#include <stdio.h>
int main()
{
	
	int a = 10;
	p = &a;

	int* p = NULL;
	//....

	if (p != NULL)
	{
		*p = 20;
	}
	return 0;
}

4. Pointer operation

• Pointer + - integer
• Pointer pointer
• Relational operation of pointer

4.1 pointer + - integer

#define N_VALUES 5
float values[N_VALUES];
float *vp;
//Pointer + - integer; Relational operation of pointer
for (vp = &values[0]; vp < &values[N_VALUES];)
{
     *vp++ = 0;
}

4.2 pointer - pointer

#include <stdio.h>
int main()
{
	int arr[10] = { 0 };

	printf("%d\n", &arr[0] - &arr[9]);

	return 0;
}
//Premise: pointer pointer is that two pointers must point to the same space
//Pointer - pointer, which gets the number of elements between the pointer and the pointer

4.3 pointer relation operation

for(vp = &values[N_VALUES]; vp > &values[0];)
{
    *--vp = 0; 
}

for(vp = &values[N_VALUES-1]; vp >= &values[0];vp--)
{
    *vp = 0; 
}

In fact, most compilers can successfully complete the task, but we should avoid writing this, because the standard does not guarantee its feasibility.

Standard provisions:

A pointer to an array element is allowed to be compared with a pointer to the memory location after the last element of the array, but not with

A pointer to the memory location before the first element is compared.

5. Pointers and arrays

#include <stdio.h>
int main()
{
	int arr[10] = { 1,2,3,4,5,6,7,8,9,0 };
	printf("%p\n", arr);
	printf("%p\n", &arr[0]);
	return 0;
}

Operation results:

It can be seen that the array name and the address of the first element of the array are the same.

Conclusion: The array name represents the address of the first element of the array .

Then it is feasible to write code like this:

int arr[10] = {1,2,3,4,5,6,7,8,9,0};
int *p = arr;//p stores the address of the first element of the array

Since we can store the pointer as an address in the array.

For example:

#include <stdio.h>
int main()
{
	int arr[] = { 1,2,3,4,5,6,7,8,9,0 };
	int* p = arr; //Pointer to the address of the first element of the array
	int sz = sizeof(arr) / sizeof(arr[0]);
	for (i = 0; i < sz; i++)
	{
		printf("&arr[%d] = %p   <====> p+%d = %p\n", i, &arr[i], i, p + i);
	}
	return 0;
}

Operation results:

therefore p+i In fact, the calculation is an array arr Subscript i Your address.

Then we can access the array directly through the pointer.

As follows:

int main()
{
 int arr[] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 0 };
 int *p = arr; //Pointer to the address of the first element of the array
 int sz = sizeof(arr) / sizeof(arr[0]);
 int i = 0;
 for (i = 0; i<sz; i++)
 {
 printf("%d ", *(p + i));
 }
 return 0;
 }

6. Secondary pointer

Pointer variable is also a variable. If it is a variable, it has an address. Where is the address of pointer variable stored?

This is it. Secondary pointer .

The operations of the secondary pointer are:

*ppa Pass on ppa Dereference the address in, so that you can find pa ， *ppa In fact, the interview is pa .

int b = 20; 
*ppa = &b;//Equivalent to PA = & B;

**ppa Pass first *ppa find pa , Then yes pa To dereference: *pa , what did you find a .

**ppa = 30;
//Equivalent to * pa = 30;
//Equivalent to a = 30;

7. Pointer array

Is a pointer array a pointer or an array?

Answer: array. Is an array of pointers.

Array, we already know the integer array, the character array.

int arr1[5];
char arr2[6];

What about array pointers?

int* arr3[5];//What is it?

arr3 Is an array with five elements, each of which is an integer pointer.