Data file handling


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Data file handling

  1. 1. Data File Handling in C++ Edited by: Dr. Muhammad Tanvir Afzal Computer Science Muhammad Ali Jinnah University Islamabad A A A A A A
  2. 2. All rights reserved 2 Topics - Agenda • Introduction • Opening & closing of files • Stream state member functions • File operations • Binary file operations • Random access file operations • Conclusion
  3. 3. All rights reserved 3 Introduction • Computer programs are associated to work with files as it helps in storing data & information permanently. • File - itself a bunch of bytes stored on some storage devices. • In C++ this is achieved through a component header file called fstream.h • The I/O library manages two aspects- as interface and for transfer of data. • The library predefine a set of operations for all file related handling through certain classes.
  4. 4. The fstream.h header file Streams act as an interface between files and programs. They represent as a sequence of bytes and deals with the flow of data. Every stream is associated with a class having member functions and operations for a particular kind of data flow. File  Program ( Input stream) - reads Program  File (Output stream) – write All designed into fstream.h and hence needs to be included in all file handling programs. Diagrammatically as shown in next slide
  5. 5. All rights reserved 5
  6. 6. File Handling Classes Hierarchy Diagram
  7. 7. Why to use Files: •Convenient way to deal large quantities of data. •Store data permanently (until file is deleted). •Avoid typing data into program multiple times. •Share data between programs. We need to know: how to "connect" file to program how to tell the program to read data how to tell the program to write data error checking and handling EOF
  8. 8. All rights reserved 9 // Initial experience reading and writing files #include <fstream.h> #include <iostream.h> #include <stdlib.h> int main() { ifstream in_stream; ofstream out_stream; int num;"numbers.dat"); if ( { cout << "Input file could not be opened.n"; exit(1); }"squares.dat"); if ( { cout <<"Output file could not opened.n"; exit(1); } in_stream >> num; out_stream << "The square of " << num << " is " <<num * num; in_stream.close(); out_stream.close(); }
  9. 9. File Handling Classes • When working with files in C++, the following classes can be used: – ofstream – writing to a file – ifstream – reading for a file – fstream – reading / writing • What does it all have to do with cout? – When ever we include <iostream.h>, an ostream object, pointing to stdout is automatically defined – this object is cout. • ofstream inherits from the class ostream (standard output class). • ostream overloaded the operator >> for standard output.…thus an ofstream object can use methods and operators defined in ostream.
  10. 10. Opening & Closing a File  A file can be open by the method “open()” or immediately in the constructor (the natural and preferred way). void ofstream / ifstream::open(const char* filename, int mode);  filename – file to open (full path or local)  mode – how to open (1 or more of following – using | ) ios::app – append ios::ate – open with marker at the end of the file ios::in / ios::out – (the defaults of ifstream and ofstream) ios:nocreate / ios::noreplace – open only if the file exists / doesn’t exist ios::trunc – open an empty file ios::binary – open a binary file (default is textual)  Don’t forget to close the file using the method “close()”
  11. 11. All rights reserved 12 1: To access file handling routines: #include <fstream.h> 2: To declare variables that can be used to access file: ifstream in_stream; ofstream out_stream; 3: To connect your program's variable (its internal name) to an external file (i.e., on the Unix file system):"infile.dat");"outfile.dat"); 4: To see if the file opened successfully: if ( { cout << "Input file open failedn"; exit(1); // requires <stdlib.h>}
  12. 12. All rights reserved 13 5: To get data from a file (one option), must declare a variable to hold the data and then read it using the extraction operator: int num; in_stream >> num; [Compare: cin >> num;] 6: To put data into a file, use insertion operator: out_stream << num; [Compare: cout << num;] NOTE: Streams are sequential – data is read and written in order – generally can't back up. 7: When done with the file: in_stream.close(); out_stream.close();
  13. 13. Stream state member functions •In C++, file stream classes inherit a stream state member from the ios class, which gives out the information regarding the status of the stream. For e.g.: –eof() –used to check the end of file character –fail()- used to check the status of file at opening for I/O –bad()- used to check whether invalid file operations or unrecoverable error . –good()- used to check whether the previous file operation has been successful
