#define preprocessor

The #define preprocessor allows us to define symbolic names, constants, and macros. A quick example

	#define PI 3.14159

This statement will translate every occurance of PI in the program to 3.14159. Here is a more complete example:

	#define PI 3.14159
	main()
	{
   	   int r=10;
	   float cir;
	   cir = PI * (r*r);
	}

This feature is partically usefull for defining 'magic' numbers.

Always use () around any expression to avoid errors of order of operation when the define is expanded. For example:

#define THREE 1 + 2
#define FOUR (2 + 2)
	main() {
		x = THREE * 2; //x is now 1 + 2 * 2 = 5, not 6
		y = FOUR * 2; //y is now (2 + 2) * 2 = 8
		}
	

Use #undef to remove a definition.

See Also:

• A related C keyword is enum which can be used to build lists of aliases. enums often work better in debugging code as the name is still associated with the value, where in a define, the name is lost during compile. enum's also have scope, but can only be ints. e.g. you can't define PI as an enum.

• Another related C keyword is const.

• An advanced use of #define is in the creation of macros.

Examples:

example program.

Conflict between enum and #define.

See also:

• http://gcc.gnu.org/onlinedocs/gcc-2.95.3/cpp_1.html#SEC10
• http://www.mikeash.com/pyblog/friday-qa-2010-12-31-c-macro-tips-and-tricks.html Interesting tutorial and some useful macros.