How to use define to define constants

define definition constant usage: 1. Define numeric constants, "#define PI value"; 2. Define string constants, "#define GREETING "string""; 3. Define expression constants, " #define MAX(a, b) ((a) > (b) ? (a) : (b))".

`#define` can be used to define constants, making it more convenient and readable when using this constant in code. Common usages are as follows:

1. Define numeric constants:

#define PI 3.14159

When using `PI` in the code, the preprocessor will replace it with `3.14159`. In this way, using `PI` in your code is equivalent to using `3.14159` directly.

2. Define string constants:

#define GREETING "Hello, World!"

When using `GREETING` in the code, the preprocessor will replace it with `"Hello, World!"`. In this way, using `GREETING` in your code is equivalent to using `"Hello, World!"` directly.

3. Define expression constants:

#define MAX(a, b) ((a) > (b) ? (a) : (b))

When using `MAX(5, 10)` in the code, the preprocessor will replace it with `((5) > ( 10) ? (5) : (10))`, that is, `10`. In this way, you can easily use macros to define some commonly used expressions, such as maximum value, minimum value, etc.

It should be noted that the constants defined by `#define` are global and have no scope restrictions. Throughout the code, whenever the preprocessor encounters a macro name, it replaces it with the corresponding replacement text. Therefore, when using macros to define constants, avoid conflicts with other identifiers and carefully consider possible side effects.

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