The ever-changing pointer ----- C language

1、 Character pointer

1、 Brief introduction

Before writing pointer , Let's first recall the knowledge of the primary pointer .

1.  A pointer is a variable , It's used to store the address , Address unique identification one 
     Block memory space .
2.  The size of the pointer is fixed 4/8 Bytes （32 Bit platform /64 position 
    platform ）.
3.  Pointers are typed , The type of pointer determines the type of pointer +- The integer 
     step , The permission of pointer dereference operation .

This is the most classic use of character pointers , Change the value of a variable by address .
Look at another usage

Print string by address , Instead of putting the entire string address into , Instead, put the address of the first element of the string into the character pointer , Find the whole string through the first element , To print the entire string .

2、 Example explanation

Let's look at a simple topic
What is the result of running the following code ？

The answer is ：
First of all to see str1 And str2, They are all arrays , You need to open up a piece of memory first , To store data . and str3 And str4 It stores the same string “hello world” First element h The address of , therefore str3 And str4 equal

2、 Pointer array and array pointer

1、 What is pointer array

int arr[ ]： An integer array is an array that holds the integer ;
char arr[ ]： Character array is an array of characters ;
therefore , Pointer array is an array of pointers ;

2、 What is an array pointer

An array pointer is a pointer to an array , It's essentially a pointer , Its shape is like int (*p2)[ ],

p2 It's an array pointer ,p2 Can point to an array , In the array 10 Types are int The elements of . Of course , If you want to know more about array pointers , We must first figure out the array name and & Differences between array names .

3、 Array names and & Array name

In general , The array name represents the address of the first element , But there are also two special cases .
1、sizeof（ Array name )： At this point, the array name represents the address of the entire element .

2、& Array name ： here , The array name represents the address of the whole array , So what we take out is the address of the whole array .

From this picture, we can't see the difference between the three , Only know that the three points to the same address .
Next , Look at the next picture

From this picture, we can clearly see the difference between the three ,


Be careful ： Unit is byte .

Next, return to the focus , Array pointer and pointer array

Shaping pointer is used to store shaping pointer
Character pointer is a pointer used to store characters
Array pointers are pointers used to store arrays

Be careful ：

4、 The use of array pointers

Unconventional use （ Not recommended ）

Print 2D array , Common method

Let's think about a problem ,arr Is the address of the first element of the array , So two-dimensional arrays arr What is the address of the first element of ？ Two dimensional array arr The address of the first element is the address of the first line , therefore , What we pass the parameters is the address of the first row of the array

	void print2(int(*p)[5], int r, int c)
	{
    
		for (int i = 0; i < r; i++)
	    {
    
			for (int j = 0; j < c; j++)
			{
    
				printf("%d ", *(*(p + i) + j));
	//p It points to the address of the first row of the array ,
	//*(p+i) It points to an array arr The first i The address of the line 
	//*(*(p+i)+j) It points to an array arr Of the i Xing di j The elements of the column 
	
			}
			printf("\n");
	    }
	}
	int main()
	{
    
		int arr[3][5] = {
     1,2,3,4,5,3,4,5,6,7,4,5,6,7,8 };
		//print1(arr, 3, 5);
		print2(arr, 3, 5);
		// Pass the address of the first row of the array ,
		// So we need to use array pointer to receive 

    }

Let's review the meaning of the following code

int arr[5];  //arr It's an integer array 
int *parr1[10];  //parr1 Is an array of integer pointers 
int (*parr2)[10];  //parr2 It's an array pointer 
int (*parr3[10])[5];  //parr3 Is an array that holds array pointers 

4、 Array parameters 、 Pointer parameter

1、 Two dimensional array parameters

void test(int* arr)// error
// Parameters should be received with an array pointer 

void test(int *arr[5])//error
//int *arr[5]  It's an array of Pointers , It's not a pointer 

void test(int(*arr)[5])//ok
// The array pointer is correct 

void test(int** arr)//error
// The secondary pointer is a variable that puts the address of the primary pointer , Cannot receive the address of a line of elements 


int main()
{
    
	int arr[3][5] = {
     0 };
	test(arr);// Pass the address of the first row of the array 
	return 0;
}

2、 First level pointer parameter transfer

void print(int* p)

int main()
{
    
	// When my parameter part is a pointer , We can pass the reference in this way 
	int a = 10;
	int* ptr = &a;
	int arr[10];


	print(&a);
	print(ptr);
	print(arr);

}

3、 The secondary pointer transmits parameters

// If the formal parameter of the function is a secondary pointer .
// The calling function can pass parameters like this 
void test(int** p)

int main()
{
    
	int* p1;
	int** p2;
	int* arr[10];

	test(&p1);
	test(p2);
	test(arr);
}

5、 A function pointer

1、 Function pointer Introduction

Array pointer ： Pointer to array ;
A function pointer ： Pointer to function ;

