#include <stdio.h>

int mystr_idx(char *str, char *substr) {
    for (int i = 0; str[i]; i++) {
        int j = 0;
        while (str[i + j] && substr[j] && str[i + j]  == substr[j]) {
            j++;
        }
        if (!substr[j]) {
            return i;
        }
    }
    return -1;
}
int main() {
    char str[] = "Hello, world!";
    char substr[] = "world";
    int index = mystr_idx(str, substr);
    printf("substring index '%s' in string '%s': %d\n", substr, str, index);
    return 0;
}