working file handling in cpp overview

Contents
11.1 Introduction
11.2 Classes for File Stream Operation
11.3 Opening & Closing Files
11.4 Detection of End of File
11.5 More about Open( ): File modes
11.6 File pointer & manipulator
11.7 Sequential Input & output Operation
11.8 Updating a File : Random Access
11.9 Command Line Arguments
2By:-Gourav Kottawar

File Input and Output Stream
 C++ uses file streams as an interface
between the programs and the data
files.
Disk Files Program
Output Stream
Data output
Data input
Input Stream
Write Data
Read Data

File Input and Output Stream
Disk Files Program
Output Stream
Data output
Data input
Input Stream
Write Data
Read Data
The stream that
supplies data to the
program is known as
input stream
The stream that
receives data from the
program is known as
output stream
Input stream extracts (or read)
data from file
Output stream inserts (or
writes) data to the file

Stream classes for file operations
contained fstream

Opening Files
 For opening a file, we must first create a file
stream and then link it to the filename.
 A file stream can be defined using the classes
ifstream, ofstream, and fstream that are
contained in the header file fstream.
 The class to be used depends on read or
write.
 A file can be open in two ways:
Using the constructor function of the class.
○ Useful when we use only one file in the stream.
Using the member function open( ) of the class.
○ Use to manage multiple files using one stream.

Opening Files Using Constructor
 This involves two steps:
Create a file stream object to manage the
stream using appropriate class.
○ The class ofstream used to create output
stream.
○ The class ifstream to create input stream.
Initialize the file object with the desired
filename.

Opening Files Using Constructor
 ofstream outfile (“results”);
 ifstream infile(“data”);
continue …
Program
Output Stream
outfile
infile
Input Stream
Disk
Result
File
Data
File

Opening Files Using Open( )
 The open( ) can be used to open
multiple files that use the same stream
object.
 For processing a set files sequentially.
file-stream-class stream-object;
stream-object.open ( “file_name” );

Opening Files Using Open( )
ofstream outfile;
outfile.open(“DATA1”);
……..
outfile.close( );
outfile.open(“DATA2”);
………
outfile.close( );
.........
 The above program segment opens two files in
sequence for writing the data.
 The first file is closed before opening the second one.
 A stream can be connected to only one file at a time.
continue …

Opening file using constructor

#include<iostream.h>
#include<stdio.h>
#include<conio.h>
#include<fstream.h>
void main()
{
char c,fname[10];
ofstream out;
cout<<"Enter File name:";
cin>>fname;
out.open(fname);
cout<<"Enter contents to store in file (Enter # at end):n";
while((c=getchar())!='#')
{
out<<c;
}
out.close();
getch();
}
Opening file using open()

// Creating file with constructor
#include<fstream.h>
int main()
{
ofstream outf(“Product”);
cout<<“|n Enetr product name:”;
char name[20];
cin>>name;
outf<<name<<“n”;
cout<<“Enter item cost”;
float cost;
cin>>cost;
outf<<cost <<“n”;
outf.close();
ifsteam inf(“Product”);
inf>>name;
inf>>cost;
cout<<endl;
cout<<“Item name :”<<name <<endl;
cout<<“Item cost :”<<cost<<“n”;
inf.close();
}

// Creating file with constructor
#include<fstream.h>
int main()
{
ofstream outf(“Product”);
cout<<“|n Enetr product name:”;
char name[20];
cin>>name;
outf<<name<<“n”;
cout<<“Enter Product cost”;
float cost;
cin>>cost;
outf<<cost <<“n”;
outf.close();
ifsteam inf(“Product”);
inf>>name;
inf>>cost;
cout<<endl;
cout<<“Product name :”<<name <<endl;
cout<<“IProduct cost :” <<cost<<“n”;
inf.close();
}
//Output
Enter product name : HDD
Enter product cost : 4000
Product Name :HDD
Product cost :4000

#inlcude<fstream.h>
int main()
{
Ofstream fout;
fout.open(“Brand”);
fout<<“Marutin”;
fout<<“Volks n”;
fout<<“Hondan”;
fout.close();
fout.open(“Car”);
fout<<“Swiftn”;
fout<<“Polon”;
fout<<“Amazen”;
fout..close();
//Reading the files
Const int N = 80;
Chat line[N];
ifstream fin;
fin.open(“Brand”);
cout<<“nContents of Brand filen”;
while(fin)
{
fin.getline(line,N);
cout<<line;
}
fin.close();
fin.open(“Car”);
cout<<“n Contents of carfile n”;
while(fin)
{
fin.getline(line,N);
cout<<line;
}
fin.close();
return 0;
}

