Introduction to C / C++ Programming
Input and Output

Basic Input and Output

• The most common input and output commands are:
  • cout << "Quoted strings and " << variables;
  • cin >> variable;

The 3 Standard I/O Streams

• cin - Standard input, normally the keyboard.
• cout - Standard output, normally the screen.
  • Only the RESULTS of the program ( useful work product ) should be sent to cout.
• cerr - Standard error, also normally the screen.
  • Error messages should be sent to cerr instead of cout.
  • Other messages, such as "Please enter X " should also really be sent to cerr, so that they can be separated from the results.
• Data sent to cout is buffered, and may not be written until the buffer is full or flushed ( e.g. with endl ), whereas any data sent to cerr is transmitted immediately.

Formatting Output

• cout.setf( format flags );
  • ios::showpoint - Show the decimal point, even for integral numbers
  • ios::fixed - Fixed notation, opposite of scientific notation. Also holds number of displayed digits to a fixed amount ( in conjunction with setprecision ) even if trailing zeros must be kept.
  • ios::scientific - Scientific notation, e.g. 6.02E23
  • ios::right - Right justified
  • ios::left - Left justified
• manipulators added to the output stream
  • Must #include <iomanip> to use these manipulators
  • Fed into the output stream along with data to be printed.
    • Example: cout << "X = " << fixed << setprecision( 2 ) << X << endl;
  • showpoint - Forces the decimal point to be shown, even if there are no fractions, e.g. 3.0 instead of 3
  • fixed - Opposite of scientific notation. Also holds number of displayed digits to a fixed amount ( in conjunction with setprecision ) even if trailing zeros must be kept.
  • scientific - Scientific notation, i.e. 6.02E23
  • setprecision( n ) - Set the number of digits after the decimal point, e.g. 2 for money
  • setw( n ) - Set the total width of the field used to hold the number.
  • right - Right justified
  • left - Left justified
• Note that setf( format flags ) sets the condition of the output stream permanently ( until changed with another setf( ) or an unsetf( ) command ), whereas the manipulators only affect the next value to be output. Unfortunately there are not setf( ) equivalents for all the manipulators, e.g. setw( n ).
• C has traditionally formatted input and output using printf and scanf, which are also available under C++. See separate page for details.

File I/O

• See separate page.