File Handling Basics of C++.......

Writing in .txt mode:

#include <iostream>

#include <fstream>  //// header file for file input and output 

using namespace std; 
void main()

{
     int i = 10; 

     char c = 'A'; 

     float f = 2.222; 
    ofstream infile("First.txt");   // creating file          

                                              // Here infile is file handle 

    infile<<i<<c<<f;                  // writing to file using “<<”(insertion operator)    

    infile.close();                        // closing file opened with file handle infile          

                                             // if you open a file in your program then it

                                             //is necessary to close that file using             

                                            //built in function under fstream header file. 

 

    cout<<"Data written to file"<<endl; 

    system("pause");

}

 
If you open the First.txt file it will looklike this..
output:

10A2.222

You can see here value of i which is 10, c is A and f is 2.222 all these values are written in Fist.txt file. There are three standards to open any file in C or C++’s program. Header file used for all file handling purpose is #include <fstream>

ofstream:
                Open file to write data in file.If the file exist then it opens the existing file otherwise create a new file to write in it.

 

Ifstream:

               Open file to read data from file. If the file exists then it opens the existing file otherwise it will do nothing.

fstream: 
            Open file to perform both read and write operations on file.If the file exists then it opens the existing file for both read and write otherwise create new file to perform operations.

common thing in three options is :
                                                      All these functions are include in fstream .

 

Reading from .txt file:

 // reading data from file

 #include <iostream>

#include <fstream>  // header file for file input and output 

using namespace std; 
void main()

{ 

int i; 

char c; 

float f; 
ifstream infile("First.txt");   // opening existing file  

infile>>i>>c>>f;            // reading data using extraction(>>) operator   

infile.close();       // closing file using file handle(infile) 
cout<<"Data Written in file is"<<endl; 

cout<<i;  cout<<c;  cout<<f;

system("pause");

}

Writing character array to .txt file :

void main()

 { 

char arr[] = "Writing to file";  // char array to write in file 

ofstream infile("First.txt");   // creating file 

infile<<arr;             // writing to file 

 infile<<"I am in first semester of BS"<<endl; 

infile<<"I am studying at UOG"<<endl; 

system("pause");

}

 
Data written in file of above programme: >>

output:

writing to file

I am in first semester of BS

I am studying at UOG

 

You can see value of arr and other two strings written in the file. Like .exe file endl is used to move on next line which is equalent to ‘\n’ character.

 

 Reading array from above file:

void main()

{ 

 char arr[30]; 

ifstream infile("First.txt");              // creating file 

while(!infile.eof())                         // checking end of file 

{

 infile.getline(arr,30);                   // getting input from file line by line

                                                    //written in file.  

 cout<<arr<<endl; 

} 

 system("pause");

 }

Output:

 writing to file

I am in first semester of BS

I am studying at UOG

Press any key to continue......

 

!infile.eof( ) checks for the end of file opened by handle infile and in while condition remains true until the end of file is not reached. Loop iterates and prints all record on .exe file.

If we declare file handle with this method instead of given in above programs.

 fstream file;

 file.open(“filename.txt”);

 Then we use open( ) then we can use different modes to open the file.

 

ios::in	Open file for reading (default for ifstream)
ios::out	Open file for writing (default for ofstream)
ios::ate	Start reading and writing at the end of file
ios::app	Start writing at the end of file
ios::binary	Open file to perform read and write in binay
ios::trunc	Truncate file to zero length if exist

 

void main()

{

 char arr[30] = "Writing to file"; 

 fstream infile;              // declaring file handle 

 infile.open("First.txt",ios::app | ios::in | ios::out);  // creating file 

 if(!infile)                 // !infile check file is opend or not 

{

  cout<<"Unable to openfile"<<endl;  

 system("pause");  

exit(0);                    // include in process.h or stdlib.h 

} 

 infile<<arr<<endl; 

infile<<"I am in first semester of BS"<<endl; 

infile<<"I am studying at UOG"<<endl; 

infile<<"Zafar Mehmood Khattak"<<endl; 

 infile.seekp(0);  // moving file pointer at start of file. 

 while(!infile.eof()) 

{  

infile.getline(arr,30);             // getting input from file  

cout<<arr<<endl; 

} 

infile.close();   

system("pause");

} 

Now if you run program again and again then size of file goes on increasing and after every run output will become large and large.

cin.getline, cin.get() 

To overcome the trouble with cin being word oriented, with that I mean it reads single words from input and if words consist of more then one word, like New York, cin buffers York, and use New to store in for example your char array variable. 

Luckily the developers of the library add some inteligent functions to the cin object. Created from the class isstream. 

cin.getline(), and cin.get(), both read a line of input at a time until they find the '\n' character, but cin.getline() then discards the newline character, whereas cin.get() leaves it in the input queue. 

cin.getline() takes two arguments. The first argument is the name of the array destined to hold the line of input, and the second argument is a limit on the number of characters to be read. If this limit is, say, 20, the function reads no more then 19 characters, leaving room to automaticaly add the null character at the end. The getline() member function stops reading input when it reaches this numeric limit or when it reads the newline character, whichever comes first. 

example: 

cin.getline(name,20); 

This reads the entire line into name array, provided that the line consists of 19 or fewer characters.

Writing and Reading String data to/from file:

 

#include <fstream>

#include <iostream>

using namespace std;  

int main ()

{        

char data[100];    

 // open a file in write mode.  

   ofstream outfile;   

   outfile.open("afile.dat"); 

   cout << "Writing to the file" << endl;   

   cout << "Enter your name: ";    

   cin.getline(data, 100); 

// write inputted data into the file.   

   outfile << data << endl; 

   cout << "Enter your age: ";    

   cin >> data;  

   cin.ignore();

// again write inputted data into the file.   

   outfile << data << endl; 

// close the opened file.   

   outfile.close(); 

// open a file in read mode.   

   ifstream infile;    

   infile.open("afile.dat");     

   cout << "Reading from the file" << endl; 

   infile >> data;  

// write the data at the screen.   

   cout << data << endl;       

// again read the data from the file and display it.   

    infile >> data;     cout << data << endl;  

// close the opened file.   

     infile.close(); 

     return 0;

}