C++ files and streams. Lec 28-31

Transcription

1 C++ files and streams Lec 28-31

2 Introduction So far, we have been using the iostream standard library, which provides cin and cout methods for reading from standard input and writing to standard output respectively. To perform file processing in C++, header files <iostream> and <fstream> must be included in your C++ source file.

3 Files A file is a collection on information, usually stored on a computer s disk. Information can be saved to files and then later reused. All files are assigned a name that is used for identification purposes by the operating system File Name and Extension File Contents and the user. MYPROG.BAS MENU.BAT INSTALL.DOC CRUNCH.EXE BOB.HTML 3DMODEL.JAVA INVENT.OBJ PROG1.PRJ ANSI.SYS README.TXT BASIC program DOS Batch File Documentation File Executable File HTML (Hypertext Markup Language) File Java program or applet Object File Borland C++ Project File System Device Driver Text File 3

4 Process of Using a File Using a file in a program is a simple three-step process The file must be opened. If the file does not yet exits, opening it means creating it. Information is then saved to the file, read from the file, or both. When the program is finished using the file, the file must be closed. 4

5 Contd 5

6 File Input/Output Before file I/O can be performed, a C++ program must be set up properly. File access requires the inclusion of fstream.h Before data can be written to or read from a file, the file must be opened. ifstream inputfile; inputfile.open( customer.dat ); 6

7 Example This program demonstrates the declaration of an fstream object and the opening of a file. 1. #include <iostream> 2. #include <fstream> 3. using namespace std; 4. int main() 5. { 6. fstream datafile; // Declare file stream object 7. char filename[81]; 8. cout << "Enter the name of a file you wish to open\n"; 9. cout << "or create: "; 10. cin.getline(filename, 81); 11. datafile.open(filename, ios::out); 12. cout << "The file " << filename << " was opened.\n"; Output: 13. return 0; 14. } Enter the name of a file you wish to open or create: mystuff.dat The file mystuff.dat was opened. 7

8 Opening a File at Declaration fstream datafile( names.dat, ios::in ios::out); This program demonstrates the opening of a file at the time the file stream object is declared. 1. #include <iostream> 2. #include <fstream> 3. using namespace std; 4. int main() 5. { 6. fstream datafile("names.dat", ios::in ios::out); 7. cout << "The file names.dat was opened.\n"; 8. return 0; 9. } Output: The file names.dat was opened. 8

9 Testing for Open Errors datafile.open( cust.dat, ios::in); if (!datafile) { cout << Error opening file.\n ; } datafile.open( cust.dat, ios::in); if (datafile.fail()) { cout << Error opening file.\n ; } 9

10 Closing a File A file should be closed when a program is finished using it. This program demonstrates the close function. 1. #include <iostream> 2. #include <fstream> 3. using namespace std; 4. int main() 5. { fstream datafile; Output: File was created successfully. Now closing the file. 6. datafile.open("testfile.txt", ios::out); 7. if (!datafile) 8. { } cout << "File open error!" << endl; return 0; 9. cout << "File was created successfully.\n"; 10. cout << "Now closing the file.\n"; 11. datafile.close(); 12. return 0; } 10

11 File Default Open Modes File Type ofstream Default Open Mode The file is opened for output only. (Information may be written to the file, but not read from the file.) If the file does not exist, it is created. If the file already exists, its contents are deleted (the file is truncated). ifstream The file is opened for input only. (Information may be read from the file, but not written to it.) The file s contents will be read from its beginning. If the file does not exist, the open function fails. 11

12 File Mode Flag Meaning File Mode Flags ios::app ios::ate ios::binary ios::in ios::nocreate ios::noreplace ios::out ios::trunc Append mode. If the file already exists, its contents are preserved and all output is written to the end of the file. By default, this flag causes the file to be created if it does not exist. If the file already exists, the program goes directly to the end of it. Output may be written anywhere in the file. Binary mode. When a file is opened in binary mode, information is written to or read from it in pure binary format. (The default mode is text.) Input mode. Information will be read from the file. If the file does not exist, it will not be created and the open function will fail. If the file does not already exist, this flag will cause the open function to fail. (The file will not be created.) If the file already exists, this flag will cause the open function to fail. (The existing file will not be opened.) Output mode. Information will be written to the file. By default, the file s contents will be deleted if it already exists. If the file already exists, its contents will be deleted (truncated). This is the default mode used by ios::out. 12

13 Write on file The stream insertion operator (<<) may be used to write information to a file. outputfile << I love C++ programming!

