Reading & Writing to Files

1. Reading & Writing to Files The Ohio State University

2. helloworld.cpp #include <iostream> using namespace std; int main() { cout << "Hello World!" << endl; cout << "Goodbye World!" << endl; return 0; } The Ohio State University

3. helloworld.cpp #include <iostream> using namespace std; int main() { cout << "Hello World!" << endl; cout << "Goodbye World!" << endl; return 0; } >helloworld.exe Hello World! Goodbye World! > The Ohio State University

4. Unix: Redirecting Output to a File • On Unix we can redirect output to a file using the ‘>’ symbol: > helloworld.exe > hellofile.txt > The Ohio State University

5. Writing to Files The Ohio State University

6. I/O File Streams • A File Stream is a one-way transmission path used to connect a file stored on a physical device (disk, CD-ROM, etc) to a program. • Two directions: • Input File Stream (read from a file) • Output File Stream (write to a file) • To use file stream operations we #include <fstream> The Ohio State University

7. Writing to Files: Output File Stream • To write a file: • Create an output stream object • Establish connection to a file • Check Validity • Start writing • Close the file The Ohio State University

8. writeFile1.cpp #include <cstdlib> // function exit() is in cstdlib #include <fstream> // class ofstream() is in fstream #include <iostream> using namespace std; int main() { ofstream fout; // declare an output file stream fout.open("hellofile.txt", ios::out); // open file file_name for output if (!fout.is_open()) // check if file is opened for output { cerr << "Unable to open file hellofile.txt." << endl; exit(10); } The Ohio State University

9. writeFile1.cpp (cont) cout << "Writing to file hellofile.txt." << endl; // write text to the file fout << "Hello World!" << endl; fout << "Goodbye World!" << endl; // close file stream fout fout.close(); return 0; } The Ohio State University

10. Writing to Files (1) • “Create an output stream object” • Make sure we included the <fstream> class. • Create a variable of type ofstream. For example, ofstream fout; • The fout variable is what we would also call an “output file handler” The Ohio State University

11. Writing to Files (2) • “Establish a connection to a file” • Open the file “hellofile.txt” for output: fout.open(“hellofile.txt”, ios::out); The Ohio State University

12. Writing to Files (3) • “Check Validity” • Many things can prevent our file from being opened. • For instance, we may not have permission to write to hellofile.txt. These runtime errors must be handled before we do any writing! if (!fout.is_open()) // check if file is open { cerr << "Unable to open file hellofile.txt." << endl; exit(10); } The Ohio State University

13. Writing to Files (3) • “Check Validity” (continued…) • In the previous code, exit() is a function that terminates program execution. • So, why not just use return 0; as we have been all this time to indicate termination of main()? • We could have, but exit() actually performs some routine maintenance before termination (including file stream cleanup). • To use exit() we must first #include <cstdlib> The Ohio State University

14. Writing to Files (4) • “Start writing” • Now that we are certain our file handler, fout, is valid we can start writing to the file that is associated with it. • This should look familiar... fout << “Hello World” << endl; • This will print “Hello World” to hellofile.txt The Ohio State University

15. Writing to Files (5) • “Close the file” • After we are finished writing to our file, it is always important to close the file: fout.close(); • Why is this a good idea? • Can now reuse fout handler for other file writes • If we are not careful, our program could be keeping many files open… which can lead to very bad things. The Ohio State University

16. Formatted Writing to Files • Notice the similarities between cout and fout. • In fact, everything we can do with cout also applies to fout. Including I/O manipulation: setprecision(), setw(), etc. • Though we named our file handler fout to strike name similarities with cout, we could have actually named it anything arbitrary (just like other variables). The Ohio State University

17. writeFile2.cpp #include <cstdlib> // function exit() is in cstdlib #include <fstream> // class ofstream() is in fstream #include <iomanip> #include <iostream> using namespace std; int main() { ofstream fout; // declare an output file stream fout.open("sample.txt" , ios::out); // open file file_name for output if (!fout.is_open()) // check if file is opened for output { cerr << "Unable to open file sample.txt." << endl; exit(10); } The Ohio State University

18. writeFile2.cpp (cont) cout << "Writing to file sample.txt." << endl; // write to the file fout << "Test file output." << endl; fout << "100.0/3.0 = " << 100.0/3.0 << endl; fout.precision(12); fout << "100.0/3.0 = " << 100.0/3.0 << endl; fout << "100.0/3.0 = " << fixed << 100.0/3.0 << endl; // close file stream fout fout.close(); return 0; } The Ohio State University

