What are macros?

I'm not sure what macros are. For example the following macro 开发者_JAVA百科is found in the cocos2d framework written in objective-c. What exactly is a macro,why are they useful, and how do I define them? Thanks.

#define CCRANDOM_0_1() ((random() / (float)0x7fffffff ))

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Here you'll find everything you need: http://gcc.gnu.org/onlinedocs/cpp/Macros.html

But to get a basic idea, they are pieces of code that replace the macro name when you write them in your code.

In your example, if you wrote:

int i = CCRANDOM_0_1();

It would be the same as:

int i =  ((random() / (float)0x7fffffff ));

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it just replaceCCRANDOM_0_1() with ((random() / (float)0x7fffffff )) in your code

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Whenever CCRANDOM+0_1() is seen in the program (by the compiler) it will be replaced by ((random() / (float)0x7fffffff ))