Study Material for Class XII. Data File Handling

Size: px

Start display at page:

Download "Study Material for Class XII. Data File Handling"

Amie Fletcher
5 years ago
Views:

1 Study Material for Class XII Page 1 of 5 Data File Handling Components of C++ to be used with handling: Header s: fstream.h Classes: ifstream, ofstream, fstream File modes: in, out, in out Uses of cascaded operators ( << & >>) for writing text to the and reading text from the. Functions like: open(), close(), get(), getline(), put(), tellg(), seekg(), tellp() and seekp() Detecting end of s ( With or without using eof() function) Opening a binary using in, out and in out modes. Header : fstream.h Classes : ifstream, ofstream and fstream As we all know all system functions and tools in c++ is provided in a header. For example string.h contains all system functions required for handling such as strcmp(), strcpy() etc. Like wise when we dealing with handling all necessary functions have been clubbed together in fstream.h header. fstream.h header besides the other thing (classes and functions) contains three very important classes. These classes are : ifstream : To open a data in read only mode ofstream : To open a data in write only mode fstream : To open a in read/write mode The last one fstream class is more flexible one but it increases the compiler overheads and therefore it is always advised that we should use the most appropriate class for a data to be opened. The following program opens a data (a text ) and after reading its content displays on to the monitor. Progam#1 : Program for reading a Program#2 : Program for writing on to a #include <fsteam.h> // Using header while(afile.eof()) // checking for EOF #include <fsteam.h> // Using header ofstream afile( StName.txt ); // opening a afile.get(ch); // reading while(ch!= * ) // checking for * to end data writing ch=getch(); // reading afile.put(ch); // displaying read data Note: Here StName.txt is a text containing name of students that we are reading and displaying Note: Here we are first reading a character from the user and then writing on to the StName.txt File Modes And opening a with modes: This is an information given at the time of opening or creation of a. It decides that what we can do with the. In the above program it is important to note that we have not mentioned that whether the is to be opened in reading or writing mode but the program runs successfully. This is because of the default mode associated with ifstream and ofstream classes respectfully. Following are the modes used in C++:

2 Page 2 of 5 in : Read only mode. Used for reading purposes. This is default mode for ifstream class with this mode an existing is opened with all its data intact. But if the doesn t exist then a blank will be opened. out: Write only mode. Used for writing purposes. This is default mode for ofstream class. With this mode an existing with same name is replaced with a new blank and the blank is opened. In such a situation we will lost the old existing. If the doesn t exit straight way a new blank is opened. app: To append (add) data at the end of the. This mode is used along with out mode. ate: To fix the pointer at the end the of when it is opened. With this we write data any where in the even on the existing data by shifting the pointer position. nocreate: To open only an existing. This mode can be used along with both in and out modes. noreplace: To open only a non-existing ( i.e. only a new ). This mode can be used along with both in and out modes. trunc: With this mode all the data of the is deleted and a blank is opened. From the above description of modes we can also conclude that we can use one or more than modes when we create or open a. In the above program opening statements can also be given as Progam#1 : Program for reading a ifstream afile( StName.txt, ios::in); // opening a Program#2 : Program for writing on to a ofstream afile( StName.txt, ios::out); // opening Few examples of opening statements: ifstream afile( StName.txt ); Here mode is ios::in which has been passed implicitly. ifstream afile( StName.txt, ios::in); Here explicitly mode ios::in has been specified. ofstream X( Story.dat ); Here mode is ios::out which has been passed implicitly. Here a new blank Story.txt will be created and if it is already existing then all the data of the will be lost. ofstream X( Story.dat,ios::out); Here mode is ios::out which has been passed explicitly. Here a new blank Story.txt will be created and if it is already existing then all the data of the will be lost. fstream X( Story.dat ); This one is a wong Satement as we have not given the mode. It is important to note that whenever we open or create a with the help of fstream class we must always pass in, out or in out (both) modes. i.e. there is no default mode for the class fstream. Correct statement for the above will be as fstream X( Story.dat,ios::in ios::out); // Here we can both read from and write onto the fstream X( Story.dat,ios::in ios::nocreate); Here a Story.dat will be opened for reading but only when the is existing. fstream X( Story.dat,ios::out ios::noreplace); Here a new Story.dat will be opened for writing but only when the is non-existing. The entire above said concept works equally good for binary also. a Uses of cascaded operator (<< & >>) for reading and writing onto a text : Perhaps the easiest method to read and write from/to a is with the help of cascaded << (insertion) and >> (extraction) operators. << : Used for writing onto the >> : Used for reading from the The following shows how we use these two operators.