19. writeMultiFile.cpp . . . ofstream fout1; // declare output file stream 1 ofstream fout2; // declare output file stream 2 fout1.open("book1.txt" , ios::out); // open book1.txt fout2.open("book2.txt", ios::out); // open book2.txt if (!fout1.is_open()) { cerr << "Unable to open file book1.txt." << endl; exit(10); } if (!fout2.is_open()) { cerr << "Unable to open file book2.txt." << endl; exit(15); } The Ohio State University

20. writeMultiFile.cpp (cont) cout << "Writing to files book1.txt and book2.txt." << endl; // write to book1.txt fout1 << "Assets: $" << 10000 << endl; fout1 << "Liabilities: $" << 15000 << endl; // write to book2.txt fout2 << "Assets: $" << 12000 << endl; fout2 << "Liabilities: $" << 9000 << endl; fout1.close(); // close file stream fout1 fout2.close(); // close file stream fout2 return 0; } The Ohio State University

21. Asking for the File Name • Read the input file name into a string: #include <string> string file_name; cout << “Enter file name: “; cin >> file_name; • Unfortunately, the open() function only takes C style strings. Convert a C++ string into a C style string: file_name.c_str(); • Call fout.open() using the C style string: fout.open(file_name.c_str(), ios::out); The Ohio State University

22. writeFile3.cpp . . . #include <fstream> #include <string> // type string is in file “string” . . . int main() { ofstream fout; // declare an output file stream string file_name; cout << "Enter file name: "; cin >> file_name; // file_name.c_str() returns a C style string fout.open(file_name.c_str(), ios::out); // open file file_name for output if (!fout.is_open()) // check if file is opened for output { cerr << "Unable to open file " << file_name << endl; exit(10); } The Ohio State University

23. writeFile3.cpp (cont) cout << "Writing to file " << file_name << endl; // write text to the file fout << "Hello World!" << endl; fout << "Goodbye World!" << endl; // close file stream fout fout.close(); return 0; } The Ohio State University

24. Reading from Files The Ohio State University

25. Reading from Files: Input File Stream • To read from a file: • We need a-priori knowledge of the file format • Create an input stream object • Establish connection to a file • Check Validity • Start reading • Close the file The Ohio State University

26. readFile1.cpp #include <cstdlib> // function exit() is in cstdlib #include <fstream> // class ofstream() is in fstream #include <iostream> using namespace std; int main() { ifstream fin; // declare an input file stream int x; fin.open("intList1.txt" , ios::in); // open file intList.txt for input if (!fin.is_open()) // check if file is open for input { cerr << "Unable to open file intList1.txt." << endl; exit(10); } ... The Ohio State University

27. readFile1.cpp (cont) // read text from file fin >> x; cout << "Read integer: " << x << endl; // close file stream fin fin.close(); return 0; } The Ohio State University

28. … // read text from file fin >> x; cout << "Read integer: " << x << endl; … > readFile1.exe Read integer: 77 > File: intList1.txt 77 65 28 33 112 The Ohio State University

29. Reading from Files (1) • “Have prior knowledge of file format” • Does the input file contain integers, floating point numbers, or strings? • Files can be some combination of different types, e.g.: 523 Warren Harding 3.89 334 William McKinley 3.21 … • For each row, the first item is an integer, the second is a string, the third is a string, and the last is a double. The Ohio State University

30. Reading from Files (2) 2. “Create an input stream object” • Again, make sure we included the <fstream> class. • Create a variable of type ifstream. For example, ifstream fin; • The fin variable is what we would also call an “input file handler” The Ohio State University

31. Reading from Files (3) 3. “Establish a connection to a file” • Open the file “intList1.txt”: fin.open(“intList1.txt”, ios::in); The Ohio State University

32. Reading from Files (4) 4. “Check Validity” • Again, we should check that the file is able to be opened and read. // check if file is open for input if (!fin.is_open()) { cerr << "Unable to open file " << file_name << endl; exit(10); } The Ohio State University

33. Reading from Files (5) 5. “Start reading” • Because we know that the file contains integers, we read the data into a variable of type int : int x; // read text from file fin >> x; The Ohio State University

34. Reading from Files (6) 6. “Close the file” • Like before, we close the file after we are done using it: fin.close(); The Ohio State University

