string fsubstr(string str, int pos, int len)
{
    if (len > str.length()) //length larger than string
        return "";
    else if (len <= 0) //invalid length
        return "";
    else if ((pos > (str.length() - 1)) || (pos < 0)) //invalid position
        return "";
    else if ((pos + len) > (str.length()-1)) // range out of string range
        return "";
    else
    {
        string result;
        for (int i = pos; i < (pos+len); i++)
        {
            result.push_back(str[i]);
        }
        return result;
    }
}