silence compiler warning about singed vs. unsigned

    // str may not be empty and may only consist of digits or 'x'

    int len = strlen(str);

    size_t len = strlen(str);

    if (len < 1) valid = false;

    for (int i=0; i < len; ++i)

    for (size_t i=0; i < len; ++i)

      if (isdigit(str[i]) || str[i] == 'x') continue;

      else valid = false;