In depth analysis of C language - variable error prone knowledge points ＃ dry goods inventory ＃

Variables are very basic content , But be sure to master !

3.1. What is a variable

Log in and copy

 Open up a specific size of space in memory , To hold data 

     

      
1.
     

---

3.2. How to define variables - How to use it?

Log in and copy

int x = 10;
char c = 'a';
double d = 3.14;
 type   Variable name  =  The default value is 

     

      
1.
      
2.
      
3.
      
4.
     

---

3.3 Why define variables

Computers are born to solve the problem of people's lack of computing power . namely , Computers are for calculation .
And calculation , You need data .
And to calculate , Any moment , Not all data should be calculated immediately .

Why variables are needed ？ Because there is data that needs to be saved temporarily , Waiting for further processing

---

3.4 The essence of variable definition

• The program runs , It needs to be loaded into memory

2. Program calculation , You need to use variables

The essence of variable definition ： Open up a space in memory , To hold data （ Why must it be memory ： Because defining variables , It's also part of program logic , The program has been loaded into memory ）

---

3.5 The nature of variable declarations

Declared variables are usually added extern keyword ：

Be careful ： Cannot initialize when declaring

Log in and copy

extern int a ;	// Declare an external integer variable a

      

       
1.
      

The declaration even tells the compiler , This variable is defined elsewhere , Declarations do not allocate memory space to variables ！！

---

3.6 About the difference between declaration and definition

difference ： The definition has memory allocation , Declaration has no memory allocation

Be careful ： Cannot initialize when declaring

---

Statement

The declaration just tells the compiler that there is such a variable or function , This function or variable is defined in other places , So in this process, no memory is allocated again , Therefore, a declaration can be declared more than once ;

Definition

A program opens up memory space for its variables or functions at run time , Definition can only be defined once .
As for initialization , Is to complete the development of memory space , Fill in the specified value for the memory opened up .

---