文章目录
所需头文件：stdio.h stdlib.h string.h
（1）strlen字符串求长度
int strlen_my(const char* src_str)
{
if (NULL == src_str)
{
return -1;
}
const char* p = src_str;
while (*p != '\0')
{
p++;
}
return p - &(src_str[0]);
}
//递归
int strlen_my2(const char* src_str)
{
if (src_str == NULL)
return -1;
if (*src_str)
{
return strlen_my2(src_str + 1) + 1;
}
else
{
return 0;
}
}
int main()
{
const char* p = "hello";
printf("%d \n",strlen_my(p));
return 0;
}
（2）strcmp / strncmp字符串比较
int strcmp_my(const char* str_a, const char* str_b)
{
while ((*str_a != '\0' || *str_b != '\0') && *str_a == *str_b )
{
str_a++;
str_b++;
}
return *str_a - *str_b > 0 ? 1 : (*str_a - * str_b < 0 ? -1 : 0);
}
int strcmp_my2(const char* str_a, const char* str_b)
{
int sub = 0;
while ((sub = *str_a - *str_b) == 0 && *str_a++ && *str_b++); //先赋值，指针再++
return sub > 0 ? 1 : (sub < 0 ? -1 : 0);
}
int strncmp_my(const char* str_a, const char* str_b, size_t front_len)
{
while (--front_len && *str_a != '\0' && *str_b != '\0'
&& *str_a == *str_b)
{
str_a++;
str_b++;
}
return *str_a - *str_b > 0 ? 1 : (*str_a - *str_b < 0 ? -1 : 0);
}
int main()
{
const char* p = "hella";
const char* q = "hell";
printf("%d \n", strncmp_my(p, q, 4));
return 0;
}
（3）strchr / strrchr 字符串中查找字符ch
- 查找第一个出现的字符ch
//strchr 返回字符串中第一个出现的字符ch
char* strchr_my(const char* src_str, char ch)
{
if (src_str == NULL)
{
return NULL;
}
while (*src_str != '\0' && *src_str != ch)
{
src_str++;
}
return *src_str == '\0' ? NULL : src_str;
}
int main()
{
const char* p = "hello";
p = strchr_my(p, 'e');
if (p == NULL)
return 0;
printf("%c\n", *p);
}
- 查找最后一个出现的字符ch
//strrchr查找字符串中最后一个出现的字母ch
char* strrchr_my(const char* src_str, char ch)
{
if (NULL == src_str) return NULL;
const char* p = src_str + strlen(src_str) - 1;
while ((p - src_str) >= 0 && *p != ch)
{
p--;
}
if (p - src_str == -1)
{
return NULL;
}
return p;
}
int main()
{
const char* p = "hello";
//p = strchr_my(p, 'q');
p = strrchr_my(p, 'l');
if (p == NULL)
return 0;
printf("%c\n", *p);
}
（4）strstr 字符串匹配：src_str中查找des_str，返回匹配的起始位置，没有为NULL（BF算法）
char* strstr_my(const char* src_str, const char* des_str)
{
if (NULL == src_str || NULL == des_str)
{
return NULL;
}
int len_src = strlen(src_str);
int len_des = strlen(des_str);
int i = 0; //src_str
int j = 0; //des_str
while (i < len_src && j < len_des)
{
if (src_str[i] == des_str[j])
{
i++;
j++;
}
else
{
i = i - j + 1;
j = 0;
}
}
if (j == len_des)
{
return src_str + i - j;
}
return NULL;
}
int main()
{
const char* p = "hello";
const char* q = "llo";
const char* rt = strstr_my(p, q);
if (rt != NULL)
{
printf("%s\n", rt);
return 0;
}
return 0;
}
（5）strcpy / strncpy字符串拷贝
char* strcpy_my(char* des_str, const char* src_str)
{
if (des_str == NULL || src_str == NULL)
return NULL;
char* p = des_str;
while (*src_str != '\0')
{
*p++ = *src_str++;
}
*p = '\0';
return des_str;
}
char* strncpy_my(char* des_str, const char* src_str, size_t len)
{
if (des_str == NULL || src_str == NULL || len <= 0)
return NULL;
char* p = des_str;
//源字符串实际长度
int cur_len = strlen(src_str) <= len ? len : strlen(src_str);
while (cur_len--)
{
*p++ = *src_str++;
}
*p = '\0';
return des_str;
}
int main()
{
const char* p = "hello";
char q[10] = {
0};
if (strncpy_my(q, p, 1) != NULL)
{
printf("%s\n", q);
}
return 0;
}
（6）strcat / strncat字符串的粘贴
char* strcat_my(char* des_str, const char* src_str)
{
if (des_str == NULL || src_str == NULL)
return NULL;
int len_des = strlen(des_str);
char* p = des_str + len_des;
while (*src_str != '\0')
{
*p++ = *src_str++;
}
return des_str;
}
char* strncat_my(char* des_str, const char* src_str, size_t len)
{
if (des_str == NULL || src_str == NULL || len < 0)
return NULL;
int len_des = strlen(des_str);
char* p = des_str + len_des;
while (len--)
{
*p++ = *src_str++;
}
return des_str;
}
int main()
{
char p[20] = "hello";
const char* q = "world";
printf("p = %s\n", p);
printf("q = %s\n", q);
if (strncat_my(p, q, 3) != NULL)
{
printf("p = %s\n", p);
}
printf("q = %s\n", q);
return 0;
}
（7）strdup 字符串申请堆区空间存放字符串的副本
char* strdup_my(const char* src_str)
{
if (NULL == src_str)
return NULL;
char* newspace = (char*)malloc((strlen(src_str) + 1 ) * sizeof(char));
if (newspace == NULL)
return NULL;
int i = 0;
while (*src_str != '\0')
{
newspace[i++] = *src_str++;
}
newspace[i] = '\0';
return newspace;
}
int main()
{
const char* p = "hello";
char* q = strdup_my(p);
if (q != NULL)
{
printf("%s\n", q);
free(q);
q == NULL;
}
return 0;
}