35. readFile2.cpp . . . #include <fstream> #include <string> . . . int main() { ifstream fin; // declare an input file stream string file_name; int x; cout << "Enter file name: "; cin >> file_name; fin.open(file_name.c_str(), ios::in); // open file file_name for input if (!fin.is_open()) // check if file is open for input { cerr << "Unable to open file " << file_name << endl; exit(10); } The Ohio State University

36. readFile2.cpp (cont) // read text from file for (int i = 1; i <= 5; i++) { fin >> x; cout << "Read integer: " << x << endl; } // close file stream fin fin.close(); return 0; } The Ohio State University

37. … // read text from file for (inti = 1; i <= 5; i++) { fin >> x; cout << "Read integer: " << x << endl; } … File: intList1.txt 77 65 28 33 112 > readFile2.exe Enter file name: intList1.txt Read integer: 77 Read integer: 65 Read integer: 28 Read integer: 33 Read integer: 112 > The Ohio State University

38. … if (!fin.is_open()) // check if file is open for input { cerr << "Unable to open file " << file_name << endl; exit(10); } … > readFile2.exe Enter file name: missing.txt Unable to open file missing.txt > The Ohio State University

39. … // read text from file for (inti = 1; i <= 5; i++) { fin >> x; cout << "Read integer: " << x << endl; } … File: intList3.txt 10 20 > readFile2.exe Enter file name: intList2.txt Read integer: 10 Read integer: 20 Read integer: 20 Read integer: 20 Read integer: 20 > The Ohio State University

40. readFile3.cpp . . . int main() { ifstream fin; // declare an input file stream string file_name; int x; cout << "Enter file name: "; cin >> file_name; fin.open(file_name.c_str(), ios::in); // open file file_name for input if (!fin.is_open()) // check if file is open for input { cerr << "Unable to open file " << file_name << endl; exit(10); } The Ohio State University

41. readFile3.cpp (cont) // read text from file fin >> x; while (!fin.fail()) { cout << "Read integer: " << x << endl; fin >> x; } // close file stream fin fin.close(); return 0; } The Ohio State University

42. fail() • The fail() function is true when the read fails. • A read fails because: • The end of file is reached. • Read error: Trying to read a character string as an integer. • Once an input operation fails, all subsequent input operations will fail. The Ohio State University

43. fail() • To read all integers in a file: fin >> x; while (!fin.fail()) { // Process x... fin >> x; } The Ohio State University

44. … // read text from file fin >> x; while (!fin.fail()) { cout << "Read integer: " << x << endl; fin >> x; } … File: intList1.txt 77 65 28 33 112 > readFile3.exe Enter file name: intList1.txt Read integer: 77 Read integer: 65 Read integer: 28 Read integer: 33 Read integer: 112 > The Ohio State University

45. … // read text from file fin >> x; while (!fin.fail()) { cout << "Read integer: " << x << endl; fin >> x; } … File: intList1.txt 10 20 30 40 50 60 70 > readFile3.exe Enter file name: intList2.txt Read integer: 10 Read integer: 20 Read integer: 30 Read integer: 40 Read integer: 50 Read integer: 60 Read integer: 70 > The Ohio State University

46. … // read text from file fin >> x; while (!fin.fail()) { cout << "Read integer: " << x << endl; fin >> x; } … File: intList3.txt 10 20 > readFile3.exe Enter file name: intList3.txt Read integer: 10 Read integer: 20 > The Ohio State University

47. readFile4.cpp . . . int main() { ifstream fin; // declare an input file stream string file_name; int x; cout << "Enter file name: "; cin >> file_name; fin.open(file_name.c_str(), ios::in); // open file file_name for input if (!fin.is_open()) // check if file is open for input { cerr << "Unable to open file " << file_name << endl; exit(10); } The Ohio State University

48. readFile4.cpp (cont) // read text from file fin >> x; while (fin) // equivalent to while (!fin.fail()) { cout << "Read integer: " << x << endl; fin >> x; } // close file stream fin fin.close(); return 0; } The Ohio State University

49. fail() ifstream fin; • The condition: while(fin) is equivalent to: while(!fin.fail()) The Ohio State University

50. fail() • The fail() function is true when the read fails. • A read fails because: • The end of file is reached. • Read error: Trying to read a character string as an integer. • Once an input operation fails, all subsequent input operations will fail. The Ohio State University