  14. 14. File operations The following member functions are used for reading and writing a character from a specified file. get()- is used to read an alphanumeric character from a file. put()- is used to write a character to a specified file or a specified output stream
  15. 15. Reading /Writing from/to Binary Files • To write n bytes: – write (const unsigned char* buffer, int n); • To read n bytes (to a pre-allocated buffer): – read (unsighed char* buffer, int num) #include <fstream.h> main() { int array[] = {10,23,3,7,9,11,253}; ofstream OutBinaryFile("my_b_file.txt“, ios::out | ios::binary); OutBinaryFile.write((char*) array, sizeof(array)); OutBinaryFile.close(); }
  16. 16. All rights reserved 17 C++ has some low-level facilities for character I/O. char next1, next2, next3; cin.get(next1); Gets the next character from the keyboard. Does not skip over blanks or newline (n). Can check for newline (next == 'n') Example: cin.get(next1); cin.get(next2); cin.get(next3); Predefined character functions must #include <ctype.h> and can be used to convert between upper and lower case test whether in upper or lower case test whether alphabetic character or digit test for space CHARACTER I/O
  17. 17. All rights reserved 18 //#include, prototypes, void main() omitted for space ifstream fin; char Chem1, Chem2; double ratio;"input.dat"); // open error check omitted for space fin.get(Chem1); while (!fin.eof()) { if (isdigit(Chem1)) cout << "Test Code: " << Chem1 << endl; else { fin >> ratio; fin.get(Chem2); Chem2 = toupper(Chem2); cout << "Ratio of " << Chem1 << " to " << Chem2 << " is " << ratio << endl; } new_line(fin); fin.get(Chem1); } } void new_line(istream& in) { char symbol; do { in.get(symbol); } while (symbol != 'n'); }
  18. 18. Reading /Writing from/to Textual Files • To write: – put() – writing single character – << operator – writing an object • To read: – get() – reading a single character of a buffer – getline() – reading a single line – >> operator – reading a object #include <fstream.h> main() { // Writing to file ofstream OutFile("my_file.txt"); OutFile<<"Hello "<<5<<endl; OutFile.close(); int number; char dummy[15]; // Reading from file ifstream InFile("my_file.txt"); InFile>>dummy>>number; InFile.seekg(0); InFile.getline(dummy,sizeof(dummy)); InFile.close(); }
  19. 19. All rights reserved 20 Binary file operations In connection with a binary file, the file mode must contain the ios::binary mode along with other mode(s) To read & write a or on to a binary file, as the case may be blocks of data are accessed through the use of C++ read() and write() respectively.
  20. 20. All rights reserved 21 Random access file operations Every file maintains two internal pointers: get_pointer and put_pointer They enable to attain the random access in file otherwise which is sequential in nature. In C++ randomness is achieved by manipulating certain functions
  21. 21. Moving within the File • seekg() / seekp() – moving the reading (get) / writing (put) marker – two parameters: offset and anchor • tellg() / tellp() – getting the position of the reading (get) / writing (put) marker
  22. 22. All rights reserved 23 EOF and File I/O Within Functions When using a file within a function, the file parameter MUST BE a reference parameter: int read_file(ifstream& infile); As with keyboard input, it is often desirable to process some unknown amount of data. We use the end-of-file (EOF) to accomplish this. EOF is automatically included in text files we create. Can test for EOF in several ways. while (in_stream >> num) OR in_stream >> num; // priming read while (! in_stream.eof()) {loop body in_stream >> num;}
  23. 23. All rights reserved 24 Summary • Files in C++ are interpreted as a sequence of bytes stored on some storage media. • Bases classes are used to perform I/O operations. • The data of a file is stored in either readable form or in binary code called as text file or binary file. • The flow of data from any source to a sink is called as a stream.
  24. 24. All rights reserved 25 1. When getting data from a file, there is no need to prompt for input. 2. One program may have multiple input and/or output files, and may intermix keyboard/ display I/O with file I/O. 3. The file name may be obtained from the user, rather than hard coded in the program. 4. The layout of a program's output is called the format. A variety of options are available for controlling the appearance of the output. 5. Flags to control floating point display: • out_stream.setf(ios::fixed); • out_stream.setf(ios::showpoint); • out_stream.precision(2); 6. rd_state() – returns a variable with one or more (check with AND) of the following options: • ios::goodbit – OK • ios::eofbit – marker on EOF • ios::failbit – illegal action, but alright to continue • ios:badbit – corrupted file, cannot be used. 7. We can also access the bit we wish to check with eof(), good(), fail(), bad(), 8. clear() is used to clear the status bits (after they were checked). More Information on File I/O