1.二維數組的本質
二維數組的本質是一個數組指針,放寬來說多元數組的本質也是一個數組指針
int arr[i][j]
(arr+i)代表第i行的位址 二級指針
*(arr+i)代表第i行首元素的位址 一級指針 ,第i行位址和第i行首元素位址雖然是相同的
但是指針的表示形式不同
*(arr+i)+j == arr[i][j] 代表第i行第j列元素的位址
((arr+i)+j) 代表第i行第j列元素的值
![](https://img.laitimes.com/img/_0nNw4CM6IyYiwiM6ICdiwiIn5GcuIWOwU2M1MGMxMjMxUGOmJjM1MWNjNDZxI2Y3MDN4ETYfdWbp9CXt92Yu4GZjlGbh5SZslmZxl3Lc9CX6MHc0RHaiojIsJye.png)
二維數組與指針.png
#define _CRT_SECURE_NO_WARNINGS
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
void main() {
int arr[3][5];
int i = 0;
int j = 0;
int temp = 0;
for (i = 0; i < 3; i++) {
for (j = 0; j < 5; j++) {
arr[i][j] = temp++;
}
}
for (i = 0; i < 3; i++) {
for (j = 0; j < 5; j++) {
printf("%d ", arr[i][j]);
}
}
printf("\n");
//觀察這兩行列印,看位址有什麼特點,arr+1相對于arr移動了
//20,這剛好等于二維數組一行的位元組數(5*4(一個int4個位元組))
printf("arr %d,arr+1 %d\n",arr,arr+1);
//&arr+1相對于&arr移動了60個位元組,剛好等于一個二維數組
//的總位元組數(5*4*3)
printf("&arr %d,&arr+1 %d\n",&arr,&arr+1);
//定義一個指向數組的指針變量,指向以int[5]為元素的二維數組
int(*p)[5];
//指向arr
p = arr;
//列印,你會發現p[i][j]和arr[i][j]列印結果是一樣的,其實二維數組的本質
//就是一個數組指針,arr和p是等價的
for (i = 0; i < 3; i++) {
for (j = 0; j < 5; j++) {
printf("%d ", p[i][j]);
}
}
//(arr+i)代表第i行的位址 二級指針
//*(arr+i)代表第i行首元素的位址 一級指針 ,第i行位址和第i行首元素位址雖然是相同的
//但是指針的表示形式不同
//*(arr+i)+j == arr[i][j] 代表第i行第j列元素的位址
//*(*(arr+i)+j) 代表第i行第j列元素的值
system("pause");
}
2.數組做函數參數的退化問題
1.C語言中隻會以機械式的值拷貝方式傳遞參數(實參把值傳遞給形參)。
下邊兩個方法參數不同,但是列印結果是相同的,sizeof(數組)得到的值都是4,為什麼?一個char 一個int,這是因為一維數組做函數參數會退化為一個一維指針,是以結果是一樣的,解釋一下C語言中隻會以機械式的值拷貝方式傳遞參數這句話,當一個數組作為位址傳遞到一個方法中,實參向形參傳遞值的時候,并沒有在記憶體中重新建立一個數組,将數組元素一個一個拷貝進去,而是将實參數組的位址傳遞給了形參,形參也指向了那個數組,這就是值傳遞,位址值的傳遞
原因:c語言的高效性展現在這裡,這樣做更高效
int fun(char a[20],size_t b){
printf("%d %d",b,sizeof(a));
}
int fun(int a[20],size_t b){
printf("%d %d",b,sizeof(a));
}
2.二維數組做參數
//一維數組做參數退化過程
void fun(int a[5]) ---> void fun(inta[]) ---> void fun(int *a)
//二維數組做參數退化過程
void fun(int a[3][5]) ---> void fun(int a[][5]) --->void fun(int (*a)[5])
數組做函數參數的等效性
一維數組 char a[30] 等價的指針參數 char*a
指針數組 char *a[30] 等價的指針參數 char *a
二維數組 char a[20][30] 等價的指針參數 char(a)[30]
4.三種判斷字元串到達結尾的方式
'\0' 數字0 NULL都可以作為判斷字元串結尾的方式,原因在于stdlib.h中有這樣的定義
NULL定義.png
#define _CRT_SECURE_NO_WARNINGS
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
void main() {
char *a[5] = {
"helloe",
"world",
"\0"
};
char *b[5] = {
"today",
"yesterday",
0
};
char *c[5] = {
"good",
"bad",
NULL
};
for (int i = 0; a[i] != NULL;i++) {
printf("%s", a[i]);
}
printf("\n");
for (int i = 0; b[i] != NULL; i++) {
printf("%s", b[i]);
}
printf("\n");
for (int i = 0; c[i] != NULL; i++) {
printf("%s", c[i]);
}
system("pause");
}
5.結構體做形參和結構體指針做形參
看下邊的函數,為什麼通過結構體指派失敗,而通過結構體指針就成功了,我來畫一下記憶體結構示意圖,就明白了
#define _CRT_SECURE_NO_WARNINGS
