My advanced learning notes of C language ----- keywords

Preface

This is my own learning c After the foundation of language , Interested in further study , So let's start with an article to record , This time , This worst time , It is also the best time .

keyword

c Language standards define 32 Key words

keyword      significance
auto     Declare automatic variables , By default, the compiler generally defaults to auto
int     Declare the integer variable
double     Declare a double variable
long     Declare a long integer variable
char     Declare character variables
float     Declare floating point variables
short     Declare a short integer variable
signed     Declare a variable of signed type
unsigned     Declare an unsigned variable
struct     Declare struct variables
union     Declare the consortium data type
enum     Declare enumeration type
static     Declare static variables
switch     For switch statements
case     Switch statement Branch
default     In the switch statement “ other ” Branch
break     Jump out of current loop
register     Declare register variables
const     Declare read-only variable
volatile     Explain that variables can be changed implicitly during program execution
typedef     It is mainly used to alias data types
extern     Declared variables are declared in other files
return     Subroutine return statement （ With parameters , It can also be done without parameters ）
void     Declares that the function returns no value or takes no arguments , Declare null type pointer
continue     End the current cycle , Start next cycle
do     The body of a loop statement
while     The loop condition of a loop statement
if     Conditional statements
else     Conditional statement negates Branch （ And if Continuous use ）
for     A circulatory body
goto     Jump statements without conditions
sizeof     Calculate the memory space occupied by the object
(22 Bar message ) C Language standards define 32 Key words _dengchuanjing The blog of -CSDN Blog _c Language user identifier 32 individual https://blog.csdn.net/dengchuanjing/article/details/79265323

What is the definition , What is a statement ?

Definition is that the compiler creates an object , Allocate a heap of memory for this object and give it a name , Variable name and object name .

Statement ： The first meaning is to tell the compiler , The name has been matched to a piece of memory , Next time, you can use the variable name directly , Statements can appear multiple times . The second meaning is to tell the compiler , The name has been reserved , You can't use it as a variable name anywhere else , Note that it is used as a variable name instead of . such as

int i; Definition

extern int i; Statement

Other keywords

auto

Just think he doesn't exist , The compiler defaults that all variables are auto

register

register Ask the compiler to keep variables as long as possible cpu In the internal register , Instead of searching for addresses through memory .

Fastest call

The register is ,cpu And memory module ,cpu The access speed is fast but the process cost is high

meanwhile register Variables cannot be used & To get register Variable address

static

First to speak

Static global variables ： Scope is limited to the file in which the variable is defined , Other files even if used extern Statements can't use it

Static local variables ： Define in function body , Can only be used in this function . Even if this function ends , The value of this static variable will not be destroyed , Function can use this value next time .

static int i;// Global variables

void fun1（void）

{ 

        static i++;// Static local
}

Modify function , Become a static function , The advantage is that you don't have to worry about whether your defined function will have the same name as other file functions .

Basic data type

 

（1） Short short（ In memory 2 Bytes ）
yes short int Abbreviation .
Value range ：-32768 ~ + 32767 (2 Bytes)

Define constants ：short a = 5;

（2） integer int(long int) （4 byte ）
long yes long int Abbreviation .
Value range ：-2147483648 ~ [phone] (4 Bytes)

Definition ：
int a = 0; // Definitions are best initialized
long a = 0;
// The above two definitions are the same .

（3） Long integer long long （8 byte ）

PS:（1）C The language standard is stipulated as follows ：int least 16 position （2 byte ）,long It can't be compared with int short ,short It can't be compared with int Long , The specific bit length is determined by the compiler developer according to various situations .
（2）32 Under the platform long yes 4 Bytes ,long long yes 8 byte ; however 64 Under the bit platform, it is all 8 byte . Therefore, in order to ensure the universality of the platform , Try not to use... In the program long data type .

2、 floating-point
（1） Single precision float
The basic floating point type of the system . At least it can express exactly after the decimal point 6 Significant digits .
One float Type take up 4 Storage bits of bytes .
The highest bit is the sign bit , Then 8 Bit refers to digit , The rest 23 Bits are the last digits .
Format specifier ：%f

（2） Double precision double
Double precision floating point type . At least it can express exactly after the decimal point 12 Significant digits .
One double Type take up 8 Storage bits of bytes .
The highest bit is the sign bit , Then 8 Bit refers to digit , The rest 52 Bits are the last digits .
Format specifier ：%lf
 

（3） Character

Array

Is a series of values of the same type stored in sequence , Such as 10 individual char Type of character or 15 individual int Type value . The entire array has an array name , Accessing individual items or elements in an array through integer subscripts .
for example ： The following statement ：
float debts[20];// Statement debts Is a content 20 Array of elements , Each element can store float Type value . The first element of the array is debts[0], The first 2 The element is debts[1], And so on , until debts[19]. Be careful , The elements of the array are numbered from 0 Start , Not from 1 Start . Each element can be assigned float Type value .
for example , It can be written like this ：debts[5] = 32.54;debts[6] = 1.2e + 21; actually , Using array elements is the same as using variables of the same type .

Structure

Is a custom composite data type . For example, this data type will be used to store student information .

Shared body

It's also called a consortium , Make several different types of variables occupy a total of memory ( Cover each other ), In other words, members of the community share a piece of memory , The data of the assigned member variable is the effective data of the community , Because the previous assignment has been overwritten . The memory occupied by the community can accommodate at least the space required for the largest member variables