//Reads the files
#include <iostream.h>
#include<fstream.h>
#include<stdlib.h>
int main()
{
const int SIZE = 80;
char line[SIZE];
ifstream fin1,fin2;
fin1.open(“Brand”);
fin2.open(“Car”);
for(int i= 1;i<=10;i++)
{
if(fin1.eof()!=0)
{ cout<<“Exit from Brand n”);
exit(1);
}
fin1.getline(line,SIZE);
cout<<“Car of “<<line;
if(fin2.eof() !=0)
{
cout<<“Exit from Car”;
exit(1);
} 20By:-Gourav Kottawar

cout<<line<<“n”;
}
return 0;
}

cout<<line<<“n”;
}
return 0;
}
//Output
Car of Maruti
Swift
Car of Volks wagan
Polo
Car of Honda
Amaze

Detecting End-Of-File
 while (fin)
An ifstream object fin returns a value 0 if any error
occurs in the file operation including the end-of-file
condition.
So the while loop may terminates when fin returns a
value of zero on reaching the end-of-file condition.
 if(fin1.eof() != 0) {exit(1);}
eof( ) is a member of ios class.
It returns a non-zero value if the end-of-file (EOF)
condition is encountered, and a zero, otherwise.

File Modes
 stream-object.open(“file_name”, mode);
The second argument mode specifies the
purpose for which the file is opened.
Default values for these second parameters:
○ ios::in – for ifstream - reading only
○ ios::out – for ofstream - writing only

File Modes
 ios::app  Append to end-of-file
 ios::ate  Go to end-of-file on opening
 ios::binary  Binary file
 ios::in  Open file for reading only
 ios::nocreate  Open fails if the file does not
exist
 ios::noreplace  Open files if the file already
exists
 ios::out  Open file for writing only
 ios::trunc  Delete the contents of the file if it exists
fout.open(“data”, ios::app | ios :: nocreate)
continue …

File Pointer
 Input Pointer (get pointer)
The input pointer is used for reading contents of
a given file location.
 Output Pointer (put pointer)
The output pointer is used for writing to a given
file location.
 Each time an input or output operation
takes place, the appropriate pointer is
automatically advanced.

File Pointer – Default Actions
 When a file opened in read-only mode, the
input pointer is automatically set at the
beginning of the file.
 When a file is opened in write-only mode, the
existing contents are deleted and the output
pointer is set at the beginning.
 When a file is opened in append mode, the
output pointer moves to the end of file.

Functions for Manipulations of File Pointers
 seekg( )  Moves get pointer (input) to a
specified location.
 seekp( )  Moves put pointer(output) to a
specified location.
 tellg( )  Gives the current position of the
get pointer.
 tellp( )  Gives the current position of the
put pointer.

Seek Function with Absolute Position
 infile.seekg(10);
Moves the file pointer to the byte number 10.
The bytes in a file are numbered beginning
from zero. Therefore, the pointer pointing to
the 11th
byte in the file.

Seek Function with Specifying the Offset
 seekg( offset, refposition);
 seekp( offset, refposition);
The parameter offset represents the number of bytes the
file pointer is to be moved from the location specified by
the parameter refposition.
The refposition takes one of the following three constants
defined in the ios class:
 ios : : beg  start of the file
 ios : : cur  current position of the pointer
 ios : : end  end of the file

Seek Function with Specifying the Offset
fout.seekg(0, ios : : beg);fout.seekg(0, ios : : beg); Go to startGo to start
fout.seekg(0, ios : : cur);fout.seekg(0, ios : : cur); Stay at the current positionStay at the current position
fout.seekg(0, ios : : end);fout.seekg(0, ios : : end); Go to the end of fileGo to the end of file
fout.seekg(m, ios : : beg);fout.seekg(m, ios : : beg); Move to (m+1)th byte in the fileMove to (m+1)th byte in the file
fout.seekg(m, ios : : cur);fout.seekg(m, ios : : cur); Go forward by m bytes from the current positionGo forward by m bytes from the current position
fout.seekg(-m, ios : : cur);fout.seekg(-m, ios : : cur); Go backward by m bytes from the current positionGo backward by m bytes from the current position
fout.seekg(-m, ios : : end)fout.seekg(-m, ios : : end) Go backward by m bytes from the endGo backward by m bytes from the end

Sequential Input and Output Operations
 put( ) and get( ) Functions
The function put( ) writes a single character
to the associated stream.
The function get( ) reads a single charracter
from the associated stream.