As can be seen from the above figure ,& What the function name takes out is indeed the address of the function
For functions ,& Function name and function name get the address of the function
The function pointer is shaped like ：

int Add(int x, int y)
{
    
	return (x + y);
}
int main()
{
    
	int (*pf)(int, int) = &Add;// A function pointer 
	// When we want to call the function pointer 
	int ret = (*pf)(2,3);
	// You can also write like this  int ret = pf(2,3);
	// You can also write like this  int ret = Add(2,3);
	printf("%d",ret);// 5
}

2、 Use function pointer to realize simple calculator

Let's first look at how to implement a calculator in a common way

// First write out each function 
int add(int a, int b)
{
    
	return a + b;
}
int sub(int a, int b)
{
    
	return a - b;
}
int mul(int a, int b)
{
    
	return a * b;
}
int div(int a, int b) 
{
    
	return a / b;
}
int main()
{
    
	int x, y;
	int input = 1;
	int ret = 0;
	do
	{
    
		printf("*************************\n");
		printf(" 1:add 2:sub \n");
		printf(" 3:mul 4:div \n");
		printf("*************************\n");
		printf(" Please select ：");
		scanf("%d", &input);
		switch (input)
		{
    
			case 1:
				printf(" Enter the operands ：");
				scanf("%d %d", &x, &y);
				ret = add(x, y);
				printf("ret = %d\n", ret);
				break;
			case 2:
				printf(" Enter the operands ：");
				scanf("%d %d", &x, &y);
				ret = sub(x, y);
				printf("ret = %d\n", ret);
				break;
			case 3:
				printf(" Enter the operands ：");
				scanf("%d %d", &x, &y);
				ret = mul(x, y);
				printf("ret = %d\n", ret);
				break;
			case 4:
				printf(" Enter the operands ：");
				scanf("%d %d", &x, &y);
				ret = div(x, y);
				printf("ret = %d\n", ret);
				break;
			case 0:
				printf(" Exit procedure \n");
				break;
			default:
				printf(" Wrong choice \n");
				break;
		}
	} while (input);

	return 0;
}

Next, use function pointer to realize simple calculator

int add(int a, int b) 
{
    
	return a + b;
}
int sub(int a, int b) 
{
    
	return a - b;
}
int mul(int a, int b) 
{
    
	return a * b;
}
int div(int a, int b) 
{
    
	return a / b;
}
void Calc(int(*pf)(int x, int y))
// No matter what function comes up , We use function pointer to receive 
{
    
	int x, y;
	int ret = 0;
	printf(" Please enter two operands ：>\n");
	scanf("%d %d", &x, &y);
	ret = pf(x, y);
	printf("%d\n", ret);
}
int main()
{
    
	int input = 0;

	do
	{
    
		printf("*************************\n");
		printf(" 1:add 2:sub \n");
		printf(" 3:mul 4:div \n");
		printf("*************************\n");
		printf(" Please select ：");
		scanf("%d", &input);
		switch (input)
		{
    
			case 1:
				Calc(add);
				break;
			case 2:
				Calc(sub);
				break;
			case 3:
				Calc(mul);
				break;
			case 4:
				Calc(div);
				break;
			case 0:
				printf(" Exit procedure \n");
				break;
			default:
				printf(" Wrong choice \n");
				break;
		}
	} while (input);

	return 0;
}

6、 Pointer array topic resolution

Look directly at the code

int main()
{
    
	int a[] = {
     1,2,3,4 };

	printf("%d\n", sizeof(&a + 1));//4/8
	//&a What we get is the address of the array 
	//&a--> int(*)[4]
	//&a+1  Is from an array a The address of skipped a backward （4 Of an integer element ） Size of array 
	//&a+1 Or the address , Yes, the address is 4/8 byte 
	//
	printf("%d\n", sizeof(&a[0]));//4/8
	//&a[0] It's the address of the first element 
	// It calculates the size of the address 
	printf("%d\n", sizeof(&a[0] + 1));//4/8
	//&a[0]+1 Is the address of the second element 
	// Size is 4/8 Bytes 
	//&a[0]+1 --> &a[1]
	//

	printf("%d\n", sizeof(a));//16
	//sizeof( Array name ), The array name represents the entire array , It calculates the size of the entire array , Unit is byte 
	printf("%d\n", sizeof(a + 0));//4
	//a Not alone sizeof Inside , No address , therefore a Is the address of the first element ,a+0 Or the address of the first element 
	// It's the address , Size is 4/8 Bytes 
	printf("%d\n", sizeof(*a));//4
	//*a Medium a Is the address of the first element of the array ,*a Is to dereference the address of the first element , What you find is the first element 
	// The size of the first element is 4 Bytes 
	printf("%d\n", sizeof(a + 1));
	// there a Is the address of the first element of the array 
	//a+1 Is the address of the second element 
	//sizeof(a+1) Is the size of the address 
	printf("%d\n", sizeof(a[1]));//4
	// It calculates the size of the second element 
	printf("%d\n", sizeof(&a));//4/8
	//&a The address of the extracted array , Address of array , It's just an address 
	printf("%d\n", sizeof(*&a));//16
	//&a----> int(*)[4]
	//&a What you get is the address of the array name , The type is  int(*)[4], Is an array pointer 
	// Array pointer dereference finds an array 
	//*&a ---> a
	// 
	//& and * Offset 
	//*&a ---> a
	return 0;
}

