Detailed pointer advanced 1

Catalog

One Character pointer

  Two pointer array

3、 ... and & Array and array name

Four Array pointer

5、 ... and Array parameters 、 Pointer parameter

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Preface

First know the pointer through the previous study , We know Pointer to the variable It's used to store the address , understand Secondary pointer variables It is used to store the address of the first level pointer variable , Wild pointer Cause of formation, etc , Next we will continue to learn more about pointer .

One Character pointer

Among the pointer types, we know that one pointer type is character pointer char*

#include<stdio.h>

int main()
{
	char* p = "abcdefg";
	return 0;
}

For this code , I believe many people will ask , Isn't the pointer variable storing the address ？ Yes? Character pointer You can store a string . Actually char*p What's in it is The first address of the string （a The address of the character ）.

  Now let's look at an interview question

#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;
}

  From this interview question, we can see , Different arrays store the same elements , The initial address of the element is different .

Character pointers store the same elements , The first address is the same , This kind of string is also called constant string .

  Two pointer array

Pointer array So it's called Siyi The essence is array , Is an array of pointers .

#define  _CRT_SECURE_NO_WARNINGS

#include<stdio.h>

int main()
{
	int arr1[] = { 1,2,3,4,5,6 };
	int arr2[] = { 2,4,5,6,8,7 };
	int arr3[] = { 3,4,5,6,8,9 };
	int* ppr[3] = { arr1,arr2,arr3 };
	return 0;
}

Among them ppr Is the pointer array .

3、 ... and & Array and array name

arr and &arr What's the difference ？ We know arr It's an array name , The array name indicates the address of the first element of the array . that &arr What is the array name

Visible array name and & The address printed by the array name is the same . Are the two the same ？

We are looking at the following code

According to the above code, we find that , Actually &arr and arr, Although the values are the same , But the meaning is different . actually ： &arr Represents the address of the array , Instead of the address of the first element of the array .（ Feel it carefully ） In this case &arr The type is ： int(*)[10] , Is an array pointer type Address of array +1, Skip the size of the entire array , therefore &arr+1 be relative to &arr The difference is 40.

The array name is the first element, and the address is usually .

But there are two special cases ：

1sizeof( Array name ) The array name here represents the entire array , The calculation is The size of the entire array .

2& Array name , The array name here still represents Entire array .

We mentioned array pointers above , You may not know much about , Next we will start with what is array pointer , How can array pointers be used to explain .

Four Array pointer

Array pointer Essence pointer , Array pointer Is able to Pointer to array .

Now we can see the following code

#include<stdio.h>

int main()
{
	int* p1[10];
	int(*p2)[10];
	return 0;
}

about p1 Come because [] Is greater than * therefore p1 The first and [] combination , therefore p1 Is an array .

about p2 Come on *p2 By () therefore * and p2 First combine so p2 Is a pointer .

We know that array pointers are pointers , It is a pointer to an array . So how do we use it anyway ？

We also know that the first element of a two-dimensional array is its first line , That is, the address in the first line , Then we can use array pointers to receive . The code is as follows ：

        

#include<stdio.h>

void print(int(*arr)[5], int row, int col)
{
    int i = 0;
    for (i = 0; i < row; i++)
    {
        int j = 0;
        for (j = 0; j < col; j++)
        {
            printf("%d ", arr[i][j]);// Print the elements of each line 
        }
        printf("\n");
    }

}

int main()
{
	int arr[3][5] = { 1,2,3,4,5,6,7,8,9,10,11,12,13,14,15 };
	print(arr, 3, 5);
    //arr It is the address of line 
	return 0;
}

So the array pointer can be used to receive the address of the first row of the two-dimensional array .

