DISK FILE PROGRAM. ios. ofstream

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1 [1] DEFINITION OF FILE A file is a bunch of bytes stored on some storage media like magnetic disk, optical disk or solid state media like pen-drive. In C++ a file, at its lowest level is interpreted simply as a sequence, or stream of bytes AND, a file the user level is sequence of intermixed data types characters, mathematical values, class objects etc. FILE STREAM CLASSES Stream is a flow of data which is a sequence of bytes. 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. The stream that supplies data to the program is known as input stream. It reads the data from the file and hands it over to the program. The stream that receives data from the program is known as output stream. It writes the received data to the file. It clear form the following figure: DISK FILE Output stream Input stream Data output PROGRAM Data input to Program The file I/O system contains a set of classes that define the file handling method (functions). These classes, designed to manage the disk files are declared in fstream.h, therefore, we must include this file in a program that works with files. The classes defined inside fstream.h are derived from classes under iostream.h (This header file manages console I/O operations). The following figure shows the stream class hierarchy. get( ), getline( ), read( ) seekg( ), tellg( ) ios Put( ), write( ), seekp( ),tellp( ) Console I/O istream ostream streambuf iostream iostream.h file File I/O fstreambase ifstream fstream ofstream filebuf close( ) open( ) DATA FILE The data files are the files that store data permanently pertaining to a specific application, for later use. The data files cab be stored as text files or as binary files. This tutorial is written by: Er. A. Nasra Page-1

2 [2] A text file stores information in ASCII characters. Each line of text is terminated or delimited with special character known as EOL character. Internal translation takes palace when EOL character is read or written. A binary file is just a file that contains information in the same format in which the information is held in memory. There is no delimiter in binary file, and no translation takes place. As a result, binary files are faster and easier for a program to access, read and write than text files. Unless we specify, files are by default are considered and treated as text files. OPENING AND CLOSING A FILE(TEXT FILE) Opening of a file is done in two ways: (i) Using the constructor function of the stream class (for single file) (ii) Using the function open( ) (for multiple files) Closing of a file is done by disconnectint it with the stream it is associated with. This is accomplished by the following statement: stream_object close( ) ; For example, if a file hdfile is connected with an ofstream, then it can be closed as: hdfile.close( ) ; The function close( ) flushes the buffer befor terminating the connection of the file with the stream. OPENING A FILE USING CONSTRUCTORS Let us suppose that we have to open a file named think.dat containing following text: Take your happiness to new level with, Transcend s new PF730 Digital Phot Frame. We open this file by using the constructgor of ofstream class, with thye help of following statement: ofstream hdfile( think.dat ) This statement opens an output stream called hdfile and initialize it with the name think.dat. The think.dat is the name of the file lying on the hard-disk, but in program it will be recognized as HD File. Hence, whatever is written in hdfile stream in the program, will actually be written into the file think.dat. The following program creates the required fiel: #include<iostream.h> //for file I/O main( ) { ofstream hdfile( think.dat ) ; if(!hdfile) { cout<< Can t open this file ; hdfile<< Take your happiness to new level with,\n ; hdfile<< Transcend s new PF730 Digital Phot Frame. ; hdfile.close( ) ; //closing the file THE FUNCTION get( ) This function is same as the operator >>, with the exception that it accepts white or blank spaces in the input string. It stops at \n. This function is used in two ways: (i) get(ch) ; which reads one character at a time form the stream. (ii) get(ch, n) ; which reads a string of character of length (n 1). Example: // This program reads a file named message.dat. int main( ) { ifstream yfile( message.dat ) ; char row[90], ch ; int i ; if(!yile) This tutorial is written by: Er. A. Nasra Page-2

