << endl; 41 exit( 1 ); 42 } // end if 43 44 int request; 45 int account; 46 char name[ 30 ]; 47 double balance; 48 49 // get user's request (e.g., zero, credit or debit balance) 50 request = getRequest(); 51 52 // process user's request 53 while ( request != END ) 54 { 55 switch ( request ) 56 { 57 case ZERO_BALANCE: 58 cout << " Accounts with zero balances: "; 59 break; 60 case CREDIT_BALANCE: 61 cout << " Accounts with credit balances: "; 62 break; 63 case DEBIT_BALANCE: 64 cout << " Accounts with debit balances: "; 65 break; 66 } // end switch 67 68 // read account, name and balance from file 69 inClientFile >> account >> name >> balance; 70 71 // display file contents (until eof) 72 while ( !inClientFile.eof() ) 73 { 74 // display record 75 if ( shouldDisplay( request, balance ) ) 76 outputLine( account, name, balance ); 77 78 // read account, name and balance from file 79 inClientFile >> account >> name >> balance; 80 } // end inner while 81 82 inClientFile.clear(); // reset eof for next input 83 inClientFile.seekg( 0 ); // reposition to beginning of file 84 request = getRequest(); // get additional request from user 85 } // end outer while 86 87 cout << "End of run." << endl; 88 return 0; // ifstream destructor closes the file 89 } // end main 90 91 // obtain request from user 92 int getRequest() 93 { 94 int request; // request from user 95 96 // display request options 97 cout << " Enter request" << endl 98 << " 1 - List accounts with zero balances" << endl 99 << " 2 - List accounts with credit balances" << endl 100 << " 3 - List accounts with debit balances" << endl 101 << " 4 - End of run" << fixed << showpoint; 102 103 do // input user request 104 { 105 cout << " ? "; 106 cin >> request; 107 } while ( request < ZERO_BALANCE && request > END ); 108 109 return request; 110 } // end function getRequest 111 112 // determine whether to display given record 113 bool shouldDisplay( int type, double balance ) 114 { 115 // determine whether to display zero balances 116 if ( type == ZERO_BALANCE && balance == 0 ) 117 return true; 118 119 // determine whether to display credit balances 120 if ( type == CREDIT_BALANCE && balance < 0 ) 121 return true; 122 123 // determine whether to display debit balances 124 if ( type == DEBIT_BALANCE && balance > 0 ) 125 return true; 126 127 return false; 128 } // end function shouldDisplay 129 130 // display single record from file 131 void outputLine( int account, const string name, double balance ) 132 { 133 cout << left << setw( 10 ) << account << setw( 13 ) << name 134 << setw( 7 ) << setprecision( 2 ) << right << balance << endl; 135 } // end function outputLine
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