5、 ... and Array parameters 、 Pointer parameter

  When we write code, we will pass the parameters of array or pointer to the function , Then how to design the parameters of the function ？

We can see the following code ：

One dimensional array parameters

#include <stdio.h>
void test(int arr[])// The way 1
{}

void test(int arr[10])// The way 2
{}

void test(int* arr)// The way 3
{}

void test2(int* arr[20])// The way 1
{}

void test2(int** arr)// The way 2
{}

int main()
{
	int arr[10] = { 0 };
	int* arr2[20] = { 0 };
	test(arr);
	test2(arr2);
}

about test1() function Come on , We can easily understand the way 1 And way 2 Writing , But for the way 3, Because we know that arr Is the address of the first element , We use pointers to receive , This is also very easy to understand .

about test2() function Come on , It's an array of Pointers , The way 1 There is no problem receiving by array pointer , But for the way 2 It's also possible , Then how will we understand it ？ The way 2 It is used. Secondary pointer mode To receive , The secondary pointer stores the address of the primary pointer ,test2 Function passed arr2 To come over （ Address of the first element of the array ), This array is a little special , Each of his elements is int*( Equivalent to a first level pointer ), So we use secondary pointer to receive parameters arr2 Yes. .

  We know that the parameters of one-dimensional array can be used Receive in the form of an array , It can also be used. Receive in the form of a pointer , So what can be used to receive the parameters of a two-dimensional array ？

  Two dimensional array parameters

void test(int arr[3][5])// The way 1
{}
void test(int arr[][])// The way 2
{}
void test(int arr[][5])// The way 3
{}
// summary ： Two dimensional array parameters , Function parameter design can only omit the first [] The number of .
// Because for a two-dimensional array , I don't know how many lines there are , But you have to know how many elements in a row .
// So it's easy to calculate .
void test(int* arr)// The way 4
{}
void test(int* arr[5])// The way 5
{}
void test(int(*arr)[5])// The way 6
{}
void test(int** arr)// The way 7
{}
int main()
{
	int arr[3][5] = { 0 };
	test(arr);
}

For the above two-dimensional array , Only way 1, The way 3, The way 6 Parameter design is feasible , Yes test Function of I passed. arr( The first 1 The address of the line ), The way 1 And way 3 All parameters are designed in the way of array （ As long as the group column must be written out ）, For the way 6 Come on , Array pointer is the way to receive （ I've said that before ）.

  First level pointer parameter transfer

For the first level pointer to pass parameters , We pass a pointer to the function , Functions need to receive with pointers , Like the following code ：

#include <stdio.h>
void print(int *p, int sz)
{
 int i = 0;
 for(i=0; i<sz; i++)
 {
 printf("%d\n", *(p+i));
 }
}
int main()
{
 int arr[10] = {1,2,3,4,5,6,7,8,9};
 int *p = arr;
 int sz = sizeof(arr)/sizeof(arr[0]);
 // First level pointer p, Pass to function 
 print(p, sz);
 return 0;
}

  So how can we use pointers to achieve the functions we want ？ We can dereference the pointer to find the element we want , Point to another element by adding and subtracting pointers .

Now we can continue to think about a problem When the parameter part of a function is a first-order pointer , What parameters can a function take ？

void test1(int *p)
 {} //test1 What parameters can a function take ？

among test1 You can receive The pointer , Address of the first element of the array .

The secondary pointer transmits parameters

Is there any difference between the parameters of the secondary pointer ？ In fact, when I use the first level pointer to transmit parameters, I need to use the first level pointer to receive , Then if I use the secondary pointer to transmit parameters, I need to use the secondary pointer to receive .

#include <stdio.h>
void test(int** ptr)
{
	printf("num = %d\n", **ptr);
}
int main()
{
	int n = 10;
	int* p = &n;
	int** pp = &p;
	test(pp);
	test(&p);
	return 0;
}

So when the parameter of the function is a secondary pointer , What parameters can be received ？

We can pass The secondary pointer , Be able to pass Pointer array .

summary

Here we know what a character pointer is , Pointer array , Array pointer , We also know how one-dimensional and two-dimensional arrays and pointers transfer parameters , In the next blog, I will continue to share the advanced knowledge of pointer .