file_handling_in_c.ppt......................................

File handling in C++
BCA Sem III
K.I.R.A.S

Using Input/Output Files
 Files in C++ are interpreted as a sequence of bytes stored on some
storage media.
 The data of a file is stored in either readable form or in binary code
called as text file or binary file.
 The flow of data from any source to a sink is called as a stream
 Computer programs are associated to work with files as it helps in
storing data & information permanently.
 File - itself a bunch of bytes stored on some storage devices.
 In C++ this is achieved through a component header file called
fstream.h
 The I/O library manages two aspects- as interface and for transfer
of data.
 The library predefine a set of operations for all file related handling
through certain classes.

A computer file
A computer file
 is stored on a secondary storage device (e.g.,
is stored on a secondary storage device (e.g.,
disk);
disk);
 is permanent;
is permanent;
 can be used to provide input data to a
can be used to provide input data to a
program or receive output data from a
program or receive output data from a
program, or both;
program, or both;
 must be opened before it is used.
must be opened before it is used.

General File I/O Steps
Declare a file name variable
 Associate the file name variable with the
disk file name
 Open the file
 Use the file
 Close the file

 Streams act as an interface between files and programs. In C++ . A
stream is used to refer to the flow of data from a particular device to
the program’s variablesThe device here refers to files, keyboard,
console, memory arrays. In C++ these streams are treated as
objects to support consistent access interface.
 They represent as a sequence of bytes and deals with the flow of
data.
 Every stream is associated with a class having member functions
and operations for a particular kind of data flow.
 File -> Program ( Input stream) - reads
 Program -> File (Output stream) – write
 All designed into fstream.h and hence needs to be included in all file
handling programs.
 Diagrammatically as shown in next slide

file_handling_in_c.ppt......................................

 stream
stream - a sequence of characters
- a sequence of characters
 interactive (iostream)
interactive (iostream)

 cin
cin - input stream associated with
- input stream associated with keyboard.
keyboard.

 cout
cout - output stream associated with
- output stream associated with display.
display.
 file (fstream)
file (fstream)

 ifstream
ifstream - defines new input stream (normally
- defines new input stream (normally
associated with a file).

 ofstream
ofstream - defines new output stream (normally
- defines new output stream (normally
• Stream of bytes to do input and output to different devices.
• Stream is the basic concepts which can be attached to files, strings,
console and other devices.
• User can also create their own stream to cater specific device or user

Streams
 A stream is a series of bytes, which act either as a
source from which data can be extracted or as a
destination to which the output can be sent. Streams
resemble the producer and consumer model
 The producer produces the items to be consumed by the
consumer. The producer and the consumers are
connected by the C++ operators >> or <<. For instance ,
the keyboard exhibits the nature of only a
producer,printer or monitor screen exhibit the nature of
only a consumer. Whereas , a file stored on the disk ,
can behave as a producer or consumer, depending upon
the operation initiated on it.

Predefined console streams
C++ contains several predefined streams that are
opened automatically when the execution of a
program starts.
1) cin :standard input (usually keyboard) corresponding
to stdio in C
2) cout :standard output (usually screen) corresponding
to stdout in C
3) cerr :standard error output (usually screen)
corresponding to stderr in C
4) clog : A fully buffered version of cerr (No C
equivalent)

Why to use Files
 Convenient way to deal large quantities of data.
 Store data permanently (until file is deleted).
 Avoid typing data into program multiple times.
 Share data between programs.
 We need to know:
 how to "connect" file to program
 how to tell the program to read data
 how to tell the program to write data
 error checking and handling EOF

Classes for Stream I/O in C++
ios is the base class.
ios is the base class.
istream and ostream inherit from
istream and ostream inherit from
ios
ios
ifstream inherits from istream
ifstream inherits from istream
(and ios)
(and ios)
ofstream inherits from ostream
ofstream inherits from ostream
(and ios)
(and ios)
iostream inherits from istream
iostream inherits from istream
and ostream (& ios)
and ostream (& ios)
fstream inherits from ifstream,
fstream inherits from ifstream,
iostream, and ofstream
iostream, and ofstream
When working with files in C++, the
following classes can be used:
ofstream – writing to a file
ifstream – reading for a file
fstream – reading / writing
When ever we include <iostream.h>, an
ostream object, is automatically defined –
this object is cout.
ofstream inherits from the class ostream
(standard output class).
ostream overloaded the operator >> for
standard output.…thus an ofstream object
can use methods and operators defined in
ostream.

