The pound sign at the beginning of the line indicates that it is a C pre-processor statement.

#include

• The line

#include <iostream>
  
  tells the pre-processor to read in the file named iostream before the code is sent to the compiler.
  
  
• This file contains the definitions for cout, cin, and endl.
  
  Note: The only type of file in a #include should be header files (.h files).

#define

• The #define directive is a way to make your code much more readable.

• There are many uses of #define, but the basic one follows this syntax:

  #define TOKEN new-value
  

• A #define tells the pre-processor that when it sees the TOKEN in the code, replace it with new-value before sending the source to the compiler.

• By convention, the token in a #define is in all caps.

• So, you could write code like this to define RED and BLUE:

  #define	DEFAULT_COLOR	1
  #define	RED	2
  #define	BLUE	3
  

  as ints.

• This tells the pre-processor to replace every DEFAULT_COLOR with the number 1, every RED with the number 2, and and every BLUE with the number 3 before the code is sent to the compiler.
  
  
• Then write your code using RED, BLUE, and DEFAULT_COLOR (much easier for a person), even though the compiler needs something like 1, 2, and 3.
  
  
• Here's an example:

  #define	DEFAULT_COLOR	1
  #define	RED		2
  #define	BLUE		3
  
  int	enter_color();
  
  int main()
  {
  	...
  	int	color_entered;
  
  	color_entered = enter_color();
  
  	if (color_entered == RED)
  		cout << "The RGB value is: 0xFF0000\n";
  	else if (color_entered == BLUE)
  		cout << "The RGB value is: 0x0000FF\n";
  
  	...
  }
  
  int	enter_color()
  {
  	string	answer;
  
  	cout << "Enter a color (i.e. red): ";
  	cin >> answer;
  
  	if ((answer == "red") || (answer == "RED"))
  		return(RED);
  
  	if ((answer == "blue") || (answer == "BLUE"))
  		return(BLUE);
  
  	return(DEFAULT_COLOR);
  }