What does an identifier in C language consist of?

In C language, identifiers consist of letters (A~Z and a~z), numbers (0~9), and underscores (_), and the first character must be a letter or underscore, and cannot Begin with any other character or number; identifiers are case-sensitive, uppercase and lowercase letters are considered different characters, and identifiers cannot be keywords.

The operating environment of this tutorial: windows7 system, c99, Dell G3 computer.

Identifiers in the C language are composed of

. In high-level computer languages, they are used to identify variables, symbolic constant names, functions, arrays, Valid character sequences for naming types etc. are collectively called identifiers.

Identifier can be simply thought of as a name, used to identify variable names, constant names, function names, arrays, etc.

Variable names a, b, c, symbolic constant names PI, Pai, function names printf, scanf, etc. are all identifiers.

In C language, the identifier can be customized , but certain rules need to be followed.

The C language stipulates that an identifier can be a string consisting of letters, numbers, and underscores (A～Z, a～z, 0~9, _), and the first character must be a letter or underline.

(1) Identifier can only start with English uppercase and lowercase letters, and underscore (_) , and cannot start with any other characters or numbers .

For example, the following is an incorrect example:

int %a;//错误，标识符不能以符号开头
int 8C[8];//错误，不能以数字开头

(2) The identifier can only contain English uppercase and lowercase letters, underscores and Arabic numerals , but cannot start with an Arabic numeral .

int a8;//正确
int _adf589S_5;//正确

(3)IdentifierCase-sensitive

In C language, uppercase letters and lowercase letters are considered different characters, Therefore, the identifiers expressed by ABC and abc are different (Acd and acd, caDd and cadd, _8Fc9 and _8fc9 are all different identifiers).

(4)Identifier cannot be a keyword.

int float;//错误，float是关键字。
int Float;//正确

(5)ANSI standard stipulates that the identifier can be of any length , but the external name must be uniquely distinguishable by at least the first 6 characters, and no case sensitive. This is because some compilers (such as MS C for the IBM PC) only recognize the first 6 characters.

In addition to these hard rules, it is best to name identifiers with specific meanings to facilitate observation, reading, and maintenance.

//定义某长方体的长度、宽度和高度
int a;//高度
int b;//宽度
int c;//高度

can be changed to the following definition:

int c_Long;
int c_Width;
int c_Height;

Keywords in C language

C language does not allow the definition of variables The name is a keyword used by the C language.

Type	Keywords	Description
Data type keyword	char	Character type
	short	Short integer type
	int	Integer type
	long	Long integer type
	float	Single precision floating point type
	double	Double precision floating point type
	signed	yes Symbol type
	unsigned	Unsigned type
	enum	Enumeration type
	struct	Structure type
	union	Union type
	void	Empty type
Control statement key Words	if	Conditional judgment statement
	else	Conditional judgment statement
	for	Loop statement
	do	Loop statement
	while	Loop statement
	switch	Select branch statement
	break	End loop
	continue	Skip the current loop
	case	Select branch statement
	default	Default branch statement
	goto	Unconditional jump statement
	return	Return statement
Storage type keyword	extern	External declaration reference
	static	static type
	auto	##Auto type
	register	Register type
	main	Main function name
Other keywords	sizeof	Calculation type capacity
	const	Constant type
	typedef	Define alias
	volatile	Cancel compiler optimization and do not use cache

C language provides these keywords to design programs, then, we are When defining a variable name (identifier), it cannot be the same as the keyword provided by the C language.

Related recommendations: "C Video Tutorial"

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