14 Example This program uses the << operator to write information to a file. 1. #include <iostream> 2. #include <fstream> 3. using namespace std; 4. int main() 5. { fstream datafile; 6. char line[81]; Output: File opened successfully. Now writing information to the file. Done. 7. datafile.open("demofile.txt", ios::out); 8. if (!datafile) 9. { cout << "File open error!" << endl; return 0; } 10. cout << "File opened successfully.\n"; 11. cout << "Now writing information to the file.\n"; 12. datafile << "Jones\n"; 13. datafile << "Smith\n"; 14. datafile.close(); 15. cout << "Done.\n"; return 0; } 14

15 Example This program writes information to a file, closes the file, then reopens it and appends more information. 1. #include <iostream> 2. #include <fstream> 3. using namespace std; 4. int main() 5. { fstream datafile; 6. datafile.open("demofile.txt", ios::out); 7. datafile << "Jones\n"; 8. datafile << "Smith\n"; 9. datafile.close(); 10. datafile.open("demofile.txt", ios::app); 11. datafile << "Willis\n"; 12. datafile << "Davis\n"; 13. datafile.close(); 14. return 0; } 15

16 Read from file The stream extraction operator (>>) may be used to read information from a file. 16

17 Example This program uses the >> operator to read information from a file. 1. #include <iostream> 2. #include <fstream> 3. using namespace std; 4. int main() 5. { fstream datafile; 6. char name[81]; Output: File opened successfully. Now reading information from the file. Jones Smith Willis Davis Done. 7. datafile.open("demofile.txt", ios::in); 8. if (!datafile) 9. { cout << "File open error!" << endl; return 0; } 10. cout << "File opened successfully.\n"; 11. cout << "Now reading information from the file.\n"; 12. for (int count = 0; count < 4; count++) 13. { datafile >> name; cout << name << endl; } 14. datafile.close(); 15. cout << "Done.\n"; 16. return 0; } 17

18 Detecting the End of a File The eof() member function reports when the end of a file has been encountered. if (infile.eof()) infile.close(); In C++, end of file doesn t mean the program is at the last piece of information in the file, but beyond it. The eof() function returns true when there is no more information to be read. 18

19 Example This program uses the file stream object's eof() member function to detect the end of the file. 1. #include <iostream> 2. #include <fstream> 3. using namespace std; 4. int main() 5. { fstream datafile; 6. char name[81]; 7. datafile.open("demofile.txt", ios::in); 8. if (!datafile) Done. 9. { cout << "File open error!" << endl; return 0; } 10. cout << "File opened successfully.\n"; 11. cout << "Reading information from the file.\n"; 12. datafile >> name; // Read first name from the file 13. while (!datafile.eof()) 14. { cout << name << endl; 15. datafile >> name; } 16. datafile.close(); 17. cout << "\ndone.\n"; 18. return 0; } 19 File opened successfully. Reading information from the file. Jones Smith Willis Davis

20 Member Functions for Reading and Writing Files File stream objects have member functions for more specialized file reading and writing. 20

21 Example This program uses the file stream object's eof() member function to detect the end of the file. 1. #include <iostream> Output: 2. #include <fstream> Jones 3. using namespace std; Smith 4. int main() Willis 5. { fstream namefile; Davis 6. char input[81]; namefile.open("demofile.txt", ios::in); 7. if (!namefile) 8. { cout << "File open error!" << endl; return 0; } 9. namefile >> input; 10. while (!namefile.eof()) 11. { cout << input << endl; namefile >> input; } 12. namefile.close(); 13. return 0; 21

22 The getline Member Function datafile.getline(str, 81, \n ); str This is the name of a character array, or a pointer to a section of memory. The information read from the file will be stored here. 81 This number is one greater than the maximum number of characters to be read. In this example, a maximum of 80 characters will be read. \n This is a delimiter character of your choice. If this delimiter is encountered, it will cause the function to stop reading before it has read the maximum number of characters. (This argument is optional. If it s left our, \n is the default.) 22

23 Example This program uses the file stream object's getline member function to read a line of information from the file. Output: 1. #include <iostream> Jones 2. #include <fstream> Smith 3. using namespace std; 4. int main() Willis 5. { fstream namefile; Davis 6. char input[81]; namefile.open("demofile.txt", ios::in); 7. if (!namefile) 8. { cout << "File open error!" << endl; return 0; } 9. namefile.getline(input, 81); // use \n as a delimiter 10. while (!namefile.eof()) 11. { cout << input << endl; 12. namefile.getline(input, 81); // use \n as a delimiter } 13. namefile.close(); 14. return 0; } 23

