1 #define _CRT_SECURE_NO_WARNINGS 1
  2 #include <stdio.h>
  3 #include <string.h>
  4 #include <assert.h>
  5 #include <stdlib.h>
  6 #include <ctype.h>
  7 //模拟实现 - strlen
  8 int my_strlen(const char* str)
  9 {
 10     int count = 0;
 11     assert(str != NULL);//断言，不能等于空指针
 12     while (*str != '\0')//==>while(*str)
 13     {
 14         count++;
 15         str++;
 16     }
 17     return count;
 18 }
 19 int main()
 20 {
 21     //int len = strlen("abcdef");
 22     char arr[] = { 'a', 'b', 'c', 'd', 'e', 'f' };
 23     int len = strlen(arr);
 24     printf("%d\n", len);
 25     system("pause");
 26     return 0;
 27 }
 28 
 29 //写一个自己的strlen函数
 30 //1.计数器
 31 //2.递归 - 不创建临时变量
 32 //3.指针-指针
 33 
 34 int main()
 35 {
 36     if (strlen("abc") - strlen("abcdef") > 0)
 37     {
 38         printf("hehe\n");
 39     }
 40     else{
 41         printf("haha\n");
 42     }
 43     system("pause");
 44     return 0;
 45 }
 46 
 47 //模拟实现 - strcpy
 48 char* my_strcpy(char* dest, const char*src)
 49 {
 50     //保证指针有效性
 51     assert(dest != NULL);
 52     assert(src != NULL);
 53     //返回的是目的地的起始位置
 54     char* ret = dest;
 55     //拷贝src指向的字符串到dest指向的空间，包含'\0'
 56     while (*dest++ = *src++)
 57     {
 58         ;
 59         //dest++;
 60         //src++;
 61     }
 62     //返回目的空间的起始位置
 63     return ret;
 64     //拷贝的是 "\0"
 65     //*dest = *src;
 66 }
 67 int main()
 68 {
 69     char arr1[] = "abcdefghi";
 70     char arr2[] = "bit";
 71     my_strcpy(arr1, arr2);
 72     printf("%s\n", arr1);
 73     system("pause");
 74     return 0;
 75 }
 76 
 77 模拟实现 - strcat
 78 char* my_strcat(char* dest, const char*src)
 79 {
 80     char* ret = dest;
 81     assert(dest!=NULL);//断言
 82     assert(src);//断言
 83     //1.找到目的字符串的'\0'
 84     while (*dest != '\0')
 85     {
 86         dest++;
 87     }
 88     //2.追加
 89     while (*dest++ = *src++)
 90     {
 91         ;
 92     }
 93     return ret;
 94 }
 95 int main()
 96 {
 97     char arr1[30] = "hello";
 98     char arr2[] = "world";
 99     my_strcat(arr1, arr2);//将arr2的内容追加到arr1中去
100     printf("%s\n", arr1);
101     system("pause");
102     return 0;
103 }
104 
105 
106 //模拟实现 - strcmp
107 int main()
108 {
109     char* p1 = "abcdef";
110     char* p2 = "sqwer";
111     //int ret = (strcmp(p1, p2);
112     if (strcmp(p1, p2) == 1)
113     {
114         printf("p1>p2\n");
115     }
116     else if (strcmp(p1, p2) == 0)
117     {
118         printf("p1=p2\n");
119     }
120     else {
121         printf("p1<p2\n");
122     }
123     printf("%d\n", ret);
124     system("pause");
125     return 0;
126 }
127 
128 int my_strcmp(const char* str1, const char* str2)
129 {
130     assert(str1 && str2);//断言
131     //比较
132     while (*str1 == *str2)
133     {
134         if (*str1 == '\0')
135         {
136             return 0;//相等
137         }
138         str1++;
139         str2++;
140     }
141     if (*str1 > *str2)
142     {
143         return 1;//大于
144     }
145     else {
146         return -1;//小于
147     }
148     //return (*str1-*str2);
149 }
150 int main()
151 {
152     char* p1 = "abcdef";
153     char* p2 = "abqwe";
154     int ret = my_strcmp(p1, p2);
155     printf("%d\n", ret);
156     system("pause");
157     return 0;
158 }
159 
