Sequential Read and Write Operations

Sequential Read / Write Operations

•  In C++, sequential read and write operations are commonly performed on files using the fstream library. These operations mean data is accessed in the same order it is stored.

 

1. Writing to the file sequentially

•  For writing to the file, we use the insertion operator << with an ofstream object.

•  That means << is used to send data into the file stream.

 •  The ofstream is the output file stream class and hence we have to write a line #include<fstream> at the beginning of our C++ program.

C++ Program

#include<iostream>

#include<fstream>

using namespace std;

int main() {

ofstream outFile("myfile.txt");

if(!outFile) {

    cout<<"Error opening File!!!"<<endl;

    return 1;

}

outFile<<"This is first line"<<endl;

outFile<<"This is second line"<<endl;

outFile <<"This is the last line"<<endl;

outFile.close();

cout<<"Data written successfully!!!"<<endl;

return 0;

}

Output

Data written successfully!!!

 

2. Reading from the file sequentially

•  For reading a file in C++, we use an object of the ifstream class.

•  The ifstream class is defined in the <fstream> header file, so we must include it at the beginning of the program.

•  We need to open the file by passing the file name in the constructor

•  Then, we can read from the file using getline(). This function is useful to read the file line by line.

• Finally, we should close the file with .close() after reading.

Demo example for sequential file handling pago

For updating the contents of the file, we must position the file pointer at the desired record's position. For that purpose we use seekg command is used. Similarly, we can update a file using delete and append operations. The delete operation deletes the existing record. The append operation inserts the record at the end of the file.

Following program shows various operations such as update, delete, append

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

Program for performing various operations on Sequential File organization.

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

#include<iostream>

#include<iomanip>

#include<fstream>

#include<cstring>

#include<cstdlib>

using namespace std;

class EMP_CLASS

{

typedef struct EMPLOYEE  

{

char name[10];

int emp_id;

int salary;

}Rec;

Rec Records;

public:

void Create();

void Display();

void Update();

void Delete();

void Append();

int Search();

};

void EMP_CLASS::Create()

{

   char ch='y';

   fstream seqfile;

   seqfile.open("EMP.DAT",ios::in | ios::out | ios::binary);

   do

   {

      //User must enter the desired data in the members of te structure Records

       cout<<"\n Enter Name:

      cin>>Records.name;

      cout<<"\n Enter Emp_ID: ";

      cin>>Records.emp_id;

      cout<<"\n Enter Salary: ";

       cin>>Records.salary;

   //then write the record containing this data in the file

   seqfile.write((char*)&Records,sizeof(Records));

   cout<<"\nDo you want to add more records?";

   cin>>ch;

} while(ch=='y');

seqfile.close();

}

void EMP_CLASS::Display()

{

fstream seqfile;

int n,m,i;

seqfile.open("EMP.DAT",ios::in | ios::out | ios::binary);

//positioning the pointer in the file at the beginning

seqfile.seekg(0,ios::beg);

cout<<"\n The Contents of file are... ..."<<endl;

//read the records sequentially

while(seqfile.read((char *)&Records,sizeof(Records)))

{

if(Records.emp_id!= ‒1) edo biose

{

   cout<<"\nName: "<<Records.name;

   cout<<"\nEmp_ID: "<<Records.emp_id;

   cout<<"\nSalary: "<<Records.salary;

   cout<<"\n";

}

}

int last_rec=seqfile.tellg();//last record position

//formula for computing total number of objects in the file

n=last_rec/(sizeof(Rec));

cout<<"\n\n Total number of objects are "<<n<<"(considering logical deletion)";

seqfile.close();

}

void EMP_CLASS::Update()

{

   int pos;

   cout<<"\n For updation,";

   fstream seqfile;

   seqfile.open("EMP.DAT",ios::in | ios::out | ios::binary);

   seqfile.seekg(0,ios::beg);

//obtaining the position of desired record in the file

pos=Search();

if(pos= = ‒1)

{

   cout<<"\n The record is not present in the file";

   retum;

}

//calculate the actual offset of the desired record in the file

int offset=pos* sizeof(Rec);

I/O Systems and File I/O

seqfile.seekp(offset);//seeking the desired record for modification

//New values for the updation of record

cout<<"\n Enter the values for updation...";

cout<<"\n Name: ";cin>>Records.name;

cout<<"\n Emp_Id: ";cin>>Records.emp_id;

cout<<"\n Salary: "; cin>>Records.salary;

seqfile.write((char*)&Records,sizeof(Records))<<flush;

seqfile.seekg(0);

seqfile.close();

cout<<"\n The record is updated!!!";

}

void EMP_CLASS::Delete()

{

int id,pos;

cout<<"\n For deletion,";

fstream seqfile;

seqfile.open("EMP.DAT", ios::in | ios::out | ios::binary);

seqfile.seekg(0,ios::beg);//seeking for reading purpose

pos=Search();//finding pos. for the record to be deleted

if(pos= = ‒1)