//output
File will be created at
C:TurboC++DiskTurboC3BINABC

 write( ) and read( ) Functions
The functions write( ) and read ( ) handle the data
in binary form.
The values are stored in the disk file in the same
format in which they are stored in the internal
memory.
An int takes two bytes to store its value in the
binary form, irrespective of its size.
But a 4 digit int will take four bytes to store it in
the character form.
continue …

Representing 2594
continue …
0 0 1 0 0 0 1 00 0 0 0 1 0 1 0
4952
Binary Format
Character Format
4 Bytes II
2 Bytes II

 infile.read((char *) &V, sizeof(V));
 outfile.write((char *) &V, sizeof(V));
 write( ) and read( ) functions take two
arguments.
 First is the address of the variable V
 Second is the length of that variable in bytes.
 The address of the variable must be cast to
type char * (pointer to character type).
continue …

Reading and Writing a Class Object
 The read( ) and write( ) are also used to read
from or write to the disk files objects directly.
 The read( ) and write( ) handle the entire
structure of an object as a single unit, using
the computer’s internal representation of
data.
 Only data members are written to the disk
files.

Updating A File : Random Access
 The size of each object can be obtained using the
statement
int object_length = sizeof(object);
 The location of a desired object, say mth object
int location = m * object_length;
The location gives the byte number of the first byte of
the mth object.
 Now we can use seekg( ) or seekp( ) to set the file
pointer to reach this byte.

Updating A File : Random Access
 To find the total number of objects in a file using
object_length
int n = file_size / object_length;
 The file size can be obtained using the function tellg( )
or tellp( ) when the pointer is located at the end of the
file.
continue …

Error Handling During File Operations
 A file which we are attempting to open for reading does
not exist.
 The file name used for a new file may already exist.
 We may attempt an invalid operation such as reading
past the end-of-file.
 There may not be any space in the disk for storing more
data.
 We may use an invalid file name.
 We may attempt to perform an operation when the file is
not opened for that purpose.

Error Handling During File Operations
 The C++ file stream inherits a “stream_state” member
from the class ios.
 This member records information on the status of a file
that is being currently used.
 The class ios supports several member functions that
can be used to read the status recorded in a file stream.
 eof( )
 fail( )
 bad( )
 good( ), etc.
continue …

Command – Line Arguments
 C++ supports a feature that facilitates the supply of
argument to the main( ) function.
 These arguments are supplied at the time of invoking
the program.
 They are typically used to pass the names of data files.
 Eg:- exam data result
 The command-line arguments are typed by the user and
are delimited by a space.

 The main function can take two arguments.
 main( int argc, char * argv [ ] )
 The first argument argc (argument counter) represents
the number of arguments in the command line.
 The second argument argv (argument vector) is an
array of char type pointers that points to the command
line arguments.
 The size of the array will be equal to the value of argc.
continue …

 C:> exam data results
 The value of argc would be 3 and argv would be an
array of three pointers to strings as:
 argv[0]  exam
 argv[1]  data
 argv[2]  results
○ ……
○ ……
○ infile.open(argv[1]);
○ ……
○ ……
○ outfile.open(argv[2]);
○ ……
continue …

#include <iostream>
int main(int argc, char *argv[])
{
using namespace std;
cout << "There are " << argc << " arguments:" << endl;
// Loop through each argument and print its number and value
for (int nArg=0; nArg < argc; nArg++)
cout << nArg << " " << argv[nArg] << endl;
return 0;
}

#include <iostream>
int main(int argc, char *argv[])
{
using namespace std;
cout << "There are " << argc << " arguments:" << endl;
// Loop through each argument and print its number and value
for (int nArg=0; nArg < argc; nArg++)
cout << nArg << " " << argv[nArg] << endl;
return 0;
}
//output

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