011 C language basics: C scope rules

One ： summary

In any kind of programming , Scope is the area where variables defined in a program exist , Beyond that area variables cannot be accessed .C There are three places in the language where variables can be declared ：

• Local variables inside a function or block
• Global variables outside all functions
• In the function parameter definition of formal parameters

 

Two ： local variable

Variables declared inside a function or block are called local variables . Can only be used by the function or statements inside the code . Local variables are unknowable outside the function .

 Assume all variables  a、b  and  c  yes  main()  A local variable of a function .
 example ：
	#include <stdio.h>
	int main(){
    
		//  Local variable declaration 
		int a, b;
		int c;
		//  Actual initialization 
		a = 10;
		b = 20;
		c = a + b;
		printf("value of a = %d, b = %d and c = %d \n", a, b, c);
		return 0;
	}
 result ：
	┌──(rootkali)-[~/Desktop/c_test]
	└─# vim jububianliang.c 
	┌──(rootkali)-[~/Desktop/c_test]
	└─# gcc jububianliang.c -o jububianliang   
	┌──(rootkali)-[~/Desktop/c_test]
	└─# ./jububianliang 
	value of a = 10, b = 20 and c = 30 

 

3、 ... and ： Global variables

Global variables are defined outside functions , Usually at the top of the program . Global variables are valid throughout the life of the program , You can access global variables inside any function .

Global variables can be accessed by any function . in other words , Global variables are available throughout the program after declaration .

 example ：
	#include <stdio.h>
	int c;      //  Global variable declaration 
	int main(){
    
		int a, b;   //  Local variable declaration 
		a = 10;
		b = 20;
		c = a + b;
		printf("value of a = %d, b = %d and c = %d \n", a, b, c);
		return 0;
	}
 result ：
	┌──(rootkali)-[~/Desktop/c_test]
	└─# vim quanjubianliang.c
	┌──(rootkali)-[~/Desktop/c_test]
	└─# gcc quanjubianliang.c -o quanjubianliang   
	┌──(rootkali)-[~/Desktop/c_test]
	└─# ./quanjubianliang 
	value of a = 10, b = 20 and c = 30 
 In the program , The names of local and global variables can be the same , But in a function , The value of a local variable will override the value of a global variable .

 

Four ： Formal parameters

The parameters of the function , Formal parameters , Is treated as a local variable in the function , They override global variables first .

 example ：
	#include <stdio.h>
	int a = 20;     //  Global variable declaration 

	int main(){
    
		//  Declaring local variables in the main function 
		int a = 10;
		int b = 20;
		int c = 0;
		int sum(int, int);
		
		printf("value of a in main() = %d \n", a);
		c = sum(a, b);
		printf("value of a in main() = %d \n", c);
		return 0;
	}

	//  Add a function of two integers 
	int sum(int a, int b){
    
		printf("value of a in sum() = %d \n", a);
		printf("value of a in sum() = %d \n", b);
		return a + b;
	}
 result ：
	┌──(rootkali)-[~/Desktop/c_test]
	└─# vim xingshicanshu.c
	┌──(rootkali)-[~/Desktop/c_test]
	└─# gcc xingshicanshu.c -o xingshicanshu
	┌──(rootkali)-[~/Desktop/c_test]
	└─# ./xingshicanshu  
	value of a in main() = 10 
	value of a in sum() = 10 
	value of a in sum() = 20 
	value of a in main() = 30 

 

5、 ... and ： Initializing local and global variables

When a local variable is defined , The system will not initialize it , You have to initialize it yourself .
When defining global variables , The system will automatically initialize it .

As shown below ：

data type	Initialize defaults
int	0
char	‘’
float	0
double	0
pointer	NULL