{

cout<<"\n The record is not present in the file";

return;

}

//calculate offset to locate the desired record in the file

int offset=pos* sizeof(Rec);

seqfile.seekp(offset);//seeking the desired record for deletion

strcpy(Records.name,"");

Records.emp_id= ‒1;

Records.salary= ‒1;

seqfile.write((char*)&Records,sizeof(Records))<<flush;

seqfile.seekg(0);

seqfile.close();

cout<<"\n The record is Deleted!!!";

}

void EMP_CLASS::Append()

{

fstream seqfile;

seqfile.open("EMP.DAT", ios::ate ios::in | ios::out | ios::binary);

seqfile.seekg(0,ios::beg);

int i=0;

while(seqfile.read((char *)&Records,sizeof(Records)))

{

           i++;//going through all the records

             // for reaching at the end of the file

}

//instead of above while loop

//we can also use seqfile.seekg(0,ios::end)

//for reaching at the end of the file

seqfile.clear();//turning off EOF flag

cout<<"\n Enter the record for appending";

cout<<"\nName: "; cin>>Records.name;

cout<<"\nEmp_ID: ";cin>>Records.emp_id;

cout<<"\nSalary: "; cin>>Records.salary;

seqfile.write((char*)&Records,sizeof(Records)); and

seqfile.seekg(0);//reposition to start(optional)

seqfile.close();

cout<<"\n The record is Appended!!!";

}

int EMP_CLASS::Search()

{

fstream seqfile;

int id,pos;

cout<<"\n Enter the Emp_ID for searching the record ";

cin>>id;

seqfile.open("EMP.DAT",ios::ate | ios::in | ios::out | ios::binary);

seqfile.seekg(0,ios::beg);

pos=‒1;

int i=0;

while(seqfile.read((char *)&Records,sizeof(Records)))

{

      if(id==Records.emp_id)

      {

            pos=i;

            break;

      }

      i++;

}

return pos;

}

void main()

{

EMP CLASS List;

char ans='y';

int choice,key;

clrscr();

do

{

cout<<"\n       Main Menu         "<<endl;

cout<<"\n 1.Create";

cout<<"\n 2.Display";

cout<<"\n 3.Update";

cout<<"\n 4.Delete";

cout<<"\n 5.Append";

cout<<"\n 6.Search";

cout<<"\n 7.Exit";

cout<<"\n Enter your choice ";

cin>>choice;

switch(choice)

{

case 1:List.Create();

break;

case 2:List.Display();

break;

case 3:List.Update();

      break;

case 4:List.Delete();

      break;

case 5:List.Append();

      break;

case 6:key=List.Search();

      if(key<0)

            cout<<<<<<"\n Record is not present in the file";

      else

            cout<<"\n Record is present in the file";

      break;

case 7:exit(0);

}

cout<<"\n\t Do you want to go back to Main Menu?";

cin>>ans;

}while(ans=='y');

}

Output

Main Menu

1.Create

2.Display

3.Update

4.Delete

5.Append

6.Search

7.Exit

Enter your choice 1

Enter Name: AAA

Enter Emp_ID: 10

Enter Salary: 1000

Do you want to add more records?y

Enter Name: BBB

Enter Emp ID: 20

Enter Salary: 2000

Do you want to add more records?y

Enter Name: CCC

Enter Emp_ID: 30

Enter Salary: 3000

Do you want to add more records?n

Do you want to go back to Main Menu?y

Main Menu

1.Create

2.Display

3.Update

4.Delete

5.Append

6.Search

7.Exit

Enter your choice 2

The Contents of file are …

Name: AAA

Emp_ID: 10

Salary: 1000

Name: BBB

Emp ID: 20

Salary: 2000

Name: CCC

Emp_ID: 30

Salary: 3000

Total number of objects are 3(considering logical deletion)

Do you want to go back to Main Menu?y

Main Menu

1.Create

2.Display

3.Update

4.Delete

5.Append

6.Search

7.Exit

Enter your choice 3

For updation,

Enter the Emp_ID for searching the record 20

Enter the values for updation...

Name: XXX

Emp_Id: 30

Salary: 10000

The record is updated!!!

Do you want to go back to Main Menu?n

C++ Program

#include<iostream>

#include<fstream>

#include<string>

using namespace std;

int main() {

ifstream inFile("myfile.txt");

string line;

if(!inFile) {

       cout<<"Error opening File!!!"<<endl;

       return 1;

}

cout<<"Reading file as follows ..."<<endl;

while(getline(inFile,line)){

       cout<<line<<endl;

}

inFile.close();

return 0;

}

Output

Reading file as follows...

This is first line

This is second line

This is the last line