C basic grammar

We have seen C The basic structure of the program , This will help us to understand C Other basic building blocks of language .

C The token （Token）

C The program consists of various tokens , Tokens can be keywords 、 identifier 、 Constant 、 A string value , Or a symbol . for example , Below C The statement includes five tokens ：

printf("Hello, World! \n");

The five tokens are ：

printf
(
"Hello, World! \n"
)
;

A semicolon ;

stay C In the program , Semicolons are statement terminators . in other words , Each statement must end with a semicolon . It shows the end of a logical entity .

for example , Here are two different statements ：

printf("Hello, World! \n");
return 0;

notes

C There are two ways to annotate a language ：

//  Single-line comments 

With  //  The first single line comment , This comment can take up a single line .

/*  Single-line comments  */
/* 
  Multiline comment 
  Multiline comment 
  Multiline comment 
 */

/* */  Comments in this format can be one or more lines .

You cannot embed comments in comments , Comments cannot also appear in strings or character values .

identifier

C Identifiers are used to identify variables 、 function , Or any other user-defined project name . An identifier is in letters A-Z or a-z Or the underscore _ Start , Followed by zero or more letters 、 Underline and numbers （0-9）.

C Punctuation characters... Are not allowed in identifiers , such as @、$ and %.C yes Case sensitive Programming language . therefore , stay C in ,Manpower  and  manpower  Are two different identifiers . A few valid identifiers are listed below ：

mohd       zara    abc   move_name  a_123
myname50   _temp   j     a23b9      retVal

keyword

The following table lists them C Reserved word in . These reserved words cannot be used as constant names 、 Variable name or other identifier name .

keyword	explain
auto	Declare automatic variables
break	Jump out of current loop
case	Switch statement Branch
char	Declare a character variable or function return value type
const	Define constants , If a variable is const modification , Then its value can no longer be changed
continue	End the current cycle , Start next cycle
default	In the switch statement " Other " Branch
do	The body of a loop statement
double	Declare a double precision floating-point variable or function return value type
else	Conditional statement negates Branch （ And if Continuous use ）
enum	Declare enumeration type
extern	Declare that variables or functions are defined in other files or other locations in this file
float	Declare floating point variable or function return value type
for	A circular statement
goto	Jump statements without conditions
if	Conditional statements
int	Declare an integer variable or function
long	Declare long integer variable or function return value type
register	Declare register variables
return	Subroutine return statement （ With parameters , Or without parameters ）
short	Declare short integer variables or functions
signed	Declare a symbolic type variable or function
sizeof	Calculate data type or variable length （ That is, the number of bytes ）
static	Declare static variables
struct	Declare the struct type
switch	For switch statements
typedef	Used to alias data types
unsigned	Declare an unsigned type variable or function
union	Declare the common body type
void	Declares that the function returns no value or takes no arguments , Declare no type pointer
volatile	Explain that variables can be implicitly changed during program execution
while	The loop condition of a loop statement

C99 New keyword

_Bool

_Complex

_Imaginary

inline

restrict

C11 New keyword

_Alignas	_Alignof	_Atomic	_Generic	_Noreturn
_Static_assert	_Thread_local

C Spaces in

Lines that contain only spaces , It's called a blank line , Maybe with comments ,C The compiler will completely ignore it .

stay C in , Spaces are used to describe spaces 、 tabs 、 Line breaks and comments . Spaces separate parts of a statement , Let the compiler recognize an element in a statement （ such as int） Where to end , Where does the next element start . therefore , In the following sentence ：

int age;

ad locum ,int and age There must be at least one space character between （ It's usually a blank character ）, So that the compiler can distinguish them . On the other hand , In the following sentence ：

fruit = apples + oranges;   //  The total number of fruits obtained 

fruit and =, perhaps = and apples The space character between is not required , But to enhance readability , You can add some spaces as needed .

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