File Modes
Name Description
ios::in Open file to read
ios::out Open file to write
ios::app All the date you write, is put at the end of the file.
It calls ios::out
ios::ate All the date you write, is put at the end of the file.
It does not call ios::out
ios::trunc Deletes all previous content in the file. (empties
the file)
ios::nocreate If the file does not exist, opening it with the open()
function gets impossible.
ios::noreplace If the file exists, trying to open it with the open()
function, returns an error.
ios::binary Opens the file in binary mode.

File Modes
 Opening a file in ios::out mode also opens it in the
ios::trunc mode by default. That is, if the file already
exists, it is truncated
 Both ios::app and ios::ate set the pointers to the end of
file, but they differ in terms of the types of operations
permitted on a file. The ios::app allows to add data from
end of file, whereas ios::ate mode allows to add or
modify the existing data anywhere in the file. In both the
cases the file is created if it is non existent.
 The mode ios::app can be used only with output files
 The stream classes ifstream and ofstream open files in
read and write modes by default.

File pointers
 Each file has two associated pointers known as the file pointers.
One of them is called the input pointer or get pointer.
The get pointer specifies a location from which the current reading
operation is initiated
Other is called the output pointer or put pointer.
The put pointer specifies a location from where the current writing
operation is initiated
We can use these pointers to move through the files while reading or
writing.
The input pointer is used for reading the contents of a given file location
and the output pointer is used for writing to a given file location.
Functions for manipulation 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.

File pointers
infile.seekg(10);
Moves the file pointer to the byte number 10.
The bytes in a file are numbered beginning from zero. Thus, the pointer will
be pointing to the 11th byte in the file.
Specifying the offset :
The seek functions seekg() and seekp() can also be used with two
arguments as follows:
seekg(offset, refposition);
seekp(offset, refposition);
The parameter offset represents the number of bytes the file pointer to
be moved from the location specified by the parameter refposition.
The refposition takes one of the following these constant defined in
the ios class.
ios::beg start of the file
ios::cur current position of the pointer
ios::end end of the file.

File Open Mode
#include <fstream>
int main(void)
{
ofstream outFile("file1.txt", ios::out);
outFile << "That's new!n";
outFile.close();
Return 0;
}
If you want to set more than one open mode, just use the
OR operator- |. This way:
ios::ate | ios::binary

Dealing with Binary files
Functions for binary file handling
Functions for binary file handling
get():
get(): read a byte and point to the next byte to read
read a byte and point to the next byte to read
put():
put(): write a byte and point to the next location for
write a byte and point to the next location for
write
write
read():
read(): block reading
block reading
write():
write(): block writing
block writing
flush():
flush():Save data from the buffer to the output file.
Save data from the buffer to the output file.

Binary File I/O Examples
//Example 1: Using get() and put()
#include <iostream>
#include <fstream>
void main()
{
fstream File("test_file",ios::out | ios::in | ios::binary);
char ch;
ch='o';
File.put(ch); //put the content of ch to the file
File.seekg(ios::beg); //go to the beginning of the file
File.get(ch); //read one character
cout << ch << endl; //display it
File.close();
}

File I/O Example: Writing
#include <fstream>
#include <fstream>
using namespace std;
int main(void)
int main(void)
{
{
ofstream outFile(“fout.txt");
ofstream outFile(“fout.txt");
outFile << "Hello World!";
outFile << "Hello World!";
outFile.close();
outFile.close();
return 0;
return 0;
}
}

File I/O Example: Reading
#include <iostream>
#include <iostream>
#include <fstream>
#include <fstream>
int main(void)
int main(void)
{
{
ifstream openFile(“data.txt"); //open a text file data.txt
char ch;
char ch;
while(!OpenFile.eof())
while(!OpenFile.eof())
{
{
OpenFile.get(ch);
OpenFile.get(ch);
cout << ch;
cout << ch;
}
}
OpenFile.close();
OpenFile.close();
return 0;
return 0;
}
}

