Convenient way to deal large quantities of data. Store data permanently (until file is deleted).

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1 FILE HANDLING

2 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

3 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 ofstream: Stream class to write on files ifstream: Stream class to read from files fstream: Stream class to both read and write from/to files.

6 The Stream Class Hierarchy ios istream get() getline() read() >> fstreambase iostream ostream put() write() << Ifstream Open() Tellg() Seekg() fstream Ofstream Open() Tellp() Seekp()

7 Two kinds of files: Text :: contains ASCII codes only Binary :: can contain non-ascii characters Image, audio, video, executable, etc. To check the end of file here, the file size value (also stored on disk) needs to be checked. All rights reserved 7

8 Text File Data is stored as ASCII values Strings, characters and numeric data is stored as ASCII values Each digit occupy one byte of memory space. Data is organized into lines with new-line character as terminators. Any text editor can be used to see content of file. If you use the << operator to read from the file and the >> operator to write to the file then the file will be accessed in text mode Binary File A binary file is basically any file that is not "line-oriented" Exact copies of memory content are stored. Strings, characters and numeric data are stored as binary data(8 bit value). If instead you use the put() and get() or read() and write() functions then the file will be accessed in binary mode. 8

9 ios::in ios::out ios::binary ios::noreplace ios::nocreate ios::ate ios::app ios::trunc Open for input operations. Open for output operations. Open in binary mode. Error while opening if the file exist, opening it unless ate or app mode is set. If the file does not exist, opening it with the open() function gets impossible. Set the initial position at the end of the file. If this flag is not set, the initial position is the beginning of the file. Pointer can be moved anywhere in the file. All output operations are performed at the end of the file, appending the content to the current content of the file. If the file is opened for output operations and it already existed, its previous content is deleted before opening the file.

10 Open a file open (filename, mode); Where filename is a string representing the name of the file to be opened, and mode is an optional parameter with a combination of the following flags: ofstream myfile; myfile.open ("example.txt");

11 1) ofstream file; file.open ( test.bin", ios::out ios::app ios::binary); 2) ofstream file ( test.bin", ios::out ios::app ios::binary);

12 // basic file operations #include <iostream> #include <fstream> using namespace std; int main () { ofstream myfile; myfile.open ("example.txt"); myfile << "Writing this to a file.\n"; myfile.close(); return 0; }

13 Close a file When we are finished with our input and output operations on a file we shall close it so that the operating system is notified and its resources become available again. This member function flushes the associated buffers and closes the file: file.close(); Once this member function is called, the object can be re-used to open another file, and the file is available again to be opened by other processes. In case that an object is destroyed while still associated with an open file, the destructor automatically calls the member function close.

14 #include <iostream> #include <fstream> #include <string> using namespacestd; intmain () { string line; ifstreammyfile("example.txt"); if (myfile.is_open()) { while ( getline(myfile,line) ) { cout<< line << '\n'; } myfile.close(); } else cout<< "Unable to open file"; return 0; }

15 #include <fstream> #include <iostream> using namespace std; int main () { char data[100]; // open a file in write mode. ofstream outfile; outfile.open("afile.dat"); cout << "Writing to the file" << endl; cout << "Enter your name: "; cin.getline(data, 100); // write inputted data into the file. outfile << data << endl; cout << "Enter your age: "; cin >> data; cin.ignore(); // again write inputted data into the file. outfile << data << endl; // close the opened file. outfile.close(); // open a file in read mode. ifstream infile; infile.open("afile.dat"); cout << "Reading from the file" << endl; infile >> data; // write the data at the screen. cout << data << endl; // again read the data from the file and display it. infile >> data; cout << data << endl; // close the opened file. infile.close(); return 0; }

16 Checking state flags The following member functions exist to check for specific states of a stream (all of them return a bool value): 1) bad():- Returns true if a reading or writing operation fails. For example, in the case that we try to write to a file that is not open for writing or if the device where we try to write has no space left. 2) fail():- Returns true in the same cases as bad(), but also in the case that a format error happens, like when an alphabetical character is extracted when we are trying to read an integer number. 3) eof():- Returns true if a file open for reading has reached the end. 4) good():- It is the most generic state flag: it returns false in the same cases in which calling any of the previous functions would return true. Note that good and bad are not exact opposites (good checks more state flags at once). The member function clear() can be used to reset the state flags.

17 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() fail()- used to check the status of file at opening for I/O bad() bad()- used to check whether invalid file operations or unrecoverable error. good() good()- used to check whether the previous file operation has been successful

18 app comes from 'append'- all output will be added (appended) to the end of the file. In other words you cannot write anywhere else in the file but at the end ate come form 'at end' - it sets the stream position at the end of the file when you open it but you are free to move it around (seek) and write wherever it pleases you. All rights reserved 18

19 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 eof() used to check the end of file character fail() fail()- used to check the status of file at opening for I/O bad() bad()- used to check whether invalid file operations or unrecoverable error. good() good()- used to check whether the previous file operation has been successful

20 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();

21 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): in_stream.open("infile.dat"); out_stream.open("outfile.dat"); 4: To see if the file opened successfully: if (in_stream.fail()) { cout << "Input file open failed\n"; exit(1); // requires <stdlib.h stdlib.h>}

22 The following member functions exist to check for specific states of a stream (all of them return a bool value): bad() Returns true if a reading or writing operation fails. For example, in the case that we try to write to a file that is not open for writing or if the device where we try to write has no space left. fail() Returns true in the same cases as bad(), but also in the case that a format error happens, like when an alphabetical character is extracted when we are trying to read an integer number. eof() Returns true if a file open for reading has reached the end. good() It is the most generic state flag: it returns false in the same cases in which calling any of the previous functions would return true. Note that good and bad are not exact opposites (good checks more state flags at once). The member function clear() can be used to reset the state flags. All rights reserved 22

23 All i/o streams objects keep internally -at least- one internal position: ifstream, like istream, keeps an internal get position with the location of the element to be read in the next input operation. ofstream, like ostream, keeps an internal put position with the location where the next element has to be written. Finally, fstream, keeps both, the get and the put position, like iostream. These internal stream positions point to the locations within the stream where the next reading or writing operation is performed. These positions can be observed and modified using the following member functions:

24 get read a single character from text file and store in a buffer. e.gfile.get(ch); put -writing a single character in textfilee.g. file.put(ch); getline-read a line of text from text file store in a buffer. e.g file.getline(s,80); We can also use file>>chfor reading and file<> operator does not accept white spaces.

25 ios::in get(ch) get(str) get(str,max) Open for input operations. Extract one character into ch Extract characters into array str Extract upto max characters into array str get(str,delim) Extract into array struptospecified delim((typically uptil\n). Leave delimiting character in stream. get(str,max,delim) Extract into array str untill MAX or delim(typically uptil \n). Leave delimiting character in stream. getline(str,max,delim) Extract into array str untill MAX or delim(typically uptil \n). Extract delimiting character

26 write() and read() function write() and read() functions write and read blocks of binary data. example: file.read((char *)&obj, sizeof(obj)); file.write((char *)&obj, sizeof(obj));

Developed By : Ms. K. M. Sanghavi

Developed By : Ms. K. M. Sanghavi Designing Our Own Manipulators We can design our own manipulators for certain special purpose.the general form for creating a manipulator without any arguments is: ostream