Filehandlinging cp2


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File Handeling in C++

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Filehandlinging cp2

  2. 2. Introduction  All programs we looked earlier:  input data from the keyboard.  output data to the screen.  Difficult to handle large amount of input data.  Output would also be lost as soon as we exit from the program.  How do we store data permanently?.  We can use secondary storage device.  Data is packaged up on the storage device as data structures called files.
  3. 3. Files (Streams)  Files are used to store data in a relatively permanent form, on floppy disk, hard disk, tape or other form of secondary storage. Files can hold huge amounts of data if need be. Ordinary variables (even records and arrays) are kept in main memory which is temporary and rather limited in size. Lets put it in points…………..
  4. 4. Why use files? Convenient way to deal with large quantities of data. • Store data permanently (until file is deleted). • Avoid having to type data into program multiple times. • Share data between programs. •
  5. 5. The following is a comparison of the two types of storage………..
  6. 6. Main memory • Made up of RAM chips. • Used to hold a program when it is running, including the values of its variables (whether integer, char, an array, etc.) Secondary memory • Usually a disk magnetic tape). drive (or • Used to hold files (where a file can contain data, a program, text, etc.) • Can hold rather large amounts of data.
  7. 7. Main memory Secondary memory • Can only hold relatively small amounts of data. • Can hold rather amounts of data. • Is temporary (as soon as the program is done or the power goes out all of these values are gone). • • Gives fast access to the data (all electronic). Is fairly permanent. (A file remains even if the power goes out. It will last until you erase it, as long as the disk isn't damaged, at least.)  Access large to the data is considerably slower (due to moving parts).
  8. 8. I/O in C++  I/O in C++ uses streams  A Stream is a general name given to flow of data.
  9. 9. Flow of Data…. PROGRAM Input Stream >> (Extraction operator) istream class DEVICES OR Data FILES Data Output Stream << (Insertion operator) ostream class
  10. 10. More About Files…..  Now 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
  11. 11. I/O in C++  Different streams are used to represent different kinds of data flow.  Each stream is associated with a particular class, which contains member functions and  definitions for dealing with that particular kind of data flow. 
  12. 12. File Related Classes The following classes in C++ have access to file input and output functions:  ifstream  ofstream  fstream
  13. 13. The Stream Class Hierarchy ios NOTE : UPWARD ARROWS INDICATE THE BASE CLASS istream get() getline() read() >> fstreambase Ifstream Open() Tellg() Seekg() fstream iostream ostream put() write() << Ofstream Open() Tellp() Seekp()
  14. 14. OPENING A FILE (Associating a stream with a file) 1. By using the CONSTRUCTOR of the stream class. ifstream transaction(“sales.dly”); ofstream result(“result.02”); 2. By using the open() function of the stream class ifstream transaction;“sales.dly”);
  15. 15. File Mode Parameters PARAMETER  ios::app  ios::ate  ios::binary  ios::in  ios::nocreate  ios::noreplace  ios::out  ios::trunc MEANING Append to end-of file goto end of file on opening binary file Open existing file for reading open fails if file doesn’t exist open fails if file already exists creates new file for writing on Deletes contents if it exists The mode can combine two or more modes using bit wise or ( | )
  16. 16. Checking For Successful File Opening ifstream transaction(“sales.dly”); if (transcation == NULL) { cout<<“unable to open sales.dly”; cin.get(); // waits for the operator to press any key exit(1); }
  17. 17. Closing of File Stream_name.close(); e.g., transaction.close(); fl.close(); Note : There is no need to give the physical file name at the time of closing a file.
  18. 18. Types of Files  The two basic types of files are  Text files &  Binary files
  19. 19. Text Files A text file consists of readable characters separated into lines by newline characters.  (On most PCs, the newline character is actually represented by the two-character sequence of carriage return (ASCII 13), line feed (ASCII 10). (n)
