Pointer advanced --- pointer array, array pointer

The pointer , Is the key to learn data structure well . Most students only master the basic concept of pointer , This week , Let's dig deeper knowledge of pointer . To be proficient in industry and to be idle in play , Not lazy , Don't waste , aggressive , Long term vision .

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Preface

1. What is an integer pointer ？
2. What is a secondary pointer ？
3. What is an array pointer ？
4. What is pointer array ？
5. How are their usages different ？

In this section, , Let's solve these mysteries .

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Tips ： The following is the main body of this article , The following cases can be used for reference

1. The basic concept of pointer

1. The pointer is a Variable , What is stored is the... Of the indicated variable Address , Can also pass through Pointer dereference To find the corresponding address Content
   

2. The pointer stores Address , The memory occupied by the address is 4 Bytes （x64 Environmental Science ） or 8 Bytes （x86 Environmental Science ）. The following code ,pa,pc,pf All occupy four bytes .
   

3.  Since the size of the pointer is 4/8, So why  int *pt,
   char* pc  And other types of pointers ？int *  Is an integer pointer type ,
    Indicates that the pointer points to int Data of type , char*  Empathy 

4. Pointer type function 2： Different types of pointers , Add 1, Number of bytes skipped Is different .

int *  Type pointer plus one , Skip four bytes ;
char *  Type pointer plus one , Skip a byte ;
 The same goes for other types , The number of bytes occupied by the data type is the same .

 As shown in the figure below ,int * The pointer par+1, Skip four bytes , Point to the second of the array 
 Elements .char* Pointer to type pc1 Add one , Skip a byte , Pointing to an array 
 The second element .

2. Character pointer

The character pointer is Pointing character The pointer to . here , We understand through the topic .

1. subject 1, For printing results

const char * p = "abcdef";
printf ("%s\n", p);

Let's think about it , What is the result of the above code ?

The result is that abcdef It's printed , Why? ？
here , The pointer p Deposit is The first element of the string is ’a’ The address of , When printing a string , You can start with the first address , meet ’\0’ end , Print string .
because "abcdef" Is a constant string , Add const modification , Prevent warnings .
2. subject 2. For printing results

    const char* p1 = "abcdef";
	const char* p2 = "abcdef";

	char arr1[] = "abcdef";
	char arr2[] = "abcdef";

	if (p1 == p2)
		printf("p1==p2\n");
	else
		printf("p1!=p2\n");

	if (arr1 == arr2)
		printf("arr1 == arr2\n");
	else
		printf("arr1 != arr2\n");

give the result as follows , Did you do it right ？

p1==p2
arr1 != arr2

Why? ？
because ,"abcdef" Is a constant string , Put it in the read-only area , Save only one copy . When p2 Also when pointing to a string , The program will not redefine a string ,p1 And p2 Pointing to The same area , therefore p1 == p2.

because arr1 And arr2 It is two independent spaces , although Same content , But the starting and ending addresses are different , So the two arrays are not equal .

3. Pointer array

Pointer array , It's essentially a Array , Every element of an array is Pointer types . The following code

    int arr1[] = {
     1,2,3,4,5 };
	int arr2[] = {
     2,3,4,5,6 };
	int arr3[] = {
     3,4,5,6,7 };
	int* parr[3] = {
     arr1, arr2, arr3 };

int * parr[3] ,parr Is an array , The array type is int * [3],
 Open up three spaces ,  Every element is an integer pointer . Replace 
 First element address , Save it in the pointer array .

I want to print out the elements with this pointer array , What to do ？
We know , The array name is the first element address , Then the first address of each element has , It's not difficult to print it out .

 In the pointer array parr  in ,parr[1] That is to say a Array name ,
 Namely  arr1 The first address of the array , therefore arr1[i] It can be used 
*(arr1+i) Express , It can also be used. *(parr[0]+i) Express ,
 Empathy ,arr2[3] It can be used *(parr[1]+i) Express , The specific code is as follows 

for (i = 0; i < 3; i++)
	{
    
		int j = 0;
		for (j = 0; j < 5; j++)
		{
    
			printf("%d ", *(parr[i] + j));
		}
		printf("\n");
	}

4. The meaning of array names

In general , The array name represents the address of the first element , But there are two special cases .

