File Processing Files are used for data persistance-permanent retention of large amounts of data. Computer store files on secondary storage devices,such as magnetic disks,optical disks and tapes. Create a file Update a file Process data files

Files and Stream When a file is opened, an object is created and a stream is associated with the object Cin, cout, cerr and clog are created when is included. The streams associated with these objects provide communication channels between a program and a particular file or device. Example: cin object (standard input stream object) enables a program to input data from the keyboard or from other devices

Cont… Cout object (standard output stream object) enables a program to output data to the screen. Cerr and clog objects(standard error stream objects) enable a program to output error message.

Cont… File processing – header files and must be included. Header includes the definitions for the stream class templates basic _if stream(for file input), basic _of stream (for file output) and basic_fstream(for file input and output). Each class template has a predefined template specialization that enables char I/O.

Cont… Files are opened by creating objects of these stream template specializations. These templates “derive” from class templates basic_istream, basic_ostream and basic_iostream respectively. All member functions,operators and manipulators that belong to these templates also can be applied to file streams.

Cont… Details of file stream classes: Filebuf - its purpose is to set the file buffers to read and write. Contains open() of file stream classes, also contains close() and open() as members. Fstreambase - Provides operations common to the file streams.serves as a base for fstream,ifstream and ofstream classes. Contains open() and close() functions.

Cont… Ofstream – provides output operations. Contains open() with default output mode. inherits put(),seek(),write() functions from ostream. Fstream – Provides support for simultaneous input and output operations. Contains open() with default input mode. Inherits all the functions from istream and ostream classes through iostream.

Creating a sequential file Record does not exist in c++ file. The programmer must structure files to meet the application’s requirements The file stream classes support a number of member functions for performing the input and output operations on files. Put() and get() are designed for handling a single character at a time.

Cont… The function put() writes a single character to the associated stream. The function get() reads a single character from the associated stream. Example: #include Main() { Char string(20); Cout<<“Enter a string\n”; Cin>> string; Int len = strlen(string); Fstream file; //input and output stream File.open (“Text”, ios:: in : ios::out); For (int i=0; i<len; i++) file.put(string[i] ); \\ put a character to file File.seekg(0); Char ch; While (file) { file.get(ch); \\get a character from file cout << ch; \\display it on screen Out put: Enter a string C programming input C programming output

Reading data from a sequential file Since fstream object can handle both the input and output simultaneously We can open the file in ios::in and ios::out mode. To read the entire file File.seekg(0); Files store data and it may be retrieved for processing when needed.

Cont… Creating an if stream object opens a file for input. The ifstream constructor can receive the filename and file open mode as arguments. The arguments in parentheses are passed to the ifstream constructor function, which opens the file and establishes a “line of communication” with the file.

Cont… #include Using std :: cerr; Using std :: cout; Using std :: endl; Using std :: fixed; Using std :: ios; Using std :: left; Using std :: right; Using std :: showpoint;

Cont… # include Using std :: ifstream; //input file stream # include Using std :: setw; Using std :: setprecision; #include Using std:: string; #include Using std ::exit; //exit function prototype Void outputline (int,const string,double);

Cont… Int main() { // ifstream constructor opens the file Ifstream inclientfile (“clients.dat”,ios::in); //exit program if ifstream could not open file If (!inclientfile) { cerr<<“file could not be opened”<<endl; exit(1); } //end if

Cont… Int account; Char name[30]; Double balance; Cout<<left<<setw (10)<< “account” <<setw (13) <<“name”<<“balance”<<endl<<fixed<<showp oint; //display each record in file

Cont… //display each record in file While (inclientfile>>account>>name>>balance) outputLine(account,name,balance); return 0; } //end main //display single record from file Void outputLine(int account,const string name,double balance)

Cont… { cout <<left<<setw(10) <<account <<setw(13) <<name <<setw(7) <<setprecision(2) <<right<<balance<<endl; }//end function outputLline Account Name Balance 100 Jones Doe white

Creating a binary file Pair of functions, write() and read() designed to write and read binary data The values are stored in disk file in the same format in which they are stored in the internal memory. An int takes two bytes to store its value in the binary form, irrespective of its size. The binary format is more accurate for storing the numbers as they are stored in the exact internal representation. There are no conversions while saving the data and therefore saving is much faster.

Cont… Binary format : 2 bytes Character format: 4 bytes

Cont… These infile.read((char*) &v, sizeof(v)); outfile.write((char*)&v,sizeof(v)); These functions take two arguments. The first is the address of the variable v, and the second is the length of that variable in bytes. The address of the variable must be cast to type char* (i.e. pointer to character type).

Cont… # include Const char * filename = “BINARY”; Main() { float height(4)=(20.3, 34.0, 56.7, 48.7); ofstream outfile(filename); outfile.write((char*) & height, size of (height)); outfile.close(); //close file for reading for (int i=0; i<4; i++) //clear array from memory height(i) =0; ifstream infile(filename); infile.read((char*) & height, size of (height)); for (i=0; i< 4; i++) { cout.setf(ios::showpoint); cout<<setw(10) << setprecision(2) <<height(i); } infile.close(); } Output: 20.3,34.0,56.7,48.7:

Updating sequential file Data that is formed and written to a sequential file cannot be modified without the risk of destroying other data in the file Example If the name “white” needs to be changed to “liva” the old name cannot be overwritten without corrupting the file

Random-access files, If all the records in a file be of the same fixed length. Using same-size, fixed-length records Data can be inserted into a random-access file without destroying other data in the file. Data stored previously also can be updated or deleted without rewriting the entire file.

Creating a random –access file The ostream member function write outputs a fixed number of bytes, beginning at a specific location in memory, to the specified stream. When the stream is associated with a file,function write writes the data at the location in the file specified by the “put”file-position pointer. The istream member function read inputs a fixed number of bytes from the specified stream to an area in memory beginning at a specified address.

Cont… If the stream is associated with a file,function read inputs bytes at the location in the file specified by the “get” file-position pointer Writing bytes with ostream member function write -When writing an integer number to a file, instead of using statement outfile<<number; Converting between pointer types with the reinterpert_cast operator

Writing and updating random-access file Writes data to the file and uses the combination of fstream functions seekp and write to store data at exact locations in the file. Function seekp sets the “put” file-position pointer to a specific position in the file and then write outputs the data. Example #INCLUDE Using std ::cerr; Using std ::cin; Using std ::cout; Using std :: endl; Using std ::ios; #include Using std::setw; #include Using std :: exit; #include “clientdata.h”

Cont… Int main() { int accountnumber; Char lastname(15); Char firstname(10); Double balance; Fstream outcredit(“credit.dat”,ios::in/ios::out/ios::binary); If (!outcredit) { cerr<<“file could not be opened.”<<endl; exit(1); } Cout<<“enter account number ( 1 to 100, 0 to end input)\n?”;

Cont… Clientdata client; Cin>>accountnumber; While (accountnumber> 0 && accountnumber<= 100) Cout<<“enter lastname, first name, balance\n?”; Cin>>setw(15) >> lastname; Cin>>setw(10) >> firstname; Cin>> balance; Client.setaccountnumber(accountnumber); Client.setlastname(lastname); Client.setfirstname(firstname); Client.setbalance(balance);