FILE HANDLING IN C++

All programs we looked earlier:

Introduction

input data from the keyboard.

output data to the screen. Difficult to handle large amount of input data. Output would also be lost as soon as we exit from the program. How do we store data permanently?.

We can use secondary storage device.

Data is packaged up on the storage device as data structures called files.

Files (Streams)

Files are used to store data in a relatively permanent form, on floppy disk, hard disk, tape or other form of secondary storage. Files can hold huge amounts of data if need be. Ordinary variables (even records and arrays) are kept in main memory which is temporary and rather limited in size.

Lets put it in points…………..

Why use files?

• Convenient way to deal with large quantities of data.

• Store data permanently (until file is deleted).

• Avoid having to type data into program multiple times.

• Share data between programs.

The following is a

comparison

of

the two types of storage………..

Main memory

Made up of RAM chips.

Used to hold a program when it is running, including the values of its variables (whether integer, char, an array, etc.)

Secondary memory

Usually a disk drive (or magnetic tape).

Used to hold files (where a file can contain data, a program, text, etc.)

Can hold rather large amounts of data.

Main memory

Can only hold relatively small amounts of data.

Is temporary (as soon as the program is done or the power goes out all of these values are gone).

Gives fast access to the data (all electronic).

Secondary memory

Can hold rather large amounts of data.

Is fairly permanent. (A file remains even if the power goes out. It will last until you erase it, as long as the disk isn't damaged, at least.)

Access to the data is considerably slower (due to moving parts).

I/O in C++

I/O in C++ uses streamsA Stream is a general name given to

flow of data.

Flow of Data….

PROGRAM

DEVICES OR

FILES

InputStream

>>

OutputStream

<<

Data

Data

istream class ostream class

(Insertion operator)

(Extraction operator)

More About Files…..

Now we need to know:how to "connect" file to programhow to tell the program to read datahow to tell the program to write dataerror checking and handling eof

I/O in C++

Different streams are used to represent different kinds of data flow.

Each stream is associated with a particular class, which contains

member functions and definitions for dealing with that particular

kind of data flow.

The following classes in C++ have access to file input and output functions:

ifstreamofstreamfstream

File Related Classes

The Stream Class Hierarchy

ios

istreamget()getline()read()>>

ostreamput()write()<<

fstreambase

iostream

IfstreamOpen()Tellg()Seekg()

OfstreamOpen()Tellp()Seekp()

fstream

NOTE : UPWARD ARROWS INDICATETHE BASE CLASS

OPENING A FILE

1. By using the CONSTRUCTOR of the stream class.

ifstream transaction(“sales.dly”);

ofstream result(“result.02”);

2. By using the open() function of the stream class

ifstream transaction;

transaction.open(“sales.dly”);

(Associating a stream with a file)

File Mode Parameters

PARAMETER MEANING ios::app Append to end-of file ios::ate goto end of file on opening ios::binary binary file ios::in Open existing file for reading ios::nocreate open fails if file doesn’t exist ios::noreplace open fails if file already exists ios::out creates new file for writing on ios::trunc Deletes contents if it exists

The mode can combine two or more modes using bit wise or ( | )

Checking For Successful File Opening

ifstream transaction(“sales.dly”);

if (transcation == NULL)

{

cout<<“unable to open sales.dly”;cin.get(); // waits for the operator to press any key

exit(1);

}

Closing of File

Stream_name.close();

e.g., transaction.close();

fl.close();

Note : There is no need to give the physical file name at the time of closing a file.

Types of Files

The two basic types of files areText files

& Binary files

Text Files

A text file consists of readable characters separated into lines by newline characters.

(On most PCs, the newline character is actually represented by the two-character sequence of carriage return (ASCII 13), line feed (ASCII 10). (\n)

A binary file stores data to disk in the same form in which it is represented in main memory.

If you ever try to edit a binary file containing numbers you will see that the numbers appear as nonsense characters.