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
typedef struct Worker {
char *name;
int age;
int id;
} Worker;
void copy(Worker from,Worker to){
to = from;
}
void copy2(Worker *from, Worker *to) {
*to = *from;
}
void main() {
Worker w1 = {"zhangsan",12,1};
Worker w2;
Worker w3;
//通過等号,會将結構體中的值拷貝到w2中
w2 = w1;
printf("w2.name=%s", w2.name);
//通過函數進行指派
//copy(w1, w3);
//運作直接報錯。使用了未初始化的局部變量w3,但是在copy函數中
//我們已經指派了
//printf("w3.name=%s", w3.name);
//換一種方式,通過傳遞結構體指針,列印成功
copy2(&w1,&w3);
system("pause");
}
針對第一個copy函數
結構體通過等号指派,做的是記憶體中值的拷貝,不是位址,這一點要和數組做形參差別開。因為是值拷貝,是以w2指派給to之後,二者在除了值相同,是沒有聯系的,to的改變不會影響到w2,是以列印w2失敗
結構體變量做形參.png
針對第二個copy2函數
結構體指針做形參.png
6.結構體做參數
#define _CRT_SECURE_NO_WARNINGS
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
typedef struct Worker {
char *name;
int age;
int id;
} Worker;
void printWorker(Worker *worker, int num) {
for (int i = 0; i < num; i++) {
//printf("age:%d\n", (*(worker+i)).age);
//printf("age:%d\n", (worker + i)->age);
printf("age:%d\n", worker[i].age);
}
}
void sortWorker(Worker *worker, int num) {
int i, j;
Worker tmp;
for (i = 0; i < num; i++) {
for (j = i + 1; j < num; j++) {
/*if ((worker + i)->age > (worker + j)->age) {
tmp = *(worker + i);
*(worker + i) = *(worker + j);
*(worker + j) = tmp;
}*/
if (worker[i].age >worker[j].age) {
tmp = worker[i];
worker[i] = worker[j];
worker[j] = tmp;
}
}
}
}
void main() {
int i = 0, num = 3;
Worker Array[3];
for (i = 0; i < num; i++) {
printf("請輸入age:");
scanf("%d", &(Array[i].age));
}
printf("\n排序前\n:");
printWorker(Array, num);
sortWorker(Array, num);
printf("\n排序後\n:");
printWorker(Array, num);
system("pause");
}
7.結構體二級指針做參數
注意三個createWorker方法
#define _CRT_SECURE_NO_WARNINGS
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
typedef struct Worker {
char *name;
int age;
int id;
} Worker;
Worker *createWorker1(int num) {
Worker *tmp = (Worker*)malloc(sizeof(Worker)*num);
if (tmp == NULL) {
return NULL;
}
return tmp;
}
int createWorker2(Worker **worker, int num) {
Worker *tmp = NULL;
tmp = (Worker*)malloc(sizeof(Worker)*num);
if (tmp == NULL) {
return -1;
}
//這樣做的效果是讓二級指針Worker **worker指向了tmp,并沒有改變
//Worker *pWorker這個指針的指向,可以分析一下,但函數執行時,定義了一個
//二級指針worker指向了Worker *pWorker,worker = &tmp這樣操作改變的隻是
//臨時變量worker,對pWorker沒有影響,是以後邊操作pWorker導緻崩潰
worker = &tmp;
return 0;
}
int createWorker3(Worker **worker, int num) {
Worker *tmp = NULL;
tmp = (Worker*)malloc(sizeof(Worker)*num);
if (tmp == NULL) {
return -1;
}
*worker = tmp;
return 0;
}
void printWorker(Worker *worker, int num) {
for (int i = 0; i < num; i++) {
//printf("age:%d\n", (*(worker+i)).age);
//printf("age:%d\n", (worker + i)->age);
printf("age:%d\n", worker[i].age);
}
}