3 [3] { cout<< Can t open file ; for(i = 1 ; i <= 4 ; i++) { yfile.get(row, 80) ; // read a line form the file. yfile.get(ch) ; // lread \n cout << row << /n ; // display the line on screen. yfile.close( ) ; THE FUNCTION getline( ) The format of this function is : getline(ch, size) ; It reads the characters from a line till it encounters \n or till(size 1). Example: // this program reads a file named message.dat int main( ) { ifstream yfile( message.dat ) ; char row[80] ; int i ; if(!yile) { cout<< Can t open file ; for(i = 1 ; i <= 4 ; i++) { yfile.getline(row, 80) ; // read a line form the file. cout << row << /n ; // display the line on screen. yfile.close( ) ; THE FUNCTION put( ) This function writes a single character at a time to the output stream. Fro example the statement afile.put( M ) ; will write a character M into the file named afile. Similarly, the statement bfile.put(v) ; will write the character content of character-variable v into the file named bfile. Example: //this program writes a line of text in a file character by character and displays it. #include<conio.h> int main( ) { char fname[15] ; char text[ ] = Nasra s World of Study. ; char line[30] ; int i ; clrscr( ) ; cout<< Enter the name of the file to be opened ; cin >> fname ; ofstream xfile(fname) ; for( i = 0 ; I < 30 ; i++) { xfile.put( text[i ] ) ; //write a character into the file. xfile.put( \n ) ; xfile.close( ) ; ifstream afile(fname) ; //open the file in Input mode. This tutorial is written by: Er. A. Nasra Page-3

4 [4] afile.getline(line, 30) ;//reads a line form the file. cout<< \n << line ; //display it afile.close( ) ; OPENING A FILE BY USING THE FUNCTION open( ) This function is used to open a file in a program, when we specify the stream to be connected with the file. For illustration, a file named diary.dat can be opened for output by the following set of statements: ofstream afile ; //declares afile of type ofstream (output file stream) afile.open( diary.dat ) // assigns the file stream to the file named diary.dat Thus, in the program the file diary.dat would be identified as afile. The major benefit of this method of opening a file is that more than one files can be opened at a time in a program. Example: //A program that opens an input stream called myfile and associate it with the textfile whose exact copy is to be created. An output stream called docfile will be opened and associated woth the new textfile containing the desired copy. #include<conio.h> main( ) { char infile[15], outfile[15] ; char ch ; clrscr( ) ; cout << \n Enter the input file name ; cin >> infile ; ifstream myfile ; myfile.open(infile) ; if(!myfile) { cout<< \ncan t open input file ; cout<< \n Enter the output file name ; cin >>outfile ; ofstream docfile ; docfile.open(outfile) ; if(!docfile) { cout << \ncan t open input file ; while(myfile) { myfile.get(ch) ; docfile.put(ch) ; myfile.close( ) ; docfike.close( ) ; THE FUNCTION read( ) AND write( ), (BINARY FILES) These functions read and write block of binary data. The general form of these functions are: istream & read((char*) & buf, int sizeof(buf)) ; ostream & write((char*) & buf, int sizeof(buf)) ; The read( ) function reads sizeof(buf) (it can any other integer value also) in bytes from the associated stream and puts them in the buffer pointed to by buf. The write( ) write function writes sizeof(buf) ( it can be any other integer value also) in bytes to the associated stream from the buffer pointed to by buf. This tutorial is written by: Er. A. Nasra Page-4