Binary Files

Not having to translate numbers into a readable form makes binary files somewhat more efficient.

Binary files also do not normally use anything to separate the data into lines.

Such a file is just a stream of data with nothing in particular to separate components.

Binary Files

When using a binary file we write whole record data to the file at once.

but the numbers in the binary file will not be readable in this way.

When using a text file, we write out separately each of the pieces of data about a given record.

The text file will be readable by an editor

Text Files Binary Files

for the text file we will use the usual output operator(<<) and will output each of the pieces of the record separately.

with the text file we will read each of the pieces of record from the file separately, using the usual input operator(>>)

For the binary file we will use write to write to the file,

With the binary file we will use the read function to read a whole record,

The programs to create the data files will differ in how they open the file and in how they write to the file.

:Sequential access. With this type of file access one must read the data in order, much like with a tape, whether the data is really stored on tape or not. Random access (or direct access). This type of file access lets you jump to any location in the file, then to any other, etc., all in a reasonable amount of time.

Types of File Access

FILE POINTERS

FILE POINTERS

Each file object has two integer values associated with it :get pointerput pointer

These values specify the byte number in the file where reading or writing will take place.

File pointers…..

By default reading pointer is set at the beginning.

By default writing pointer is set at the end (when you open file in ios::app mode)

There are times when you must take control of the file pointers yourself so that you can read from and write to an arbitrary location in the file.

Functions associated with file pointers :

The seekg() and tellg() functions allow you to set and examine the get pointer.

The seekp() and tellp() functions allow you to set and examine the put pointer.

seekg() function : (with one argument)

With one argument :fl.seekg(k);

fl.seekp(k);

where k is absolute position from the beginning. The start of the file is byte 0

It will result in moving the pointer as shown-

Begin File End

k bytes ^

File pointer

‘seek’ functions : (With two arguments )

Number of bytes file pointer to be moved

Location from where File pointer is to be moved

fl.seekg(offset, refposition);

fl.seekp(offset, refposition);

Refposition takes one of the following forms :

•ios::beg Start of the file•ios::cur current position of the pointer•ios::end End of the file

File Pointer offset calls

•fl.seekg(0,ios::beg); Go to start

•fl.seekg(0,ios::cur); Stay at the current position

•fl.seekg(0,ios::end); Go to the end of file

•fl.seekg(m,ios::beg);Move to (m+1)th byte in the file

File Pointer offset calls

•fl.seekg(m,ios::cur); Go forward by m bytes from current pos

•fl.seekg(-m,ios::beg); Go backward by m bytes from current pos

•fl.seekg(-m,ios::cur); Go backward by m bytes from the end

seekg() function : (With two arguments ):

Begin End

^Offset from Begin

^Offset from current position

^Offset from end

fl.seekg(-m,ios::cur);

Go backward by m bytes from the end

m bytes

fl.seekg(m,ios::cur);

Go forward by m bytes from current pos

•fl.seekg(m,ios::beg);

Move to (m+1)th byte in the file

m bytes

Begin End

EndBegin

m bytes

EXAMPLES

Creation of a text file

#include <fstream.h>

#include <conio.h>

#include <stdio.h>

void main()

{ clrscr();

char c,d,ans;

char str[80];

ofstream outfl("try.txt");

do

{ cout<<"please give the string : ";

gets(str);

outfl<<str;

cout <<"do you want to write more...<y/n> : ";

ans=getch();

} while(ans=='y');

outfl<<'\0';

outfl.close();

clrscr(); ifstream infl;getch();cout <<"reading from created file \n";infl.open("try.txt");out.open("cod.dat"); //**********************************c=infl.get();do { d=c+1; cout<<c<<d<<'\n'; out.put(d); c= infl.get(); } while (c!='\0'); out<<'\0'; infl.close(); outfl.close(); getch(); //********************************* }

Program to generate coded file……(Text File)

The End