3 二维指针

3.1 什么是二维指针？

二维指针与一维指针一样都是保存变量的地址。

• 实例：一维指针存放变量地址，二维指针存放一维指针地址。

#include <stdio.h>
int main(){
    int n = 10;
    printf("&n = %p\n",&n);
    int* p = &n;
    int** pp = &p;
    printf("%p\t%p\n",p,*pp);

    printf("%d\t%d\t%d\n",n,*p,**pp);
}

&n = 0x7ffdd4366894
0x7ffdd4366894	0x7ffdd4366894
10	10	10

实例：一维指针存放数组地址，二维指针存放指针数组地址。

#include <stdio.h>
int main(){
    //指针与数组之间的关系
    int arr[6] = {1,2,3,4,5,6};
    int* p = arr;
    for(int i = 0;i < 6;++i){
        printf("%d ",p[i]);
    }
    printf("\n");
    //二维指针与指针数组之间的关系
    int arr2[6] = {7,8,9,10,11,12};
    int* pp[] = {arr,arr2};
    int** parr = pp;
    for(int i = 0;i < 2;++i){
        for(int j = 0; j < 6;++j){
            printf("%d ",parr[i][j]);
        }
        printf("\n");
    }
}

3.2 二维指针怎么用？

3.2.1 二维指针作为函数参数

实例1：传入一维地址可以取出函数内部申请的动态内存，取出单个变量

#include <stdio.h>
#include <stdlib.h>
void Func(int** pp){
    int* p = malloc(sizeof(int));
    *p = 100;
    *pp = p;
    printf("%p\t%d\n",*pp,**pp);
}
int main(){
    int* p = NULL;
    Func(&p);
    printf("%p\t%d\n",p,*p);
    free(p);
    p = NULL;
}

实例2：传入一维地址可以取出函数内部申请的动态内存，取出一个数组

#include <stdio.h>
#include <stdlib.h>
void Func(int** pp){
    int* p = malloc(sizeof(int)*10);
    *pp = p;
    for(int i = 0;i < 10;++i){
        p[i] = i;
    }
    printf("%p\t%d\n",*pp,**pp);
}

int main(){
    int* p = NULL;
    Func(&p);
    printf("%p\t%d\n",p,*p);
    for(int i = 0;i < 10;++i){
        printf("%d ",p[i]);
    }
    printf("\n");
    free(p);
    p = NULL;
}

实例3：传入指针数组地址

#include <stdio.h>
void PrintStr(char** s,int n){
    for(int i = 0;i < 2;++i){
        printf("%s\n",s[i]);
    }
}
int main(){
    char* s[] = {
        "Hello",
        "World"
    };
    PrintStr(s,2);
}

3.2.2 二维指针作为函数返回值

• 实例：创建一个r*c的单位矩阵

#include <stdio.h>
#include <stdlib.h>
int** CreatePointer(int r,int c){
    int** p = (int**)malloc(sizeof(int*)*r);
    for(int i = 0;i < r;++i){
        p[i] = (int*)malloc(sizeof(int)*c);
        for(int j = 0;j < c;++j){
            p[i][j] = (i == j);
        }
    }
    return p;
}
int main(){
    int r,c;
    scanf("%d%d",&r,&c);
    int** pp = CreatePointer(r,c);
    for(int i = 0;i < r;++i){
        for(int j = 0;j < c;++j){
            printf("%d ",pp[i][j]);
        }
        printf("\n");
    }
    for(int i = 0;i < r;++i){
        free(pp[i]);
        pp[i] = NULL;
    }
    free(pp);
    pp = NULL;
}

• 实例：实现函数输入正数n，返回三角星号图像字符串数组。例如：n=4

#include <stdio.h>
#include <stdlib.h>

int main(){ 
    int n;
    scanf("%d",&n);
    char** pp = (char**)malloc(sizeof(char*)*n);
    for(int i = 0;i < n;++i){
        pp[i] = calloc(i+2,sizeof(char));
        for(int j = 0;j <= i;++j){
            pp[i][j] = '*';
        }
    }
    for(int i = 0;i < n;++i){
        printf("%s\n",pp[i]);
    }
    for(int i = 0;i < n;++i){
        free(pp[i]);
        pp[i] = NULL;
    }
    free(pp);
    pp = NULL;
}