data file handling points

8/12/2019 Data File Handling Points

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DATA FILE HANDLING


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INTRODUCTION Computer programs are associated to work with

files as it helps in storing data & informationpermanently.

File - itself a bunch of bytes stored on some

storage devices.

In C++ file I/O is achieved through a componentheader file called fstream.h

At lowest level in C++ file is interpreted as

sequence , or stream, of bytes.

At user level a file consists of a sequence ofintermixed data types characters, arithmetic

values etc.

The I/O library manages two aspects- as interface

and for transfer of data.


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INTRODUCTION

The fstream library predefine a set of operations

for handling all file related input/output throughcertain classes.

Stream Classes required for File I/O :

ifstream :- Stream class to read from files

ofstream :- Stream class to write on files.

fstream:- Stream class to both read and write from/to

files


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STREAM I/O

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

File Program ( Input stream) - reads data from file

Program File (Output stream)write data to file

All designed into fstream.h and hence needs to beincluded in all file handling programs.


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IFSTREAM

Input file stream Class

It provide input operation for file open() is a member function of the class ifstream

Inherited functions of ifstream class, from the class

istream are:-

Text File get()

getline()

Binary File

read()

For supporting random access

seekg()

tellg()


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DATA FILES Requirement of 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.

1. Text file : A text file stores information inASCII characters. Each line terminated byEOL ( end of line).

2. Binary file : It stores information in the same

format in which the information is held inmemory. No delimiter for line. no translationoccurs. It is faster and easier for a programread and write.


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OPENING AND CLOSING FILESUsing Constructor:

ifstream input_file(DataFile);char ch;Input_file>>ch;float amt;

input_file>>amt;

ofstream output_file(DataFile);

input_file.close( );output_file.close( );

DataFile

Input stream

input_fileProgram


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OPENING AND CLOSING FILESofstream output_file(DataFile);

Opening a file for output using stream classconstructor creates a new file if there is no file of

that name on the disk. However , if the file by that

name exists already, the act of opening it for

output scraps it off so that output starts a fresh line

Closing file :-input_file.close( );output_file.close( );

Close() does not eliminate the stream; it just

disconnects from the file.


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OPENING FILES

Using open() method

STREAM-OBJECT.open(FILENAME, mode)

Eg:-

ofstream OFILE;

OFILE.open(DATA1.TXT);

ifstream IFILE;

IFILE.open(DATA2.TXT);


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FILE MODES The File mode describes how a file is to be used:-File modeparameter

Description Stream

Type

ios::in Open file to read ifstream

ios::out Open file to write. If the file exists then, previouscontent is discarded else new file is created.

ofstream

ios::app All the data you write, is put at the end of the file. It

calls ios::out

ofstream

ios::ate All the date you write, is put at the end of the file. Itdoes not call ios::out. I/O operations can occur

in anywhere in the file

ofstream/ifstream

ios::trunc Deletes all previous content in the file. (empties the

file)

ofstream

ios::nocreate If the file does not exists, opening it with the open()function gets impossible.

ofstream

ios::noreplace If the file exists, trying to open it with the open()

function, returns an error.

ofstream

ios::binary Opens the file in binary mode. ofstream/

ifstream


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FILE MODES

All the flags can be combined using thebitwise operator OR (|). For example, if we

want to open the file example.bin in

binary mode to add data we could do it bythe following call to member function

open():

fstream FILE;

FILE.open ("EXAMPLE.BIN", ios::out | ios::app | ios::binary);


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STEPS TO PROCESS A FILE IN YOUR

PROGRAM

Determine the type of link required.

File to memory Input

Memory to File OutputBoth Input/Output

If data is to be brought in from file to memory,

then link is of type file to-memory If data is to be sent from memory to file, then

the link is memory-to-file

S


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PROGRAM

Declare a stream for the desired type of link.

In order to create file streams, include the header

file fstream.h

File to memory (IN TO the memory) ifstream fin

Memory to File(OUT FROM memory) ofstream fout

Both fstream finout


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PROGRAMNow process as required.

#includevoid main( ){ofstream fout;fout.open(mark.dat, ios::out);char ans=y;

int rollno;while(ans==y || ans==Y)

{coutrollno;fout


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OBJECTIVE : TO INSERT SOME DATA ON A

TEXT FILE

Program file SCREEN

File (.txt, .dat)#includeint main(){

ofstream fout;fout.open("abc.txt");

fout


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READING DATA FROM ATEXT FILE

#include#include#includeint main()

{ ifstream fin;char str[80];fin.open("abc.txt");fin>>str; // read only first string from file// as spaces is treated as termination point

cout


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DETECTING END OF FILEUsing EOF( ) member function Using filestream object

Syntax

Filestream_object.eof( );Example

#include

#include

#include

int main()

{

char ch;

ifstream fin;

fin.open("abc.txt");

while(!fin.eof())// using eof()function{

fin.get(ch);cout


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BASIC MODEL FOR FILE I/O

The file stream classes are simply be viewed as astream or array of uninterpreted bytes. That is, it

is like an array of bytes stored and indexed fromzero to len-1, where len is the total number of bytesin the entire file.

Each open file has two positions associated with it:

The current reading position, which is the index of thenext byte that will be read from the file. It simply pointsto the next character that the basic get method will

return. The current writing position, which is the index of the

byte location where the next output byte will be placed.It simply points to the location where the basic putmethod will place byte(s).


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RANDOM ACCESS

tellg() and tellp()

tellp( )

It return the distance of writing pointer from the beginningin bytes

Syntax

Fileobject.tellp( );

Example:long n = fout.tellp( );

tellg( )

It return the distance of reading pointer from the beginningin bytes

Syntax

Fileobject.tellg( );

Example:

long n = fout.tellg( );


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ERROR HANDLING IN FILES

The fail() function returns true (non-zero) if an error

has occurred with the current stream, false(zero)

otherwise. This can be used for checking whether theprevious operation has failed.

Examples of failures that cause fail to be set:

file not found (when opening for reading).

file cannot be created (when opening for writing).

end of file is reached before the requested data could be

read.

invalid formatting of data (e.g. letters when expecting

numbers).


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ERROR HANDLING IN FILES

The bad() function returns true((non-zero) if afatal error with the current stream has

occurred, false(zero) otherwise. Note: fatal errors do not normally occur. Even a

failure to open a file is not a fatal error.

The function eof() returns true((non-zero) if the

end of the associated input file has beenreached, false (zero)otherwise.

The function good() returns true((non-zero) if noerrors have occurred with the current stream,

false (zero)otherwise.The function clear() does two things:

it clears all io_stream_state_flags associated with thecurrent stream, and sets the flags


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DIFFERENCE BETWEEN GET()AND GETLINE() FUNCTION

Both get() and getline() can read characters from

the input stream into an array till the specified

number of characters are entered or till the

delimiting character is encountered( \n by

default). The difference is that, in case of getline()

this character is removed from the input stream

but get() leaves it in the input stream.

data file handling points

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