c语言string.h头文件字符串检验函数仿写
所需头文件:stdio.h
,stdlib.h
, string.h
strlen字符串求长度
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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; } |
strcmp / strncmp字符串比较
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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; } |
strchr / strrchr 字符串中查找字符ch
第一个出现的字符ch
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//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); } |
最后一个出现的字符ch
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//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); } |
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)
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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; } |
strcpy / strncpy字符串拷贝
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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; } |
strcat / strncat字符串的粘贴
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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; } |
strdup 字符串申请堆区空间存放字符串的副本
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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; } |
总结
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原文链接:https://blog.csdn.net/xiaoxiaoguailou/article/details/121267391