File I/O Example: Reading
#include <iostream>
#include <iostream>
#include <fstream>
#include <fstream>
#include <string>
#include <string>
int main(void)
int main(void)
{
{
string line;
string line;
if(openFile.is_open()){ //
if(openFile.is_open()){ //
while(!openFile.eof()){
while(!openFile.eof()){
getline(openFile,line);//read a line from data.txt and put it in a string
getline(openFile,line);//read a line from data.txt and put it in a string
cout << line;
cout << line;
}
}
else{
else{
cout<<“File does not exist!”<<endl;
cout<<“File does not exist!”<<endl;
exit(1);}
exit(1);}
}
}
openFile.close();
openFile.close();
return 0;
return 0;
}
}

To access file handling routines:
#include <fstream.h>
2: To declare variables that can be used to access file:
ifstream in_stream;
ofstream out_stream;
3: To connect your program's variable (its internal name) to
an external file (i.e., on the Unix file system):
in_stream.open("infile.dat");
out_stream.open("outfile.dat");
4: To see if the file opened successfully:
if (in_stream.fail())
{ cout << "Input file open failedn";
exit(1); // requires <stdlib.h>}

To get data from a file (one option), must declare a variable
to hold the data and then read it using the extraction
operator:
int num;
in_stream >> num;
[Compare: cin >> num;]
6: To put data into a file, use insertion operator:
out_stream << num;
[Compare: cout << num;]
NOTE: Streams are sequential – data is read and written in
order – generally can't back up.
7: When done with the file:
in_stream.close();
out_stream.close();

Reading /Writing from/to Binary Files
 To write n bytes:
 write (const unsigned char* buffer, int n);
 To read n bytes (to a pre-allocated buffer):
 read (unsighed char* buffer, int num)
main()
{
int array[] = {10,23,3,7,9,11,253};
ofstream OutBinaryFile("my_b_file.txt“, ios::out |
ios::binary);
OutBinaryFile.write((char*) array, sizeof(array));
OutBinaryFile.close();
}

C++ has some low-level facilities for character I/O.
C++ has some low-level facilities for character I/O.
char next1, next2, next3;
char next1, next2, next3;
cin.get(next1);
cin.get(next1);
Gets the next character from the keyboard. Does not skip over
Gets the next character from the keyboard. Does not skip over
blanks or newline (n). Can check for newline (next == 'n')
blanks or newline (n). Can check for newline (next == 'n')
Example:
Example:
 cin.get(next1);
cin.get(next1);
 cin.get(next2);
cin.get(next2);
 cin.get(next3);
cin.get(next3);
Predefined character functions must #include <ctype.h> and can be
used to
 convert between upper and lower case
 test whether in upper or lower case
 test whether alphabetic character or digit
 test for space

Reading /Writing from/to Textual Files
main()
{
// Writing to file
ofstream OutFile("my_file.txt");
OutFile<<"Hello "<<5<<endl;
OutFile.close();
int number;
char dummy[15];
// Reading from file
ifstream InFile("my_file.txt");
InFile>>dummy>>number;
InFile.seekg(0);
InFile.getline(dummy,sizeof(dummy));
InFile.close();
}
To write:
put() – writing single character
<< operator – writing an object
To read:
get() – reading a single character of a
buffer
getline() – reading a single line
>> operator – reading a object

Binary file operations
In connection with a binary file, the file mode
must contain the ios::binary mode along
with other mode(s)
To read & write a or on to a binary file,
as the case may be blocks of data are accessed
through
the use of C++ read() and write() respectively.

Handling binary data
#include <fstream>
int main(){
ifstream in(“binfile.dat”);
ofstream out(“out.dat”);
if(!in || !out) { // return}
unsigned int buf[1024];
while(!in){
in.read(buf, sizeof(unsigned
int)*1024);
out.write(buf, sizeof(unsigned
int)*1024);
}

file_handling_in_c.ppt......................................

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