  20. 20. Binary Files A binary file stores data to disk in the same form in which it is represented in main memory.  If you ever try to edit a binary file containing numbers you will see that the numbers appear as nonsense characters.
  21. 21. Binary Files  Not having to translate numbers into a readable form makes binary files somewhat more efficient.  Binary files also do not normally use anything to separate the data into lines.  Such a file is just a stream of data with nothing in particular to separate components.
  22. 22. Text Files When using a text file, we write out separately each of the pieces of data about a given record. The text file will be readable by an editor Binary Files  When using a binary file we write whole record data to the file at once.  but the numbers in the binary file will not be readable in this way.
  23. 23. The programs to create the data files will differ in how they open the file and in how they write to the file.  for the text file we will use the usual output operator(<<) and will output each of the pieces of the record separately.  For the binary file we will use write to write to the file,  with the text file we will read each of the pieces of record from the file separately, using the usual input operator(>>)  With the binary file we will use the read function to read a whole record,
  24. 24. Types of File Access :Sequential access. With this type of file access one must read the data in order, much like with a tape, whether the data is really stored on tape or not. Random access (or direct access). This type of file access lets you jump to any location in the file, then to any other, etc., all in a reasonable amount of time.
  26. 26. FILE POINTERS  Each file object has two integer values associated with it :  get pointer  put pointer  These values specify the byte number in the file where reading or writing will take place.
  27. 27. File pointers…..  By default reading pointer is set at the beginning.  By default writing pointer is set at the end (when you open file in ios::app mode)  There are times when you must take control of the file pointers yourself so that you can read from and write to an arbitrary location in the file.
  28. 28. Functions associated with file pointers :  The seekg() and tellg() functions allow you to set and examine the get pointer.  The seekp() and tellp() functions allow you to set and examine the put pointer.
  29. 29. seekg() function : (with one argument)  With one argument : fl.seekg(k); fl.seekp(k); where k is absolute position from the beginning. The start of the file is byte 0 It will result in moving the pointer as shownFile Begin k bytes ^ File pointer End
  30. 30. ‘seek’ functions : (With two arguments ) fl.seekg(offset, refposition); fl.seekp(offset, refposition); Number of bytes file pointer to be moved Location from where File pointer is to be moved Refposition takes one of the following forms : •ios::beg Start of the file •ios::cur current position of the pointer •ios::end End of the file
  31. 31. File Pointer offset calls •fl.seekg(0,ios::beg); Go to start •fl.seekg(0,ios::cur); Stay at the current position •fl.seekg(0,ios::end); Go to the end of file •fl.seekg(m,ios::beg);Move to (m+1)th byte in the file
  32. 32. File Pointer offset calls •fl.seekg(m,ios::cur); Go forward by m bytes from current pos •fl.seekg(-m,ios::beg); Go backward by m bytes from current pos •fl.seekg(-m,ios::cur); Go backward by m bytes from the end
  33. 33. seekg() function : (With two arguments ) : Begin End •fl.seekg(m,ios::beg); m bytes ^ Offset from Begin Move to (m+1)th byte in the file End Begin fl.seekg(-m,ios::cur); Offset from end ^ m bytes Go backward by m bytes from the end End Begin fl.seekg(m,ios::cur); m bytes ^ Offset from current position Go forward by m bytes from current pos
  34. 34. EXAMPLES  Creation of a text file
  35. 35. Program to generate coded file…… (Text File) #include <fstream.h> #include <conio.h> #include <stdio.h> void main() { clrscr(); char c,d,ans; char str[80]; ofstream outfl("try.txt"); do { cout<<"please give the string : "; gets(str); outfl<<str; cout <<"do you want to write more...<y/n> : "; ans=getch(); } while(ans=='y'); outfl<<'0'; outfl.close(); clrscr(); ifstream infl; getch(); cout <<"reading from created file n";"try.txt");"cod.dat"); //********************************** c=infl.get(); do { d=c+1; cout<<c<<d<<'n'; out.put(d); c= infl.get(); } while (c!='0'); out<<'0'; infl.close(); outfl.close(); getch(); //********************************* }
  36. 36. The End