Exchange variable values with and without pointers in C language

stay C In language , We often encounter the situation of exchanging two values , Generally, you will choose to exchange the addresses of two numbers through the pointer and really exchange two numbers . Some friends don't quite understand the difference between the two , Here is a detailed explanation .

Classic mistake ：

#include<stdio.h>
void swap(int x, int y);
int main()
{
	int a = 100;
	int b = 200;
	printf(" Before the exchange a=%d,b=%d", a, b);
	swap(a, b);
	printf("\n After the exchange a=%d,b=%d", a, b);
	return 0;
}
void swap(int x, int y)
{
	int temp = x;
	x = y;
	y = temp;
}

         When I first came into contact with functions and pointers, I really didn't understand why the values of two numbers didn't exchange , Even if I will a,b Set to global variable , It is still impossible to exchange two values directly .

         Later I learned that , The two values in the above code are exchanged , The function being called swap in ,a and b The values of are indeed exchanged , But when you jump out swap After the function , Everything is nothing ,a Is still a,b Is still b. amount to a and b Made a “swap” Dream , In the dream, two people exchange identities . After waking up ,a Is still a,b Is still b, All exchanges are floating clouds .

        And why change the address through the pointer , You can change two values ？ Because when the pointer points to an assigned variable , What is stored in the memory space of the address saved by the pointer is the binary form of the value assigned by the variable , That is to say, the pointer and the value are bound together , When you change the address , The variable is still the original variable , Values have been exchanged with the exchange of pointers .

        The details are as follows ：

Pointer control exchanges two values

        First understand , The relationship between variable address and variable value .

         Variable is the address of the first byte of the memory it occupies （ That is, the starting address ） To represent the address of the variable in memory . Each variable has a unique address , Is the location in memory , As shown in the figure below , Integer variables x Occupy from 0028FF1C~0028FF1F Four bytes of memory space , Variable x The address of is 0028FF1C.

        secondly , The value of each variable is stored in the memory space represented by the variable address . For instance variable x Value 10 It is stored in binary form in the compiler as x Of the four spaces allocated . You can directly access the value of a variable through its name .

Variable storage diagram

 

         and Pointer to the variable Is the variable used to save the address .  The definition of pointer is as follows ：

int a=100;
int *z=&a;

         The meaning of the above two sentences ：a It's an integer variable , The assignment is 100;z Is a pointer variable , It points to a The address of , The memory space represented by this address stores values 100 In binary form . After address exchange , The contents of the memory space represented in the address will also be exchanged .

         When we assign values to two integer variables , And set two pointer variables to point to these two integer variables respectively ：

int a=100,b=200;
int *z=&a;
int *y=&b;

         stay main Call in function a,b The address of ：

swap(&a,&b);   //&a and &b Is the actual parameter , The relay will be a,b The address of 

         When in swap When exchanging in function , The parameter must be a,b The address of ： 

void swap(int* M, int* N)    //M,N  It's a formal parameter ,M It stands for a The address of ,N representative b The address of 
{
    int temp=*M;            //*M Said by M Indirect access gets M Value , namely a Value 
    *M=*N;
    *N=temp;
}    

  The complete code is as follows ：

#include<stdio.h>
void swap(int* M, int* N);
int main()
{
	int a = 100;
	int b = 200;
	int* z = &a;
	int* y = &b;
	printf(" Before the exchange a=%d,b=%d", a, b);
	swap(&a, &b);
	printf("\n After the exchange a=%d,b=%d", a, b);
	return 0;
}
void swap(int* M, int* N)
{
	int temp = *M;
	*M = *N;
	*N= temp;
}

  The debugging results are as follows ：

I hope the above can be helpful to the understanding of some partners ！