C language - Advanced pointer

One 、 Character pointer

The pointer is a variable , It's used to store the address , Variables that store an integer address are called integer pointers , A variable that stores the address of a character is called a character pointer .
If you assign a constant string to a character pointer , The character pointer stores the address of the character , Only one character address , So you can only put the address of the first character in it , You can't put the addresses of all characters in it , Just like a single bed can only be used by one person .
int main()
{
const char* pstr = “hello bit.”;
printf(“%s\n”, pstr);
return 0;
}
Put it in pstr The address in is just h The address of .

Interview questions ：

#include <stdio.h>
int main()
{
    
    char str1[] = "hello bit.";
    char str2[] = "hello bit.";
    const char *str3 = "hello bit.";
    const char *str4 = "hello bit.";
    if(str1 ==str2)
 		printf("str1 and str2 are same\n");
    else
		 printf("str1 and str2 are not same\n");
       
    if(str3 ==str4)
		 printf("str3 and str4 are same\n");
    else
		 printf("str3 and str4 are not same\n");
       
    return 0; 
}

Output results ：

The knowledge here is that if a string is a constant string , Then the value of this string cannot be modified , Only use it , So in the case of limited memory space , Just save one copy , Avoid wasting memory space .
This can be likened to a supermarket giving a schoolbag , Schoolbag is compared to memory space , You can go to this supermarket to get anything , Then put it in this schoolbag , But when the bag is full, you can't take it , You should put an ice block inside to cool down , Ice cubes are compared to constant strings , But you will put two pieces ？ One piece is enough , Another piece will take up space , Make room for other good things .
So in the title , Pointer to the variable str3 and str4 The value of is the same , The address stored in it is the same , A constant string is stored only in memory space , and str1 and str2 It's not the same , Put the constant string into two arrays , The space requested by the two arrays is different , The addresses of these two spaces must be different , The array name represents the address of the first element , So the addresses corresponding to these two array names must be different , You can compare arrays to schoolbags , Two arrays are two schoolbags , The two schoolbags must be placed in different positions , You have to occupy a place for each to fit .

Two 、 Pointer array

If students are placed in an area , It's called school , If it's a patient , Just call the hospital .
So if you put an integer in the array , It's called an integer array , If you put characters , It is called character array .
If the array contains pointers , It is called pointer array .

3、 ... and 、 Array pointer

1、 Array pointer interpretation

The pointer is a variable , It's a container , It's used to store the address .
If you put cosmetics in a container , It's a makeup bag , If you put school supplies , It's a schoolbag .
If an integer address is placed in a pointer , It's called an integer pointer , This pointer points to an integer data .
If Inside a pointer is the address of the array , It's called array pointer , This pointer points to an array .
* This symbol indicates that a variable is a pointer variable , This symbol is like the north star , Will guide the way , The same goes for the pointer , Will point to a place .
[ ] Represents an array , Two brackets can enclose an area , It represents an array .
[ ] Than * High priority , When they appear at the same time ,[ ] Will give priority to play a role , After all [ ], The larger , It's an array , More power .
So if you want to make * Give priority to play a role , To give * Add a bracket , Give it a yellow jacket , Power came up immediately .
Example ：int(*p)[10], Given * A yellow mandarin jacket , therefore p Priority and * combination , Become a pointer , The pointer points to an array , Array has 10 Elements , The type of each element is an integer .

2、 Array name ,& Array name and sizeof（arr)

The array name usually represents the address of the first element
Here, you can imagine that an array is like a coffin , There are many emperors' bodies in it , For future generations to find them , Take the name of the coffin as the address of the first emperor's body .
But there are exceptions , Use sizeof The operator , When operating on array names , In this case, the array name represents the address of the entire array , The calculated value is the size of the entire array .
Use &, Take the address operator , Operate on the array name , At this time, the array name also represents the address of the entire array , So what we get is the address of the whole array .

3、 How array pointers are used

The array name represents the address of the first element , The elements of a two-dimensional array are each row , The first element is the first line , So the address represented by the array name of the two-dimensional array is the address of the first line , Each row can be regarded as a one-dimensional array , So the address of the first element is the address of the array , The address of the array should be put in the array pointer .
The array name represents the array , Just like our names represent us .
Pass parameter to function , Pass the array name of the two-dimensional array to the function , Use array pointers to receive , You can also use a two-dimensional array to receive .
What we imagine here is that the value of address is compared to a soul , It's the soul of a lone soul , The pointer is a body , The soul wants to reincarnate , If it is human soul , It must be thrown into the human body , If it is the soul of an animal , It must be thrown into the animal's body , The address of the array must be put in the array pointer .

#include<stdio.h>
print_arr1(int(*arr)[3],int r,int c)
{
    
	int i = 0;
	for(i = 0;i < r;i++)
	{
    
		int j = 0;
		for(j=0;j<c;j++)
		{
    
			printf("%d ",*(*(arr+i)+j));
			//arr+i, Corresponding to No i The address of the line ,*(arr+i), What you find is no i That's ok , Each row is a one-dimensional array ,
			// So what you find is an array , The array name represents an array , The array name is the address of the first element , Address +i,
			// What the representative found is i Addresses of elements .
		}
	}
}
int main()
{
    
    int arr[3][5] = {
    1,2,3,4,5,6,7,8,9,10};
 	print_arr1(arr, 3, 5);
    return 0; 
}

Four 、 Array parameter passing and pointer parameter passing

1、 One dimensional array parameters

int arr1[10]={0);
test(arr1);
Function can be used int arr[10] and int arr[ ] and int* p receive .
The number of elements can be omitted , Create according to the number passed .
The array name is the address of the first element , So you can use a pointer to receive .

2、 Two dimensional array parameters

int arr1[10][10]={0};
test(arr1);
Function can be used int arr[10][10], and int arr[ ][10] and int (*p)[10] receive .
The rows of the two-dimensional array used for reception can be omitted , Columns cannot be omitted , Omit column , Arrays will not know how to store elements .
It's like students queuing up on the playground , If you don't tell the students how many rows to line up , Only a few rows , Then you can stand at will .

3、 First level pointer parameter transfer

When the formal parameter is a first-order pointer , When passed to a function , What can be passed ？
You can pass the name of the first level pointer variable , Address , The array name of a one-dimensional array

4、 The secondary pointer transmits parameters

When the formal parameter is a secondary pointer , When passed to a function , What can be passed ？
You can pass the name of the secondary pointer variable , The address of the first level pointer variable , Array name of pointer array

5、 ... and 、 A function pointer