3 Page 3 of 5 Progam#3 : Program for reading a #include <fstream.h> ifstream X( MyFile.txt ); // Opening the char *s; while(!x.eof()) X>>s; // reading from the cout<<s; X. close(); Program#4 : Program for writing on to a #include <fstream.h> ofstream X( MyFile.txt ); // Creating the //Data writing on to the X<< \n A pointer is an object that points to the particular location in the. X<< It is created with the help of the class fstream, ifstream and ofstream. ; X. close(); Functions like: open(), close(), get(), getline(), put(), tellg(), seekg(), tellp() and seekp() Open(): This is member of all three classes we are using in handling namely ifstream, of stream and fstream. We can open/create a in ways. Either using the constructors or using the open() function. e.g. i) ifstream X( Story.txt ); // This method uses a constructor of class ifstream to create the ii) ifstream X; X. open( Story.txt ); // This method uses member function open() of class ifstream to create the close() : This one is also member of three classes namely ifstream, ofstream and fstream. This is used to close a. e.g. ofstream X( Story.txt );.. X.c lose(); get(): This function is used to read a character from a. It is member of ifstream class. e.g. ifstream X( Story.txt ); X.g et(ch); getline(): This is used for reading a line (string) from standard input device. This function receives three parameters and their uses are as follows: First parameter is a storage variable name where the string is stored. Second parameter specifies the the length of the string to be read from the input buffer. Third parameter is a delimiting character that specifies that after this character no other character from the input buffer will be read. This function can also be used for reading from a. e.g. char *s; cin.getline(s,20, * ); Case1 If we enter We are Indians. Then s will store We are Indians. Case2 If we enter We are*indians. Then s will store We are* Case3 If we enter We are Indians. And we are proud of it. Then s will store We are Indians. And ie 20 chars. Case4 If we enter We are Indians. And we are proud*of it. Then s will store We are Indians. And ie 20 chars. put(): This one is a member of ofstream class and is used for writing a single character on to a. e.g. ofstream X( Story.txt );

4 X.p ut( P ); Page 4 of 5 tellg(): It gives the current get- pointer (read mode) position in a data. tellp(): It gives the current put- pointer (write mode) position in a data. seekg(): This is used to shift get- pointer position in a data. e.g. ifstream X( Story.txt );. X.s eekg(10); Here pointer X will be shifted 10 bytes in forward direction. To move the pointer in backward direction we pass a ve no. This function receives one more parameter known as offset that can be any of the three namely ios::beg, ios::end, and ios::cur. ios::beg : To move the pointer from the beginning of the. ios::end : To move the pointer from the end of the. ios::cur : To move the pointer from the current position of the pointer. e.g. X.seekg(10,ios::beg); 10 bytes form the beginning of the. seekp(): Same as above only that it works with put- pointer and is a member of ofstream class. Detecting end of with or without using eof() function: A pointer in a data keeps on moving or changing its position with every read(get- pointer) or write(put- pointer) operation. Following are the methods to find whether the pointer has reached to end of the or not. Program#5 : Detecting EOF with eof() #include <fsteam.h> // Using header while(!afile.eof()) // checking for EOF afile.get(ch); // reading Program#6 : Detecting EOF without eof() #include <fstream.h> // Using header while(afile) // checking for EOF afile.get(ch); // reading In program#5 afile.eof() returns true if afile is at the EOF or false otherwise. And therefore we are using while (!afile.eof()) so that until and unless afile is at EOF while loop will iterate. Here note the! operatetor. In program#6 afile returns false if afile is at the EOF or true otherwise. And therefore we are using while (afile) so that until and unless afile is at EOF while loop will iterate.

5 Questions: Page 5 of 5 Q1. Differentiate between the following a) ios::app and ios::ate b) ios::nocreate and ios::noreplace c) get() and put() d) ios::trunc and ios::out e) tellg() and seekg() Q2. What is stream object or pointer? Q3. What is a stream? Q4. Given the class definition class STOCK int ITNO; char ITEM[10]; public: void GETIT() cin>>itno; gets(item); void SHOIT() cout<<itno<< <<ITEM <<endl; ; Fill in the blanks for the following program: Void read_obj() ifstream X; X. open( stock.dat, ::out ::in); If(!X) cout<< \n Cannot open ; exit(1); STOCK sk; While(!X. ) X.read( ( )&sk,sizeof( )); SHOWIT(); X.close(); Q5. Assuming that a text named FIRST.TXT contains some text written into it, write a function named vowelwords(), that reads the s FIRST.TXT and creates a new named SECOND.TXT, to contain only those words from the FIRST.TXT which start with a lower case vowel ( ie a, e, i, o, u ). For example, if the FIRST.TXT contains Carry umbrella and overcoat when it rains Then the second should contain umbrella and overcoat it Hint: Here you have to read first.txt word by word and check for the first character whether it is lowercase vowel or not. If it is then you have to write word on to the second.txt. For this purpose you should use >> operator for reading first.txt and << operator for writing on to the second.txt. Q3. Write a c++ program to replace each space of text Books.dat with *. Hint: Here has to be read character by character and if the character is a space then we have to write on to the same place with * character. For this purpose open the in R/W mode. And use seekg() or seekp() function properly to write on the correct place of the.

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