int main()
{
    
	char arr[] = {
     'a','b','c','d','e','f' };
	printf("%d\n", sizeof(arr));//6
	//sizeof( Array name )
	printf("%d\n", sizeof(arr + 0));//4/8
	//arr + 0  Is the address of the first element of the array 
	printf("%d\n", sizeof(*arr));//1
	//*arr Is the first element of the array , Size is 1 byte 
	//*arr --> arr[0]
	//*(arr+0) --> arr[0]
	printf("%d\n", sizeof(arr[1]));//1
	printf("%d\n", sizeof(&arr));//4/8
	//&arr Is the address of the array , Yes, the address is 4/8 Bytes 
	printf("%d\n", sizeof(&arr + 1));//4/8
	//&arr + 1 Is the address after the array 
	//
	printf("%d\n", sizeof(&arr[0] + 1));//4/8
	//&arr[0] + 1 Is the address of the second element 
	//
	return 0;
}

int main()
{
    
	char arr[] = {
     'a','b','c','d','e','f' };

	printf("%d\n", strlen(arr));// Random value 
	printf("%d\n", strlen(arr + 0));// Random value 

// printf("%d\n", strlen(*arr));//--> strlen('a');-->strlen(97);// Wild pointer 
// printf("%d\n", strlen(arr[1]));//-->strlen('b')-->strlen(98);

	printf("%d\n", strlen(&arr));// Random value 
	printf("%d\n", strlen(&arr + 1));// Random value -6
	printf("%d\n", strlen(&arr[0] + 1));// Random value -1

	return 0;
}

int main()
{
    
	//char arr[] = { 'a', 'b', 'c', 'd', 'e', 'f' };

	char arr[] = "abcdef";
	//strlen Is to find the length of the string , Focus on... In the string \0, The calculation is \0 The number of characters that appear before 
	//strlen It's a library function , For strings only 
	//sizeof Only pay attention to the size of memory space , Don't care what's in the memory 
	//sizeof It's the operator 

	//[a b c d e f \0]
	printf("%d\n", strlen(arr));//6
	printf("%d\n", strlen(arr + 0));//6
	//printf("%d\n", strlen(*arr));//err
	//printf("%d\n", strlen(arr[1]));//err
	printf("%d\n", strlen(&arr));//6
	printf("%d\n", strlen(&arr + 1));// Random value 
	printf("%d\n", strlen(&arr[0] + 1));//5


	//[a b c d e f \0]
	//printf("%d\n", sizeof(arr));//7
	//printf("%d\n", sizeof(arr + 0));//4/8
	//printf("%d\n", sizeof(*arr));//1
	//printf("%d\n", sizeof(arr[1]));//1
	//printf("%d\n", sizeof(&arr));//4/8
	//printf("%d\n", sizeof(&arr + 1));//4/8
	//printf("%d\n", sizeof(&arr[0] + 1));//4/8

	return 0;
}

int main()
{
    
	int a[3][4] = {
     0 };
	printf("%d\n", sizeof(a));
	printf("%d\n", sizeof(a[0][0]));
	printf("%d\n", sizeof(a[0]));
	a[0] Is the array name of this one-dimensional array in the first line , Put it alone sizeof Inside ,a[0] Represents the first entire one-dimensional array 
	sizeof(a[0]) Calculate the size of the first row 
	printf("%d\n", sizeof(a[0] + 1));
	a[0] Not alone in sizeof Inside , I didn't take the address ,a[0] It means the address of the first element 
	 Is the address of the first element of the first row of this one-dimensional array ,a[0] + 1 Is the address of the second element in the first line 
	printf("%d\n", sizeof(*(a[0] + 1)));
	a[0] + 1 Is the address of the second element in the first line 
	*(a[0] + 1)) It's the second element in the first line 
	printf("%d\n", sizeof(a + 1));//4/8
	a Although it is the address of a two-dimensional array , But it is not placed alone sizeof Inside , I didn't take the address 
	a Represents the address of the first element , The first element of a two-dimensional array is its first row ,a It's the address on the first line 
	a+1 Just skip the first line , Indicates the address of the second line 
	printf("%d\n", sizeof(*(a + 1)));//16
	*(a + 1) Is the dereference of the address in the second line , I got the second line 
	*(a+1)-->a[1]
	sizeof(*(a+1))-->sizeof(a[1])
	
	printf("%d\n", sizeof(&a[0] + 1));//4/8
	&a[0] -  Address the array name in the first row , Take out the address on the first line 
	&a[0]+1 -  What you get is the address on the second line 
	
	printf("%d\n", sizeof(*(&a[0] + 1)));//16
	printf("%d\n", sizeof(*a));//16
	a Represents the address of the first element , It's the address on the first line 
	*a Is the dereference of the address in the first line , What you get is the first line 
	
	printf("%d\n", sizeof(a[3]));//16
	printf("%d\n", sizeof(a[0]));//16

	int a = 10;
	sizeof(int);
	sizeof(a);


	return 0;
}

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