I/O in C++

1. October 7, 2008. Junaed Sattar I/O in C++

2. Stream I/O • a stream is a flow of bytes/characters/ints or any type of data • input streams: to the program • output streams: from the program • note I use plural • one program can have multiple I/O streams associated • and vice-versa

3. Input/Output • Console based, no GUI • standard streams: • cin: standard input • cout: standard output • cerr: standard error

4. Extraction/Insertion cout << “Hello world!”; cout << “The value of i is “ << i << endl; //endl puts a new line cout << “Please enter your name: “; string name; cin >> name;

5. What are cin and cout? • Stream classes • Classes have methods, as we know • so does cin and cout • some common to all I/O stream classes in C++ • File I/O, binary/text mode I/O, console I/O

6. One example • cin inputs ints, chars, null-terminated strings, string objects • but terminates when encounters space (ascii character 32)‏ • workaround? • use the “get” method

7. Inputting “i am oh so cool” cin.get gets the entire line just cin will get “I” space termination Snippet char tData[100]; // This is a method in C++ istream classes for // inputting text //including spaces cin.get( tData, 99 ); // or cin.get(tData,99,'\n');

8. Or, • Use the getline function getline( cin, name );

9. File I/O • Reading from or writing to files on disk • ifstream and ofstream classes • dedicated for input and output respectively • or, use fstream

10. Example Files Program(filesdemo) ofstream myofile; myofile.open( “sample.txt” ); myofile << “This is a sample line I'm writing\n”; myofile.close(); ... ifstream myifile; myifile.open( “sample.txt” ); string oneLine; getline( myifile, oneLine ); cout << oneLine; myifile.close();

11. Read/Write to files (files1/2)‏ • Similar to how we use cin and cout • remember, these are I/O streams too • myfile is a file stream object, then: • to write an int: • int i = 10;myfile << i; • to read an int: • int i;myfile >> i;

12. Binary files • As opposed to text files, they are unformatted as ascii. • text files stores everything as ascii text strings • even numbers • binary files do not • Example: consider outout of the program in the previous slide

13. Difference? • Example program • Accepts student ID (I input 1010) • Accepts name (I input Junaed) • Accepts CGPA (I input 4.5) • Save into two files, as text and binary

14. Storage TEXT FILE BINARY FILE

15. Binary files • Files by default are text • Different methods to write and read • requires casting (we'll see casting soon)‏ • different data format • If time permits, we'll revisit

16. Failures? • If open fails? • Check before use • if( !myifile ){ cerr << “Cannot open file!”; exit(1);} • End of file? • while( myifile.fail() ){ //do your operations here}

17. Random vs Sequential • Random access files • nonsequential, • as a result faster access times, • content must be suitable for random access • for example. not on network streams! • or console input

18. File “heads” • Access positions • one each for read and write • hence two methods: • seekg (as in “get”) for reading • seekp (as in “put”) for writing • ifstreams have seekg • ofstreams have seekp

19. seeking • seekg( position, mode) //(same for seekp)‏ • position is a long integer signed offset • mode can be • ios::beg: from the beginning • ios::end from the end • ios::cur from current position

20. telling • tellg and tellp • returns as long integer, the position of the get and put positions, respectively

21. example seeks file.seekg( 20L, ios::beg ); file.seekp( -100L, ios::cur ); long pPosition = file.tellp(); long gPosition = file.tellg();