Accessing Records Randomly

Accessing Records Randomly

•  While performing file operations, we must be able to reach at any desired position inside the file. For this purpose there are two commonly used functions‒

1) seek

The seek operation is using two functions seekg and seekp.

• seekg ‒ means get pointer of specific location for reading of record.

• seekp ‒ means get pointer of specific location for writing of record.

The syntax of seek is

seekg (offset, reference ‒ position);

seekp (offset, reference ‒ position);

where, offset is any constant specifying the location.

reference ‒ position is for specifying beginning, end or current position. It can be specified as,

ios :: beg    for beginning location

ios: end     for end of file

ios: cur     for current location

2) tell

This function tells us the current position.

For example

seqfile. tellg ()   ‒  gives current position of get pointer (for reading the record).

seqfile. tellp ()  ‒  gives current position of put pointer (for writing the record).

/************************************************************************

Program for demonstrating random access to the file

************************************************************************/

#include<iostream>

#include<fstream>

#include<cstdlib>

using namespace std;

int main()

{

ifstream in;

char Data[80];

in.open("Sample.dat");

if (lin)

{

      // Print an error and exit  

      cerr << "Sample.dat could not be opened for reading!" << endl;

      exit(1);

}

cout<<"First statement of the file ..."<<<<endl;  

in.seekg(0,ios::beg); // move to the beginning character

// Get the rest of the line and print it

in.getline(Data,80);

cout <<<"\t"<<Data << endl;

cout<<"The current position is: "<<in.tellg()<<endl;

cout<<"Moving to 3rd statement of the file ..."<<endl;

 in.seekg(13, ios::cur);

// Get rest of the line and print it

in.getline(Data,80);

cout <<"\t"<<Data << endl;

cout<<"Moving to 6th statement of the file ..."<<endl;

in.seekg(26, ios::cur);

// Get rest of the line and print it

in.getline(Data,80);

cout<<"\t"<<Data << endl;

cout<<"Moving to last statement of the file ..."<<endl;

in.seekg(‒14, ios::end);

// Get rest of the line and print it

in.getline(Data,80);

cout <<"\t"<<Data << endl;

in.close();

return 0;

}

<Sample.dat>

Statement 1

Statement 2

Statement 3

Statement 4

Statement 5

Statement 6

Statement 7

Statement 8

Statement 9

Statement 10

Output

First statement of the file ...

Statement 1

The current position is: 13

Moving to 3rd statement of the file ...

Statement 3

Moving to 6th statement of the file ...

Statement 6

Moving to last statement of the file ...

Statement 10