Notes on advanced points of C language 2

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Catalog

• Four 、 The pointer

7、 Strings and pointers
8、 Array pointer
9、 Array name and address
10、 The difference between pointer variables and array names
11、 Conversion of pointers in multidimensional arrays
12、 The pointer is used as the parameter of the function

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• Four 、 The pointer

7、 Strings and pointers

a、 char string[100] = “I love C!”
              Defines a character array string, Used to store multiple characters , And use ”I love C!” to string Array initialization
              character string “I love C！ ” Store in string in
       b、 char *str = “I love C!”
              A pointer variable is defined str, Only character address numbers can be stored ,
              So I love C！ The characters in this string cannot be stored in str In pointer variables .
              str Just stored characters I The address number of , “I love C！ ” Stored in the text constant area
       c、 char *str =(char*)malloc(10*sizeof(char));// Dynamic application 10 Bytes of storage space , First address to str assignment .
              strcpy(str, "I love C"); // The string “Ｉ love C!” copy to str Point to the memory

               A character array ： In the memory （ Stack 、 Static global area ） It opens up a space for storing strings

                              Modifiable , Can be initialized directly
               String pointer ： stay Text constant area Opened up a space to store strings , Pay the first address of the string to str

                                  Do not modify the , Can be initialized directly
               Pile up ： Use malloc The function requests space in the heap , Copy the string to the heap
                          Modifiable , Cannot initialize directly

Whether the memory pointed to by the pointer can be modified depends on the position pointed to by the pointer , Pointing to the text constant area is not modifiable , Pointing stack 、 Pile up 、 The static global area can be modified

A character array ： Use scanf perhaps strcpy

buf_aver="hello kitty"; // error , Because the name of the character array is a constant 
strcpy(buf_aver,"hello kitty"); // correct 
scanf("%s",buf_aver); // correct 

Pointer to string ：

buf_point="hello kitty"; // correct ,buf_point  Point to another string 
strcpy(buf_point,"hello kitty"); // error , read-only , Can you copy the string to  buf_piont  The memory pointed to depends on  buf_point  Where to point .

8、 Array The pointer （ Is a pointer ）

Concept ： Itself is a pointer , Point to an array , Add a jump to an array , That is, point to the next array

Define methods ： The type of array pointed to （* Pointer variable name ) [ The number of elements of the array pointed to ]

                 int (*p)[5];// Defines an array pointer variable p, p Pointing to an integer, there are 5 Array of elements , That is, each small array has five integer variables
                        p+1 Point down 5 An integer , Skip one with 5 Array of integer elements .

difference     Array pointer ：int (*p) [5]  Is a pointer , Only four bytes

                            int  *p[5]    It's an array , There are five shaping pointers

#include <stdio.h>
int main()
{
int a[3][5];// Defined a  3  That's ok  5  A two-dimensional array of columns 
int(*p)[5];// Define an array pointer variable  p, p+1  Jump one with  5  An integer array of elements 
printf("a=%p\n",a);// The first  0  Row address of row 
printf("a+1=%p\n",a+1);// The first  1  Row address of row , a  and  a +1  Bad  20  Bytes 
p=a;
printf("p=%p\n",p);
printf("p+1=%p\n",p+1);//p+1  Jump one with  5  One dimensional array of integer elements 
return 0;
}

$ One dimensional array pointer int(*p)[5] ;  There are... For each line 5 individual int A two-dimensional array of type elements

$  Two dimensional array pointer int(*p)[4][5];  Use with a three-dimensional array , A three-dimensional array consists of several 4 That's ok 5 A two-dimensional array of columns

$ ...

Confusing content ：
                                  Pointer array ： Is an array , A collection of pointers of the same type
                                                    int *p[10];
                                                    Array p Yes 10 individual int * Pointer variables of type constitute , Namely p[0] ~p[9]
                                  Array pointer ： Itself is a pointer , Point to an array , Add 1 Jump an array
                                                    int (*p)[10];
                                                    P It's a pointer , p Is an array pointer , p Add 1 Point to the next array , jump 10 A plastic .
                                  The pointer of the pointer ：
                                                    int **p;//p It's the pointer of the pointer , It can be saved int * Type of address
                                                    int *q;
                                                    p=&q;
9、 Array name and address

The pointer ： Variable takes address

The pointer of the pointer ： The pointer variable takes the address

Array pointer ： Array name and address

                  int a[10];
                         a+1 Jump an integer element , yes a[1] The address of
                         a and a+1 One element apart , 4 Bytes
                         &a It becomes a one-dimensional array pointer , yes int(*p)[10] Type of .
                         (&a) +1 and &a The difference is an array, that is 10 The elements are 40 Bytes .

c Language policy , Array name and address , Becomes an array pointer . Add 1 Jump an array

10、 The difference between pointer variables and array names

int a[10];

int *p;

p = a;

The same thing ：a and p Point to the same address , Equivalent when referring to array elements

Difference ：a Is a constant ,p It's a variable. ;p You can assign ,a Can't assign a value

              Yes a The fetch address is an array pointer , Yes p Take a pointer whose address is a pointer

11、 Conversion of pointers in multidimensional arrays

In a two-dimensional array , Line address take * Not worth it , It means that the pointer is degraded , By line address （ Array pointer ） Become the first in this line 0
Addresses of elements . take * The front and back still point to the same place , But the types of pointers are different

#include <stdio.h>
int main(int argc, char *argv[])
{
    int a[3][5];
    printf("a=%p\n",a);
    printf("a +1=%p\n",a+1);
    printf("*a =%p\n",*a);// *a  Became the second  0  Xing di  0  The address of the column element 
    printf("(*a)+1 =%p\n",(*a)+1 ); // The result is  0  Xing di  1  The address of the column element 
    return 0;
}

12、 The pointer is used as the parameter of the function

When calling a function, pass the address of the variable , Pass... In the called function *+ The address changes the value of the variable in the stagnation function

To change the value of the variable in the main function , The address of the variable must be passed in , And you have to pass *+ Address To assign , No matter what type this variable is

void fun(char **q)
{
    *q="hello kitty";
} 

int main()
{
    char *p="hello world";
    fun(&p);
    printf("%s\n",p);// The result is ： hello kitty
}

When passing an array to a function, you can't pass all the array in at one time , Can only pass the address of the array

When passing a one-dimensional array ：void fun(int p[])  perhaps    void fun(int *p)                     // The two are equivalent , It's all pointers

When transmitting a two-dimensional array ：void fun(int p[][4])    perhaps    void fun(int (*p)[4])

When passing pointer array ：

void fun(char **q) // char *q[]
{
    int i;
    for(i=0;i<3;i++)
    printf("%s\n",q[i]);
} 

int main()
{
    char *p[3]={"hello","world","kitty"}; //p[0] p[1] p[2] char *
    fun(p);
    return 0;
}