24 The get Member Function This program asks the user for a file name. The file is opened and its contents are displayed on the screen. 1. #include <iostream> 2. #include <fstream> 3. using namespace std; 4. int main() 5. { fstream file; 6. char ch, filename[51]; 7. cout << "Enter a file name: "; 8. cin >> filename; 9. file.open(filename, ios::in); 10. if (!file) 11. { cout << filename << could not be opened.\n"; return 0; } 12. file.get(ch); // Get a character 13. while (!file.eof()) 14. { cout << ch; file.get(ch); // Get another character } 15. file.close(); 16. return 0; } 24

25 The put Member Function This program demonstrates the put member function. 1. #include <iostream> 2. #include <fstream> using namespace std; 3. int main() 4. { fstream datafile("sentence.txt", ios::out); 5. char ch; cout << "Type a sentence and be sure to end it with a "; 6. cout << "period.\n"; 7. while (1) 8. { cin.get(ch); 9. datafile.put(ch); 10. if (ch == '.') 11. break; 12. } 13. datafile.close(); 14. return 0; } Type a sentence and be sure to end it with a period. I am on my way. Resulting Contents of the File SENTENCE.TXT: I am on my way. Output: 25

26 Unformatted I/O with read and write read and write member functions Unformatted I/O Input/output raw bytes to or from a character array in memory Since the data is unformatted, the functions will not terminate at a newline character for example Instead, like getline, they continue to process a designated number of characters If fewer than the designated number of characters are read, then the failbit is set.

27 File pointers to read/write from binary files To write n bytes: write (const char* buffer, int n); To read n bytes (to a pre-allocated buffer): read (char* buffer, int num)

28 Example 1. #include<iostream> 2. #include <fstream> 3. using namespace std; 4. int main() 5. { int a[] = {10,23,3,7,9,11,25}; 6. fstream fs; 7. fs.open("myfile.txt", ios::binary ios::out); 8. fs.write((char*) &a, sizeof(a)); 9. fs.close(); 10. for(int i = 0; i < 7; i++) a[i] = 0; 11. fs.open("myfile.txt", ios::in ios::binary); 12. fs.read((char*) &a, sizeof(a)); 13. for(int i = 0; i < 7; i++) cout << a[i] << " "; 14. fs.close(); }

29 Random Access Files Random Access means non-sequentially accessing information in a file. 29

30 Mode Flags Mode Flag ios::beg ios::end ios::cur Description The offset is calculated from the beginning of the file. The offset is calculated from the end of the file. The offset is calculated from the current position. 30

31 Contd Statement File.seekp(32L, ios::beg); file.seekp(-10l, ios::end); file.seekp(120l, ios::cur); file.seekg(2l, ios::beg); file.seekg(-100l, ios::end); file.seekg(40l, ios::cur); file.seekg(0l, ios::end); How it Affects the Read/Write Position Sets the write position to the 33rd byte (byte 32) from the beginning of the file. Sets the write position to the 11th byte (byte 10) from the end of the file. Sets the write position to the 121st byte (byte 120) from the current position. Sets the read position to the 3rd byte (byte 2) from the beginning of the file. Sets the read position to the 101st byte (byte 100) from the end of the file. Sets the read position to the 41st byte (byte 40) from the current position. Sets the read position to the end of the 31 file.

32 File position pointers Both istream and ostream provide member functions for repositioning the file-position pointer. These member functions are seekg ("seek get") for istream and seekp ("seek put") for ostream. The seek direction can be ios::beg (the default) for positioning relative to the beginning of a stream ios::cur for positioning relative to the current position in a stream ios::end for positioning relative to the end of a stream.

33 Continued position to the nth byte of fileobject (assumes ios::beg) fileobject.seekg( n ); position n bytes forward in fileobject fileobject.seekg( n, ios::cur ); position n bytes back from end of fileobject fileobject.seekg( n, ios::end ); position at end of fileobject fileobject.seekg( 0, ios::end );

34 Example This program demonstrates the seekg function. 1. #include <iostream> 2. #include <fstream> 3. using namespace std; 4. int main() 5. { fstream file("demofile.txt", ios::in); 6. char ch; 7. file.seekg(5l, ios::beg); 8. file.get(ch); 9. cout << "Byte 5 from beginning: " << ch << endl; 10. file.seekg(-10l, ios::end); 11. file.get(ch); 12. cout << "Byte 10 from end: " << ch << endl; 13. file.seekg(3l, ios::cur); 14. file.get(ch); 15. cout << "Byte 3 from current: " << ch << endl; 16. file.close(); 17. return 0;} 34