1. sizeof,sizeof( Array name ) Represents the memory size of the entire array .

int arr[10] = {0};
printf("%d", sizeof(a));
//   The answer is 40, ten int  Type element ,10*4=40

2. & Array name , It takes out the address of the entire array

int arr[7] = {1,2,3,4,5,6,7};
int *p = &a+1;
printf("%d", *(p-1));

What is the output result of the above code ？
The answer is 7, Why? ？
&a+1, intend Skip the entire array ,p It's an array The back position

The output is *(p-1), namely p Move an integer byte forward , Yes 7, Dereference pointer , Output 7.

5. Array pointer

Array pointer , It's essentially a pointer , Point to an array .

    int arr[10] = { 0 };
	int (*p2)[10] = &arr;

p2 Is a pointer , Point to int【10】 An array of types .

6. Array pointer application

Array pointers are mainly used for Two dimensional array . because The first element of a two-dimensional array is the element of the first row , It's a one-dimensional array , therefore , Reception time , When using the pointer , Array pointer is required to receive .

 The following code , Pass the two-dimensional array to the function with the array name, that is, the first address , Because of two dimensions 
 The first address of the array is the first line element , It's a one-dimensional array , therefore , Required number 
 Group pointer receive .p It's an array pointer , The element type pointed to is int [5], One 
 A one-dimensional array with five elements .

p Save the address in the first line ,(p+i) That is the first. i+1 The address of the line ,
*(p+i) That is the first. i+1 The address of the element at the beginning of the line ,*(p+i)+j  yes 
a[i][j] The address of ,*(*(p+0)+j ) Namely a[i][j] The elements of .

int arr[3][5] = { 1,2,3,4,5,2,3,4,5,6,3,4,5,6,7 };
print2(arr, 3, 5);
void print2(int (*p)[5], int r, int c)
{
	int i = 0;
	for (i = 0; i < r; i++)
	{
		int j = 0;
		for (j = 0; j < c; j++)
		{
			printf("%d ", *(*(p + i) + j));
			//printf("%d ", p[i][j]);
		}
		printf("\n");
	}
}

7. Array parameters

7.1 Two dimensional array parameters

int a[3][5];
fun11(a);// It is a one-dimensional array address 
fun2(a);
void fun1(int a[][5])// The number of columns must be indicated 
void fun2(int (*arr)[5])// Array pointer 

      Receive a two-dimensional array with a pointer ,int a[3][5]
      The address of the first element of the array name ,  The first element of the two-dimensional array is the first line ,
      Is the address of a one-dimensional array 
     void fun(int (*p)[5],int r,int c)
      For printing   printf("%d"*(*(p+i)+j)));
     *(p+i) Equivalent to a row of array names ,p[i]
     p, Pointer to array ,int (*)[5],p+1, skip 5 Elements 
      perhaps  void fun(int a[][5],int r,int c)
      Be careful , Rows of a two-dimensional array can be omitted , The number of columns cannot be omitted 

7.2 One dimensional integer array passes parameters

       int a[5]={
    0};
       fun1(a);
       fun2(a);
       void  fun1(int a[])
       {
    }
       void fun2(int *a)
        {
    }

7.3 One dimensional pointer array parameter transfer

One dimensional pointer array , Every element is The pointer , Pass the address of the first element of the pointer , use The secondary pointer receive . The secondary pointer , It stores the address of the first level pointer .

     int *arr[5]={
    0};
     fun1(a);
     void fun(int *arr[])
      {
    }
     void fun(int **a)// The secondary pointer can store the address of the primary pointer 
      {
    }

  int *arr[5]={0};
  fun(a);
  void fun(int *arr[])
  {}
  void fun(int **a)// The secondary pointer can store the address of the primary pointer 
  {}

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summary

There are many places where the knowledge points of pointer can be excavated , Need to be understood repeatedly . For a better future , Let's refuel ~