160 int main()
161 {
162     char arr1[5] = "abc";
163     char arr2[] = "hello bit";
164     int ret = strncpy(arr1, arr2, 4);
165     printf("%s\n", ret);
166 }
167 
168 int main()
169 {
170     //strncmp - 字符串比较
171     const char* p1 = "abcdef";
172     const char* p2 = "abcqwr";
173     //int ret = strcmp(p1, p2);
174     int ret = strncmp(p1, p2, 3);
175     printf("%d\n", ret);
176     system("pause");
177     return 0;
178 }
179 
180 
181 //模式实现 - strstr
182 char* my_strstr(const char* p1,const char* p2)
183 {
184     assert(p1 != NULL);//断言，不能等于空指针
185     assert(p2 != NULL);//断言，不能等于空指针
186     char *s1 = p1;
187     char *s2 = p2;
188     char *cur = (char*)p1;
189     if (*p2 == '\0')
190     {
191         return (char*)p1;
192     }
193     while (*p1)
194     {
195         s1 = cur;
196         s2 = (char*)cur;
197         while ((*s1 != '\0') && (*s2 != '\0') && (*s1 == *s2))
198         {
199             s1++;
200             s2++;
201         }
202         if (*s2 == '\0')
203         {
204             return cur;//找到子串
205         }
206         cur++;
207     }
208     return NULL;//找不到子串
209 }
210 int main()
211 {
212     char *p1 = "abcdef";
213     char *p2 = "def";
214     char* ret = my_strstr(p1, p2);
215     if (ret == NULL)
216     {
217         printf("子串不存在\n");
218     }
219     else{
220         printf("%s\n", ret);
221     }
222     
223     system("pause");
224     return 0;
225 }
226 
227 int main()
228 {
229     //192.168.32.121
230     //char arr[] = "dingchenxi@wspc.student";
231     char arr[] = "192.168.32.121";
232     char*p = ".";//要分割中字符串中的分割符号
233 
234     char buf[1024] = { 0 };
235     strcpy(buf, arr);
236     //切割buf中的字符串
237     char* ret = NULL;
238     for (ret = strtok(arr, p); ret != NULL; ret=strtok(NULL,p))
239     {
240         printf("%s\n", ret);
241     }
242 
243     system("pause");
244     return 0;
245 }
246 
247 int main()
248 {
249     char* str = strerror(1);
250     /*0 - No error
251     1 - Operation not permitted
252     2 - No such file or directory*/
253     //errno - 是一个全局的错误码的变量
254     //当C语言的库函数在执行过程中发生了错误，就会把对应的错误码赋值到errno中
255     printf("%s\n", str);
256     system("pause");
257     return 0;
258 }
259 
260 int main()
261 {
262     FILE * pf = fopen("test.txt", "r");
263     if (pf == NULL)
264     {
265         printf("%s\n", strerror(errno));
266     }
267     else{
268         printf("open file success\n");
269     }
270     system("pause");
271     return 0;
272 }
273 
274 int main()
275 {
276     char ch = 'w';
277     int ret = islower(ch);
278     printf("%d\n", ret);
279     system("pause");
280     return 0;
281 }
282 
283 int main()
284 {
285     char ch = tolower('Q');//小写不动，大写转小写
286     puchar(ch);
287     system("pause");
288     return 0;
289 }
290 
291 int main()
292 {
293     char ch = toupper('q');//小写转大写，大小写都可以
294     putchar(ch);
295     system("pause");
296     return 0;
297 }
298 
299 
300 int main()
301 {
302     char arr[] = "I Am A Student";
303     int i = 0;
304     while (arr[i])
305     {
306         if (isupper(arr[i]))
307         {
308             arr[i] = tolower(arr[i]);
309         }
310         i++;
311     }
312     printf("%s\n", arr);
313     system("pause");
314     return 0;
315 }