bool Code::IsValid()
// postcondition: returns true if MINLEN <= Length() < MAXLEN - 2
//                and for every k, 0 <= k < Length(),
//                myChars[k] is a digit '0'--'9' or a char 'A' -- 'Z'
{
    int k;
    
    if (Length() < MINLEN || MAXLEN - 2 < Length())
    {
	return false;
    }
    
    for(k=0; k < Length(); k++)
    {
	if (! isupper(myChars[k]) && ! isdigit(myChars[k])) return false;
    }    
    return true;
}