字符串操作的实现：strstr、strcat、strcmp和stcpy |
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几个重点强调一下： （1）注意使用assert； （2）strcat 和 strcpy最后注意赋值'\0'； （3）strcpy注意内存覆盖的情况； （4）strcmp中：while(!(ret = *(unsigned char *)s1 - *(unsigned char *)s2) && *s2 != '\0')；？？？为什么要转换类 型； （5）strstr函数中返回的类型也要为const。 #include #include #include const char* strstr(const char *s1, const char *s2) { assert(s1 != NULL && s2 != NULL); const char *tmp = s1; for(; *tmp != '\0'; ++tmp) { const char *result = tmp; const char *tmp1 = s2; while(*result == *tmp1 && *tmp1 != '\0') { ++result; //++tmp; ++tmp1; } if(*tmp1 == '\0') return tmp; //return result } return NULL; } char *strcat(char *dest, const char *src) { assert(dest != NULL || src != NULL); char *tmp = dest; while(*tmp != '\0') { ++tmp; } while(*src != '\0') { *tmp = *src; ++tmp; ++src; } *tmp = '\0'; return dest; } char *strcpy(char *dest, const char *src) { assert(dest != NULL || src != NULL); int length = strlen(src); char *result = dest; if(dest < src || src + length < dest) { while(*src != '\0') { *dest = *src; ++src; ++dest; } *dest = '\0';//不能忘了 }else { src = src + length - 1; dest = dest + length; *dest = '\0'; --dest; for(int i = 0; i < length; ++i) { *dest = *src; --src; --dest; } } return result; } int strcmp(const char *s1, const char *s2) { assert(s1 != NULL || s2 != NULL); int ret = 0; while(!(ret = *(unsigned char *)s1 - *(unsigned char *)s2) && *s2 != '\0') { ++s1; ++s2; } if(ret < 0) return -1; else if(ret > 0) return 1; return ret; } int main() { char s1[] = "shui huai li"; char s2[] = "huai"; char s3[] = "i am the best"; char s4[] = "sorry"; char s6[] = "sorryhe"; char s5[] = "hello world"; const char *result = strstr(s1, s3); if(result) printf("%s\n", result); else printf("i am sorry!!!\n"); //printf("%s\n", strcat(s1, s5)); //printf("%s\n", strcpy(s2, s1)); //printf("%d\n", strcmp(s4, s6)); return 0; }
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