5 [5] Example:// A program which creates a file with record structure (Name, Roll, Marks in 4 subject, Total marks) and stores the data of all the students of a class in the file. The name of the file is provided by the user. #include<conio.h> main( ) { struct student //declaration of structure. { char name[20] ; int roll, sub1, sub2, sub3, sub4, total ; ; student srec; //declaration of structure variable. int n, i, size ; char fname[15] ;//variable to store file name. cout<< Enter the name of the file to be created ; cin >> fname ; ofstream ofile ; ofile.open(fname) ; //opening the file in output mode. If(!ofile) { cout<< Can t open the file ; size = sizeof(student) ; //computation of size of structure. cout<< Enter the number of student of in the class ; cin >>n ; for ( i = 0 ; i < n ; i++) { clrscr( ) ; cout<< Name : ; cin >>srec.name ; cout<< Roll : ; cin >>srec.roll ; cout<< Sub1 : ; cin >>srec. Sub1 ; cout<< Sub2 : ; cin >>srec. Sub2 ; cout<< Sub3 : ; cin >>srec. Sub3 ; cout<< Sub4 : ; cin >>srec. Sub4 ; srec/tpta; = srec.sub1 + srec.sub2 + srec.sub3 + srec.sub4 ; ofile.write((char*) &srec, size) ; //writing into file. ofile.close( ) ; DETECTING EOF We can detect the end of the file (EOF) by suing the member function eof( ) which has the prototype: int eof( ) ; It returns nonzero when the end of the file has been reached, otherwise it returns zero. For instance, let us consider the following code fragment: ifstream fin ; fin.open( master ) ; while(!fine.eof( ) ) //as long as eof( ) is zero {... //that is, file s end is not reached, process the file.... if( fin.eof( ) ) // if nonzero cout<< End of file is reacher!!\n ; To detect end of file, without using eof( ), we may check whether the stream object has become Null or not e.g., This tutorial is written by: Er. A. Nasra Page-5

6 [6] ifstream fin ; fin.open ( Master ) ; while(!fin) // as long as eof( ) is zero { // that is, the file s end is not reached // process the file. THE CONCEPT OF FILE MODES The filemode describes how a file is to be used i.e. to read from it, to write to it, to append it, and so on. When a stream is associated with a file, either by initializing a file stream object with a file name or by using the open( ) method, the filemode, is of type int, and can be chosen from several constants defined in the ios class. Following table lista the filemodes available with their meaning. S.N. Filemode Parameter Meaning Stream 1. ios : : in It opens file for reading (in input mode) ifstream 2. ios : : out It opens file for writing (in output mode). This also opens the file in ios : : trunc mode, by default. This means an existing file is truncated when opened i.e. its previous contents are discarded. ofstream 3. ios : : ate This seeks to eof upon opening of the file. I/O operations can ofstream still occur anywhere within the file. ifstream 4. ios : : app This appends the data to eof. ofstream 5. ios : : trunc This causes the contents of a pre-existing file by the same name to be destroyed and truncates the file to zero length. ofstream 6. ios : : nocreate This caused the open( ) function to fail if the file does not ofstream already exist. It will not create a new file with that name. 7. ios : : noreplace This caused the open( ) function to file if the file already exists. This used when you want to create a new file and at the same time. ofstream 8. ios : : binary This causes a file to be opened in binary mode (By default, files are opened in text mode.) ofstream ifstream 9. ios : : beg Refers to the beginning of the file. 10. ios : : cur Refers to current position in the file. 11. ios : : end Refers to end of the file. Note: ifstream open( ) has default value ios : : in ofstream open( ) has default value ios : : out fstream open( ) has no default value. Two or more filemode constants can be combined using bitwise OR operator ( ). For example: ofstream fout ; fout.open( Master, ios : : app ios : : nocreate) ; will open a file in the append mode if the file exists or open( ) will fail if the file already does not exists. To open a binary file ios : : binary is specified along with the file mode, e.g., fout.open( master, ios : : app ios : : binary) ; OR fout.open( main, ios : : out ios : : binary ) ; FILE POINTERS AND RANDOM ACCESS Every file maintains two pointers called get_pointer (in input file mode) and put_pointer (in output file mode) which tell the current location in the file where writing or reading will go on. These pointers help manage random access in file. In C++ random access is achieved with the help of seekg( ), seekp( ), tellg( ) and tellp( ). This tutorial is written by: Er. A. Nasra Page-6

