Summary of basic knowledge of C language pointer (I)

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What is a pointer ？

The pointer is a variable , Its value is the address of another variable , namely Direct address of memory location . Just like other variables or constants , Before you can use pointers to store other variable addresses , On the Statement . The general form of pointer variable declaration is ：

#include <stdio.h>

int main()
{
	int *num_01;
	int a=10010;
	num_01=&a;
	printf(" The pointer num_01 The value of is ：%d\n",*num_01);
	printf(" The pointer num_01 The address for ：%#p",*num_01);
	
	return 0;
 } 

ad locum ,int Is the basic type of pointer （int type ）, It must be an effective C data type ,num01 Is the name of the pointer variable . Asterisk used to declare pointer * Same asterisk as used in multiplication . however , In this sentence , The asterisk is used to specify that a variable is a pointer .

The actual data type of the value of all pointers , Whether it's an integer 、 floating-point 、 Character , Or other data types , It's all the same , Is a long hexadecimal number representing the memory address . The only difference between pointers of different data types is , The data type of the variable or constant that the pointer points to is different .

Definition and use of pointer variables

1. The definition of a pointer variable
The definition format of pointer variables is ：
Type specifier * Pointer variable name [= initial value ];
To define two pointer variables at the same time ：
int *p1=NULL,*p2=NULL;
(NULL yes c Predefined null pointer keywords in language , It acts as a special pointer , Indicates that it does not point to any object )
Pay attention to the following points when defining pointer variables ：
（1） The type specifier indicates the data type of the variable pointed to by the pointer variable .
（2） When defining pointer variables , Pointer variable name must be preceded by “ * ” Number , Indicates that the defined variable is a pointer variable .
（3） Pointer variables are allowed to be initialized when they are defined .
2. The use of pointer variables
Two important operators ：
（1）&： Fetch address operator , Such as p=&a; be p As a variable a The address of .
（2）： The pointer Operator , Only pointer variables can be followed . Used to access the variable pointed to by the pointer variable . Such as ：*p Indicates access to pointer variables p The content of the variable pointed to .

Practice using pointers to exchange the addresses and values of two variables  

#include <stdio.h>
void temp(int *num01,int *num02); 
int main()
{
	int a=100,b=200,c;
	int *p=NULL;
	int *pa=&a;
	int *pb=&b;
	
	printf(" Before value exchange ：\n");
	printf("%d\n",a);
	printf("%d\n",b);
	printf(" Before address exchange ：\n");
	printf("%p\n",pa);
	printf("%p\n",pb); 
	p=pa;
	pa=pb;
	pb=p;
	printf(" After address exchange ：\n");
	printf("%p\n",pa);
	printf("%p\n",pb);
	temp(&a,&b);// Use function exchange  
	printf(" After value exchange ：\n");
	printf("%d\n",a);
	printf("%d\n",b);

	return 0; 
} 

void temp(int *num01,int *num02)
{
	int p;
	p=*num01;
	*num01=*num02;
	*num02=p; 	
}

Pointers and arrays

One 、 Pointer to a one-dimensional array element

c In the language array Array elements can be regarded as variables of corresponding types . As long as the type matches , You can define a pointer variable , Let this variable store the address of the array element in the array , Then the pointer points to the array element .

int a[10] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 }; // Definition a For an array containing ten data 
int *p; //p Is a pointer variable to an integer variable 
p = &a[0];// hold a[0] The address of the element is assigned to a pointer variable p
p=a;// And p = &a[0] equivalent ,p Is the value of an array a The address of the first element 

Reference one-dimensional array elements through pointers ：
1. Pointer to the first element of a one-dimensional array
Be careful !!!
（1） have access to *(p+i) Access elements a[i].
（2） because p and a Both represent the first address of the array , therefore p+i It can also be recorded as a+i, Point to elements a[i].
（3） Pointer variables pointing to arrays can also have subscripts , Such as ：p[i] And *(p+i) and *(a+i) Equivalent , Show elements a[i].
Thus we can see that ： When a pointer variable p Point to a one-dimensional array a, That is to point to the first element of a one-dimensional array a[0] after , The first of an array of i+1 The elements have the following 4 Species writing ：

a[i] ; p[i] ; *(a+i) ; *(p+i) 
a[i] The address of also corresponds to 4 Species writing ：
&a[i] ; &p[i] ;a+i ; p+i ;

#include <stdio.h>

int main()
{
	int *p;
	int arr[5]={1,2,3,4,5};
	int i;
	p=&arr[0];// Point to the first address arr[0] 
	printf(" The value of the first address of the array element is ：%d\n",*p);
	p=arr;// You can also define pointers like this p The first address of the array pointed to  
	printf(" The pointer p The address for ：%#p\n",p);
	for(i=0;i<5;i++)
	{
		printf("%d ",*p);
		p++;
	}
	
	return 0; 
}