1. Basic File I/O and Stream Objects
Andy Wang
Object Oriented Programming in C++
COP 3330

2. Input and Output to and from Files
File input and output is an essential in programming
Most software involves more than keyboard input and screen user interfaces
Data needs to be stored somewhere when a program is not running
That means writing data to storage
We need file input and file output techniques
In C++, file IOs are similar to working with cin and cout

3. Kinds of Files Formatted text vs. binary files
A text file is made of readable text characters
Looks like output printed to the screen through cout
A binary file contains unformatted data
Saved in its raw memory format
E.g., the integer is saved in a four-byte chunk

4. Kinds of Files Sequential vs. Random access files
A sequential file is written or read from start to finish
A random access file stores records of the same size
Can read or write single records in place without affecting the rest of the file
This set of slides only deals with sequential text files

5. Creating file stream objects
cout and cin are objects
cout is the standard output stream
Represents the monitor
Of the type ostream
cin is the standard input stream
Represents the keyboard
Of the type istream

6. To create file stream objects
Include the <fstream> library
#include <fstream>
using namespace std;
To declare file stream objects
ofstream outputFile;
ifstream inputFile;

7. To attach file streams to files
outputFile.open(“outfile.txt”);
inputFile.open(“inputFile.txt”);
Will open always work?
A file may not exist
You may not have the permission
The directory may not be writable
The file name may contain illegal characters

8. To check a valid file is attached
if (!inputFile) { // inputFile not attached to a valid file cout << “Sorry, bad file.”; exit(0); // abort program // need to include <cstdlib> }

9. To detach from the stream object
inputFile.close();
outputFile.close();
The stream object can be attached to another file
inputFile.open(“inputFile2.txt”);

10. Using File Streams
Once a file stream object is attached to a file, it can be used with the same syntax as cin and cout
inputFile >> x >> y >> z;
outputFile << “Hello, World” << endl;

11. Output File Example
o/set1/numout.cpp

12. numout.cpp
#include <iostream> #include <fstream> using namespace std; int main() { ofstream fout; fout.open(“datafile.txt”); if (!fout) { cerr << “Attempt to create file failed\n”; exit(1); }

13. numout.cpp
int x, y, z; cout << “Type in 3 integers: “; cin >> x >> y >> z; for (int j = 1; j <= 10; j++) { fout << x*j << ‘\t’; fout << ‘y*j << ‘\t’; fout << ‘z*j << ‘\t’; } cout << “All done!\n”;

14. numout.cpp
fout << endl; fout.close(); fout.open(“logfile.txt”); if (!fout) { exit(0); } fout << “Please disperse. Nothing to see here\n”; fout << “Please move along.\n”; fout << “Go away already!\n”; return 0; }

15. Input File Example
o/set1/numin.cpp

16. numin.cpp
#include <iostream> #include <fstream> using namespace std; int main() { ifstream fin; fin.open(“datafile.txt”); if (!fin) { cerr << “Attempt to open file failed\n”; exit(1); }

17. numin.cpp
int x, y, z; for (j = 1; j <= 10; j++) { fin >> x >> y >> z; cout << x << “ + ” << y << “ + “ << z << “ = “ << (x + y + z) << endl; } cout << “All done!\n”; fin.close(); return 0;

18. Opening a file in ‘append mode’
Default
Open an output file and begin writing from the beginning
Existing file can be opened for output
New output is tacked to the end (appending)
To open a file in append mode
ofstream fout;
Fout.open(“file.txt”, ios::app);

19. Append Mode Example
o/set1/numapp.cpp

20. numapp.cpp
#include <iostream> #include <fstream> using namespace std; int main() { ofstream fout; fout.open(“datafile.txt”, ios::app); if (!fin) { cerr << “Attempt to open file failed\n”; exit(1); }

21. numapp.cpp
int x, y, z; cout << “Type in 3 integers: “; cin >> x >> y >> z; for (j = 0; j <= 10; j++) { fout << x*j << ‘\t’; fout << y*j << ‘\t’; fout << z*j << ‘\t’; } cout << “All done!\n”;

22. numapp.cpp
fout << endl; fout.close(); return 0; }

23. User-entered file names
File names don’t have to be hard coded
To store a file name, we can declare the following
char filename[20]; // 19 char file name
A user can enter a single word name into this variable
cin >> filename;
We can use this variable in the open() function
ofstream fout;
fout.open(filename);

24. User Chosen Output File Example
o/set1/userfile.cpp

25. userfile.cpp
#include <iostream> #include <fstream> using namespace std; int main() { char ofilename[25]]; ofstream out1; do { out1.clear(); //clear status flags cout << “Name of the output file? ”; cin >> ofilename;

26. userfile.cpp
out1.open(ofilename); if (!out1) cout << “Not a valid file. Try again!\n”; } while (!out1); out1 << “Hello, World\n”; out1.close(); cout << “Processing complete\n”; return 1; }

27. User Chosen IO File Example
o/set1/userio.cpp

28. userio.cpp
#include <iostream> #include <fstream> using namespace std; int main() { char ofilename[25]]; ifstream in1; ofstream out1; do { in1.clear(); //clear status flags cout << “Name of the input file? ”; cin >> ofilename;

29. userfile.cpp
in1.open(ofilename); if (!in1) cout << “Not valid. Try again!\n”; } while (!in1); do { out1.clear(); cout << “Name of the output file? “; cin >> filename; out1.open(filename); if (!out1) cout << “Not valid, try again!\n”; } while (!out1);

30. userfile.cpp
int x, y, z; in1 >> x >> y >> z; out1 << z << endl; out2 << y << endl; out2 << z << endl; in1.close(); out1.close(); cout << “Processing complete\n”; return 1; }