I/O in C++ October 7, Junaed Sattar

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

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

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;

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

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

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'); Inputting “i am oh so cool” cin.get gets the entire line just cin will get “I”  space termination

Or, Use the getline function getline( cin, name );

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

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();

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;

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

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

Storage TEXT FILE BINARY FILE

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

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 }

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

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

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

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

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