C语言中查找字符在字符串中第n次出现位置的几种方法

#include <stdio.h>
#include <string.h>
 
char *my_strnchr(const char *str, int ch, int which) {
    char *result = NULL;
    const char *search_start = str; // 搜索起始位置
 
    // 循环查找第 which 次出现
    while (which > 0 && (result = strchr(search_start, ch)) != NULL) {
        which--;
        if (which == 0) {
            // 找到了第 which 次出现
            return result;
        }
        // 为下一次查找更新起点
        search_start = result + 1;
    }
    // 如果未找到足够次数的字符，返回 NULL
    return NULL;
}
 
int main() {
    char string[] = "192.168.0.199";
    char *pos = my_strnchr(string, '.', 3);
    if (pos != NULL) {
        printf("第3次出现的位置（指针偏移）: %ld\n", pos - string);
        printf("从该位置开始的子串: %s\n", pos);
    } else {
        printf("未找到第3次出现的字符。\n");
    }
    return 0;
}

#include <stdio.h>
 
char *find_char_nth(const char *str, char ch, int n) {
    int count = 0;
    for (int i = 0; str[i] != '\0'; i++) {
        if (str[i] == ch) {
            count++;
            if (count == n) {
                // 返回指向该字符的指针
                return (char *)(str + i);
            }
        }
    }
    return NULL;
}

#include <stdio.h>
 
char *find_char_nth_ptr(const char *str, char ch, int n) {
    const char *p = str;
    int count = 0;
    while (*p != '\0') {
        if (*p == ch) {
            count++;
            if (count == n) {
                return (char *)p;
            }
        }
        p++;
    }
    return NULL;
}