14.1-14.3 Introduction

Data Hierarchy: bits, bytes, fields, records, files.
A bit is the smallest data item in a computer. It can have a value of 0 or 1. (bit = binary digit)
A byte is eight bits. Characters are in bytes.
A field is a group of characters.
A records is composed of several fields.
A file is a group of related records.

Figure 14.1 on page 706.
 

14.3-14.4 File I/O
ofstream: output file stream, a class (a type).
ifstream: input file stream, a class (a type).

Simple Operations

// Create a sequential file
#include <fstream.h>    //ofstream and ifstream are defined in fstream.h
int main()
{
   ofstream outFile( "mydata.txt", ios::out );
    // mydata.txt resides on the harddisk.
    // outFile is an object representing mydata.txt in memory. 
    // outFile can be used in the same way as cout.
   outFile << "Hi, this is a test.";
    // The sentence is stored in mydata.txt for future retrieval, not displayed.
    outFile.close();

   cout << "Enter a character to end: ";
   char a;
   cin >> a;
   return 0;
}

// Read a sequential file
#include <iostream.h>
#include <fstream.h>    //ofstream and ifstream are defined in fstream.h
int main()
{
  ifstream inFile( "mydata.txt", ios::in );
  // mydata.txt resides on the harddisk.
  // inFile is an object representing mydata.txt in memory.
  // inFile can be used in the same way as cin.
  char a[40];
  inFile.getline(a,40);     // A sentence is retrieved from mydata.txt.
  cout << "A line has been read from mydata.txt :" << endl;
  cout << a <<endl;
  inFile.close();

  cout << "Enter a character to end: ";
  cin >> a;
 return 0;
}

Intermediate Operations

#include <iostream.h>
#include <fstream.h>
#include <stdlib.h>

int main()
{
   // ofstream constructor opens file
   ofstream outClientFile( "clients.dat", ios::out );

   if ( !outClientFile ) {  // overloaded ! operator
      cerr << "File could not be opened" << endl;
      exit( 1 );    // prototype in stdlib.h
   }

   cout << "Enter the account, name, and balance.\n"
        << "Enter end-of-file to end input.\n? ";

   int account;
   char name[ 30 ];
   float balance;

   while ( cin >> account >> name >> balance ) {
      outClientFile << account << ' ' << name
                    << ' ' << balance << '\n';
      cout << "? ";
   }

   return 0;  // ofstream destructor closes file
}
 
// Reading and printing a sequential file
#include <iostream.h>
#include <fstream.h>
#include <iomanip.h>
#include <stdlib.h>

void outputLine( int, const char *, double );

int main()
{
   // ifstream constructor opens the file
   ifstream inClientFile( "clients.dat", ios::in );

   if ( !inClientFile ) {
      cerr << "File could not be opened\n";
      exit( 1 );
   }

   int account;
   char name[ 30 ];
   double balance;

   cout << setiosflags( ios::left ) << setw( 10 ) << "Account"
        << setw( 13 ) << "Name" << "Balance\n";

   while ( inClientFile >> account >> name >> balance )
      outputLine( account, name, balance );

   return 0;  // ifstream destructor closes the file
}

void outputLine( int acct, const char *name, double bal )
{
   cout << setiosflags( ios::left ) << setw( 10 ) << acct
        << setw( 13 ) << name << setw( 7 ) << setprecision( 2 )
        << setiosflags( ios::fixed | ios::showpoint )
        << bal << '\n';
}

Advanced File I/O Operations

File open modes:
ios::in         Open a file for input
ios::out      Open a file for output
ios::app     Write all output to the end of the file
ios::ate      Open a file for output and move to the end of the file.
                  Data can be written anywhere in the file.
ios::trunc  Discard the file's contents if it exists.
ios::nocreate    If the file does not exist, the open operation fails.
ios::noreplace    If the file exists, the open operation fails.

Repositioning the file position pointer :
file position pointer: the byte number of the next byte in the file to be read or written.

istream:    seekg
ostream:    seekp

// Repositioning in a sequential file
#include <iostream.h>
#include <fstream.h>    //ofstream and ifstream are defined in fstream.h
int main()
{
  char c;
  char a[40];

  ifstream data( "mydata.txt", ios::in );
  data.getline(a,40);     // A sentence is retrieved from mydata.txt.
  cout << "A line has been read from mydata.txt :" << endl;
  cout << a << endl << endl;
  data.close();

  ifstream inFile( "mydata.txt", ios::in );
  c = inFile.get();
  cout << "The first byte in the file is: " << c << endl;
  c = inFile.get();
  cout << "The second byte in the file is: " << c << endl;

  inFile.seekg(0);        // reposition the file pointer to the first byte (the begining of the file).
  c = inFile.get();
  cout << "The first byte in the file is: " << c << endl;

  inFile.seekg(2);        // reposition the file pointer to the 3rd byte in the file.
  c = inFile.get();
  cout << "The third byte in the file is: " << c << endl;

  cout << "After reading the third byte, the file pointer is moved to the 4th byte (index 3). \n";
  cout << "The file pointer is at index poitistion : ";
  cout << inFile.tellg() << endl;

  c = cin.peek();
  cout << "The character at the position is : " << c << endl;

  cout << "The 4th byte after " << c << " is ";
  inFile.seekg(4,ios::cur);        // reposition the file pointer 4 bytes down.
  c = inFile.get();
  cout << c << endl;

  inFile.seekg(-1,ios::end);        // reposition the file pointer to the last byte in the file.
  c = inFile.get();
  cout << "The last byte in the file is: " << c << endl;

  inFile.seekg(2,ios::beg);  // same as seekg(2); The default is iso::beg.
  c = inFile.get();
  cout << "The third byte in the file is: " << c << endl;

  cout << "Enter a character to end: ";
  cin >> a;
 return 0;
}
 
// Repositioning in a sequential file
#include <iostream.h>
#include <fstream.h>    //ofstream and ifstream are defined in fstream.h
int main()
{
  char c;
  char a[40];

  ifstream data( "mydata.txt", ios::in );
  data.getline(a,40);     // A sentence is retrieved from mydata.txt.
  cout << "A line has been read from mydata.txt :" << endl;
  cout << a << endl << endl;
  data.close();

  //Hi, this is a test.
  //Ha, this is another test.
  ofstream outFile( "mydata.txt", ios::ate );
  outFile.seekp(1);  // reposition the file pointer to the first byte (the begining of the file).
  outFile.put('a');
  outFile.seekp(10,ios::cur);  // reposition the file pointer to the 10th byte from the current position.

  outFile << "another test.";
  outFile.close();

 return 0;
}

EOF