7 [7] Function seekg( ) seekp( ) tellg( ) tellp( ) Working Sets current get position in a stream. It works for ifstream object. Sets current put position in a stream. It works for ofstream object. Returns the current get position in a stream. It works for ifstream object. Returns the corrent put position in a stream. It works for ofstream object. Example: ifstreaqm fin ; ofstream fout ; fin.seekg(40) ; // will move the get_pointer to byte number 40 in the file. fout.seekg(40) ; // will move the put_pointer to byte number 40 in the file. The functions seekg( ) or seekp( ) can be used in other form also, as given below: fin.seekg(40, ios : : beg) ; // go the byte no. 40 from beginning to the file linked with fin. Fin.seekg( -4, ios : : cur) ; // back up 4 bytes form current position of get_poiinter. Fin.seekg(0, ios : : end) ; // got the end of the file. Fin.seekg( -7, ios : : end) ; // backup 7 bytes form end of the file. SEARCHING OPERATIONS ON BINARY FILE The search procedure in C++ can be done in two ways: (i) with records implemented through structures. (ii) with records implemented through classes. Let us understand this issue through following examples: Example: // program to search in a file having records maintained through structures. sturct cloth { int clcode ; //cloth code; char name[25] ; char brand[20] ; float price ; cl1 ; void main( ) { int clc ; char found = n ; //clc cloth code. ifstream cfile( cloth.dat, ios : : in) ; // file cloth.dat must exist on disk. cout << Enter cloth code to be search for: ; cin >> clc ; while(!fin.eof( )) { cfile.read((char*) & cl1, sizeof(cl1)) ; if(cl1.clcode == clc) { cout<< cl1.name <<, clcode << clc << has price of Rs. <<cl1.price << only ; found = y ; break ; if(found == n ) cout<< cloth code not found in file!!! <<endl ; cfile.close( ) ; Example: // program to search in a file having records maintained thought classes. #include<fstrea.h> class stu { int rollno ; char name[25] ; char Class[4] ; float marks ; char grade ; public: void getdata( ) { cout << Rollno = ; cin >> rollno ; This tutorial is written by: Er. A. Nasra Page-7

8 [8] cout << Name = ; cin >> name ; cout << Class = ; cin >> Class ; cout << Marks = ; cin >> marks ; if (marks >= 75) grade = A ; else if(marks.= 60) grade = B ; else if(marks.= 50) grade = C ; else if(marks.= 40) grade = D ; else grade = F ; void putdata( ) { cout << name <<, rollno << rollno << has << marks << % mrks and << grade << grade << endl ; int getrno( ) //accessro function. { return rollno ; s1 ; void main( ) { int rn ; char found = n ; ifstream fi( stu.dat, ios : : in) ; //stu.dat must exist in disk. cout << Enter rollno to be searche for ; cin >> rn ; while(!fi.eof( )) { fi.reaed((char*) &s1, sizeof(s1)) ; if(s1.getrno( ) == rn) { s1.putdata( ) ; found = y ; break ; if(found == n ) cout << Rollno not found in file!!! <<endl ; fi.close( ) ; UPDATING A FILE : RANDOM ACCESS Updating means the routine task in the maintenance of any data file. The updating includes one or more of the following tasks: Displaying the contents of a file. Modifying an existing item. Adding / Appending a new item. Deleting an existing item. Example: /* Let us first create a file called STOCK.dat and input some data. Then in the next program we will learn the updating of the same file. */ // This program illustrates how class objects can be written to and read from the disk file. #include<iostream.h> #include<iomanip.h> class INVENTORY { char name[10] ; // item name int code ; // item code float cost ; // cost of each item public: void readdata( ) ; void writedata( ) ; ; This tutorial is written by: Er. A. Nasra Page-8

9 [9] void INVENTORY : : readdata( ) // read from keyboard { cout << Enter name : ; cin >> name ; cout << Enter code : ; cin >> code ; cout << Enter cost : ; cin >> cost ; void INVERTORY : : writedata( ) // formatted display on screen { cout << setiosflags(ios : :left) << setw(10) << name ; << setiosflags(ios: : right) << setw(10) << code ; << setprecision(2) << setw(10) << cost ; << endl ; int main( ) { INVENTORY item[3] ; // declare arr of 3 objects fstream file ; // input and output file file.open( STOCK.dat, ios : : in ios : : out) ; cout << ENTER DETAILS FOR THREE ITEMS \n ; for( int i = 0 ; i < 3 ; i++) { item[ i ].readdata( ) ; file.write((char *) & item[i], sizeof(item[i])) ; file.seekg(0) ; // reset to start cout << \n OUTPUT \n\n ; for( i = 0 ; i < 3 ; i++) { file.read((char *) & item[i], sizeof(item[i])) ; item[i].writedata( ) ; file.close( ) ; The output of the above program would be: ENTER DETAILS FOR THREE ITEMS Enter name : C++ Enter code : 202 Enter cost : 1750 Enter name : FORTRAN Enter code : 204 Enter cost : 1550 Enter name : JAVA Enter code : 225 Enter cost : 2250 OUTPUT C FORTRAN JAVA /* Now, let us understand the program for the updating of the above file named STOCK.dat for five items and performing the operations of appending, modifying and displaying the contents of the file. */ #include<iostream.h> #include<iomanip.h> This tutorial is written by: Er. A. Nasra Page-9