void sortWorker(Worker *worker, int num) {
int i, j;
Worker tmp;
for (i = 0; i < num; i++) {
for (j = i + 1; j < num; j++) {
/*if ((worker + i)->age > (worker + j)->age) {
tmp = *(worker + i);
*(worker + i) = *(worker + j);
*(worker + j) = tmp;
}*/
if (worker[i].age >worker[j].age) {
tmp = worker[i];
worker[i] = worker[j];
worker[j] = tmp;
}
}
}
}
void main() {
int i = 0, num = 3;
Worker *pWorker = NULL;
//在堆記憶體中申請空間傳回記憶體位址
//pWorker = createWorker1(num);
//createWorker2(&pWorker, num);
createWorker3(&pWorker, num);
for (i = 0; i < num; i++) {
printf("請輸入age:");
scanf("%d", &(pWorker[i].age));
}
printf("\n排序前\n:");
printWorker(pWorker, num);
sortWorker(pWorker, num);
printf("\n排序後\n:");
printWorker(pWorker, num);
system("pause");
}
8.練習,将前兩個記憶體中的元素拷貝到一個新的二級指針指向的空間,并排序
#define _CRT_SECURE_NO_WARNINGS
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
/*
把第一種記憶體模型第二種記憶體模型結果copy到第三種記憶體模型中,并排序列印
*/
int sort(char **myp1, int num1, char(*myp2)[30], int num2, char ***myp3, int *num3) {
char **p3 = NULL;
int i = 0, j = 0, k = 0;
int tmplen = 0;
char *tmpP = NULL;
//p3指向的記憶體空間用于接收指向兩個數組的值,是以他的大小應該是前兩個空間的和
p3 = (char **)malloc((num1 + num2) * sizeof(char*));
if (p3 == NULL) {
return -1;
}
//将第一個數組的值拷貝到申請的空間中
for (i = 0; i < num1; i++) {
//得到myp1指向的第i個元素的長度(+1是結束符空間)
tmplen = strlen(myp1[i]) + 1;
//開辟等長的空間
p3[i] = (char *)malloc(tmplen * sizeof(char));
//拷貝到開辟的空間中
strcpy(p3[i], myp1[i]);
}
printf("i = %d\n", i);
//同樣道理拷貝myp2
for (j = 0; j < num2; j++,i++){
tmplen = strlen(myp2[j]) + 1;
p3[i] = (char *)malloc(tmplen * sizeof(char));
if (p3[i] == NULL) {
return -3;
}
strcpy(p3[i], myp2[j]);
}
//排序
tmplen = num1 + num2;
//*num3 = num1+num2;
for (i = 0; i < tmplen; i++) {
for (j = i + 1; j < tmplen; j++) {
if (strcmp(p3[i], p3[j]) > 0) {
tmpP = p3[i];
p3[i] = p3[j];
p3[j] = tmpP;
}
}
}
//間接指派
*num3 = tmplen;
*myp3 = p3;
return 0;
}
void sortFree1(char **p, int len) {
int i = 0;
if (p == NULL) {
return;
}
for (i = 0; i < len; i++) {
free(p[i]);
}
free(p);
}
/*
把二級指針指向的二維記憶體釋放掉,同時間接修改了實參的值
*/
void sortFree2(char ***myp,int len) {
int i = 0;
char **p = NULL;
if (myp == NULL) {
return;
}
//還原成二級指針
p = *myp;
if (p == NULL) {
return;
}
for (i = 0; i < len; i++) {
free(p[i]);
}
//間接指派是指針存在的最大意義
*myp = NULL;
}
void main() {
int ret = 0;
char *p1[] = {"aaaaaaa","cccccc","bbbbbb"};
char buf2[10][30] = {"11111","333333","222222"};
char **p3 = NULL;
int len1, len2, len3=0, i = 0;
len2 = 3;
len1 = sizeof(p1) / sizeof(*p1);
printf("len1=%d", len1);
ret = sort(p1, len1, buf2, len2, &p3, &len3);
if (ret != 0) {
printf("func sort err:%d \n",ret);
return ret;
}
for (i = 0; i < len3; i++) {
printf("%s\n", p3[i]);
}
system("pause");
}