1. File Processing in C++
Data is stored in files so that it may be retrieved for processing when needed

2. File Processing in C++
C++ provides the following classes to perform output and input of characters to/from files:
ofstream: Stream class to write on files
ifstream: Stream class to read from files
fstream: Stream class to both read and write from/to files.

3. Open a file
Each one of the open() member functions of the classes ofstream, ifstream and fstream has a default mode that is used if the file is opened without a second argument:
class
default mode parameter
ofstream
ios::out
ifstream
ios::in
fstream
ios::in | ios::out

4. Reading data from files using C++
The commands for reading data from a file are (for the most part) very similar to those used to read data from the keyboard.

5. Reading data from files using C++
In order for your program to read from the file, you must:
include the fstream header file
declare a variable of type ifstream
open the file
read from the file
after each read, check for end-of-file using the eof() member function
close the file when access is no longer needed (optional, but a good practice)

6. Example
Q1: Write a program that reads integer numbers from the following file and print the result in output screen.

7. Reading data from files using C++
#include<iostream>
#include <fstream>
using namespace std;
int main()
{ ifstream indata; // indata is like cin
int num; // variable for input value
indata.open(“example.txt",ios::in);
indata >> num;
while ( !indata.eof() ) {
cout << "The next number is " << num << endl; }
indata.close();
cout << "End-of-file reached.." << endl;

8. Reading data from files using C++
The result:

9. Example
Q2: Write a program that reads characters from the following file as long as the character not Q. and print the result in output screen.

10. Reading data from files using C++
#include<iostream>
#include <fstream>
using namespace std;
void main()
{ ifstream indata; // indata is like cin
char letter; // variable for input value
indata.open("example.txt“,ios::in); // opens the file
indata >> letter;
while ( letter != 'Q' ) {
cout << letter; }
cout << endl;
indata.close();
cout << "End-of-file reached.." << endl;

11. Reading data from files using C++
The result:

12. Writing data in the files using C++
In order for your program to writein the file, you must:
include the fstream header file
declare a variable of type ofstream
open the file
Writ in the file
close the file when access is no longer needed (optional, but a good practice)

13. Writing data in the files using C++
#include <iostream> #include <fstream> using namespace std; void main () { ofstream myfile; myfile.open("test1.txt",ios::out); myfile << "Writing this to a file......\n"; myfile.close(); }

14. Writing data in the files using C++

15. Example
Q3: Write a program that read 4integer numbers from the user and write them in a file .then read the numbers from a file and print them in output screen.

16. Input/Output with files
#include <iostream>
#include <fstream>
using namespace std;
void main(){
ofstream obj_out("file_05.txt",ios::out);
int x;
for(int i=0;i<=3;i++) {
cin >> x;
obj_out << x << endl; }
ifstream obj_in("file_05.txt",ios::in);
obj_in >> x;
while(!obj_in.eof())
cout << "X from the file is: " << x << endl;

18. Input/Output with files
#include <iostream>
#include <fstream>
using namespace std;
void main(){
// To write in a file:
ofstream obj_out("file_02.txt",ios::out);
int x;
while(cin >> x)
obj_out << x << endl;
// To stop writing to a file, press CTRL+Z then ENTER
// To read from a file:
ifstream obj_in("file_02.txt",ios::in);
while(obj_in >> x)
cout << "X from the file is: " << x << endl; }

19. Input/Output with files

20. Example
Q: Write a program that copies a content of a file into a one-dimensional array. Assume that the content of the file is integers.

21. Solution #include<iostream> #include<fstream>
using namespace std;
void main() {
int x, c=0;
ifstream fin1("d:\\test.txt", ios::in);
while(fin1>>x)
c++;
ifstream fin2("d:\\test.txt", ios::in);
int *a = new int[c];
for(int i=0; i<c; i++)
fin2 >> a[i];
// To print the content of the array
cout << a[i] << endl; }

22. We can use the same variable from type fstream for input and output in the same time as the following example.

23. Input/Output with files
#include <iostream>
#include <fstream>
using namespace std;
void main(){
fstream obj("file_05.txt");
int x;
for(int i=0;i<=3;i++) {
cin >> x;
obj << x << endl; }
obj.close();
obj.open("file_05.txt");
obj>> x;
while(!obj.eof())
cout << "X from the file is: " << x << endl;

24. Example
Q: Write a program that copies a content of a file into a one-dimensional array. Assume that the content of the file is string.

25. Solution
#include<iostream> #include<fstream> #include<string> using namespace std; void main() { string x; int c=0; ifstream fin1("test.txt"); while(fin1>>x) c++; fin1.close(); ifstream fin2("test.txt“); //note: we can use the same variable fin1.open("test.txt“); string *a = new string[c]; for(int i=0; i<c; i++) fin2 >> a[i]; // To print the content of the array cout << a[i] << endl; }