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This article mainly introduces the meaning of define in C language, which has certain reference value. Interested friends can refer to it. I hope you will gain a lot after reading this article. Let the editor take you to know it.

Define, macro definition, a kind of preprocessing command in C language. It is divided into non-parameter macro definition and with parameter macro definition. The general form of no-parameter macro definition is: # define macro name string; the general form of parameter macro definition is: # define macro name (parameter table) string; the define function in PHP defines a constant. Constants are similar to variables, except that the value of the constant cannot be changed after it is set. Constant names do not require the beginning of the dollar sign ($). Scope does not affect access to constants. Constant values can only be strings or numbers.

Define concept

The # define command is a macro definition command in C language, which is used to define an identifier as a string, which is called the macro name, and the defined string is called replacement text. [1] the command has two formats: one is a simple macro definition, and the other is a macro definition with parameters.

Define classification

No-parameter macro definition

The general form of no-parameter macro definition in C language is: # define macro name string [2]

The "#" indicates that this is a preprocessing command. Everything that starts with "#" is a preprocessing command. "define" defines commands for macros. "identifiers" are defined macro names. "strings" can be constants, expressions, format strings, and so on.

For example: # defineM (aqb) its function is to specify the identifier M instead of the expression (aqb). When writing the source program, all the (axib) can be replaced by M, but when compiling the source program, the macro will be replaced by the preprocessor first, that is, all the macro names M will be replaced by the (axib) expression, and then compiled.

# include

# defineM (aquib)

Intmain (void)

{

Ints,a,b

Printf ("inputnumbera&b:")

Scanf ("% d% d", & aPhoneShab)

S=M*M

Printf ("slots% d\ n", s)

}

In the above example program, we first define the macro, define M to replace the expression (axib), and make the macro call in s=M*M. After being expanded by macro during preprocessing, the statement becomes: s = (aqb) * (aqb). It is important to note, however, that there must be no less parentheses on both sides of the expression (axib) in the macro definition. Otherwise, an error will occur. Such as the following definition: # defineM (a) + (b) when the macro is expanded, you will get the following statement: s = (a) + (b) * (a) + (b).

There are also the following points for the macro definition:

1. The macro definition is to use the macro name to represent a string, and then use the string to replace the macro name when the macro is expanded, which is just a simple substitution. The string can contain any character, can be a constant, or an expression. The preprocessor does not check it. If there is an error, it can only be found when compiling the source program that has been expanded by the macro.

two。 A macro definition is not a description or statement. There is no need to add a semicolon at the end of the line. If a semicolon is added, the semicolon is replaced with it.

3. The scope of the macro definition is from the macro definition command to the end of the source program. To terminate its scope, use the # undef command.

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