Introduction to the usage of secondary pointer in C language practical algorithm series

1. Several knowledge points

• The four areas of memory: stack, global (static), and constant area. The remaining space can not be used at will for the time being; the remaining space can be used as long as it is applied for through the malloc function;
• To apply for a single intvariable on the heap :int* p = (int*)malloc(sizeof(int));
• intMethod to apply for an array on the heap :int* p = (int*)malloc(sizeof(int)*5);
• The address on the heap can be used across functions, and is released by the free function when it is not needed;

Second, the secondary pointer

Problem introduction

char* ss[4] = { "basic","fortran","C++" };//每个元素都是char*类型
For such an array of pointers, what kind of pointers should be used to traverse? The answer is: secondary pointer
char** p = ss; // 二级指针变量p，指向一个指针数组

Example 1-Define pointer arrays in the stack and on the heap respectively

char* ss[] = {
    
     "abc","dd","aaa",NULL };
char* *p = (char**)malloc(sizeof(char*) * 4);
int i = 0;
while (i < 4)
{
    
    
	p[i] = ss[i];
	++i;
}
free(p);

Example 2-Level 2 pointer traverses pointer array

#include <stdio.h>
void main()
{
    
    
	char* p = "Test";
	char* ar[4] = {
    
     "basic","fortran","C++" }; // 一共4个元素，每个元素都是char*类型
	printf("sizeof(ar)=%d\n", sizeof(ar));
	char* *pp = ar;
	int i = 0;
	while (i < sizeof(ar) / sizeof(ar[0]))
	{
    
    
		p = pp[i];
		printf("第%d个指针变量指向的内容是：%s\n", i, p); //pp[i]的类型是char*
		++i;
	}
	printf("p指向的地址是：0x%p内存空间上的内容是：%s\n", p, p);
}

operation result

Example 3

#include<stdio.h>
void main()
{
    
    
	int ar[10] = {
    
     33,55 };
	int n = 10;
	int* p = &n; //一级指针变量指向了一个普通变量，为了远程操控普通变量存储的数据
	int* q = ar;
	*p = -1;
	//int* *pp = &p; //指针变量的指针是二级指针
	//pp = &q;
	//*pp = p;//*p或p[0]远程操控一个一级指针变量，让它指向到其他地址

	int* as[10] = {
    
     ar,&n,NULL }; // 每个元素都是一个一级指针变量
	int* *pp = as; // =&as[0]; // as[0]类型是int*类型
	int i = 0;
	while (i < sizeof(as) / sizeof(as[0]))
	{
    
    
		printf("第%d条指针指向：0x%p\n", i, pp[i]); //pp[i]类型是int*类型
		++i;
	}
	printf("\n");
	int ar1[] = {
    
     32,56,93,56,-1,0 }; //每个元素都是int类型
	int* p1 = &ar1[0]; // ar; // 一级指针变量p, 指向一个普通变量的一维数组
	while (*p1)
	{
    
    
		printf("p[%d]=%d\n", p1 - ar1, *p1);
		//指针的减法: 相差字节/sizeof(int)，逻辑上是几个int
		++p1;
	}
}