10 [10] class INVENTORY { char name[10] ; // item name int code ; // item code float cost ; // cost of each item public: void getdata( ) { cout << Enter name : ; cin >> name ; cout << Enter code : ; cin >> code ; cout << Enter cost : ; cin >> cost ; void putdata( ) { cout << setiosflags(ios : :left) << setw(10) << name ; << setiosflags(ios: : right) << setw(10) << code ; << setprecision(2) << setw(10) << cost ; << endl ; ; // End of the class definition int main( ) { INVENTORY item ; fstream inoutfile ; // I/O stream inoutfile.open( STOCK.dat, ios : : ate ios : : in ios : : out ios : : binary) ; inoutfile.seekg(0, ios : : beg) ; // go to start cout << CURRENT CONTENTS OF STOCK << endl ; while(inoutfile.read((char *) & item, sizeof(item)) { item.putdata( ) ; inoutfile.clear( ) ; // turn off EOF flag /* Appending one more data */ cout << \n ADD AN ITEM \N ; item.getdata( ) ; char ch ; cin.get(ch) ; inoutfile.write((charr *) & item, sizeof(item)) ; // display the appended file inoutfile.seekg(0) ; // go to the start cout << CONTENTS OF APPENDED FIEL \N ; while(inoutile.ead((char *) & item, sizefo (item)) { item.putdata( ) ; // find number of objects in the file int last = inoutfile.tellg( ) ; int n = last/sizeof(item) ; cout << Number of objects = << n << endl ; cout << Total bytes in the file = << last << endl ; /* MODIFY THE DETAILS OF AN ITEM */ cout << Enter object number to be updated \n ; int object ; cin >> object ; cin.get(ch) ; int location = (object 1) * sizeof(item) ; if(inoutfile.eof( ) ) inoutfile.clear( ) ; inoutfile.seekg(location) ; cout << Ener new values of the object \n ; item.getdata( ) ; cin.get(ch) ; This tutorial is written by: Er. A. Nasra Page-10

11 [11] inoutile.write((char *) & item, sizeof(item)) << flush ; /* SHOW UPDATED FILE */ inoutile.seekg(0) ; // go to the start cout << CONTENTS OF UPDATED FILE \N ; while(inoutfile.read((char *) & item, sizeof(item)) { item.putdata( ) ; inoutfile.close( ) ; // end of main. The output of the above program will be: CURRENT CONTENTS OF STOCK AA BB CC DD XX ADD AN ITEM Enter name : ZZ Enter code : 33 Enter cost : 700 CONTENT OF APPENDED FILE AA BB CC DD XX ZZ Number of objects = 6 Total bytes in the files = 96 Enter object number to be updated 6 Enter new values of he object Enter name : YY Enter code : 66 Enter cost : 477 CONTENT OF UPDATED FILE AA BB CC DD XX YY This tutorial is written by: Er. A. Nasra Page-11

After going through this lesson, you would be able to: store data in a file. access data record by record from the file. move pointer within the file

After going through this lesson, you would be able to: store data in a file. access data record by record from the file. move pointer within the file 16 Files 16.1 Introduction At times it is required to store data on hard disk or floppy disk in some application program. The data is stored in these devices using the concept of file. 16.2 Objectives