12.5. The isGreater function

For basic types like int and double, there are comparison operators that compare values and determine when one is greater or less than another. These operators (< and > and the others) don’t work for user-defined types. Just as we did for the == operator, we will write a comparison function that plays the role of the > operator. Later, we will use this function to sort a deck of cards.

Some sets are totally ordered, which means that you can compare any two elements and tell which is bigger. For example, the integers and the floating-point numbers are totally ordered. Some sets are unordered, which means that there is no meaningful way to say that one element is bigger than another. For example, the fruits are unordered, which is why we cannot compare apples and oranges. As another example, the bool type is unordered; we cannot say that true is greater than false.

The set of playing cards is partially ordered, which means that sometimes we can compare cards and sometimes not. For example, I know that the 3 of Clubs is higher than the 2 of Clubs because it has higher rank, and the 3 of Diamonds is higher than the 3 of Clubs because it has higher suit. But which is better, the 3 of Clubs or the 2 of Diamonds? One has a higher rank, but the other has a higher suit.

In order to make cards comparable, we have to decide which is more important, rank or suit. To be honest, the choice is completely arbitrary. For the sake of choosing, I will say that suit is more important, because when you buy a new deck of cards, it comes sorted with all the Clubs together, followed by all the Diamonds, and so on.

With that decided, we can write isGreater. Again, the arguments (two Cards) and the return type (boolean) are obvious, and again we have to choose between a member function and a nonmember function. This time, the arguments are not symmetric. It matters whether we want to know “Is A greater than B?” or “Is B greater than A?” Therefore I think it makes more sense to write isGreater as a member function:

bool Card::isGreater (const Card& c2) const {
  // first check the suits
  if (suit > c2.suit) return true;
  if (suit < c2.suit) return false;

  // if the suits are equal, check the ranks
  if (rank > c2.rank) return true;
  if (rank < c2.rank) return false;

  // if the ranks are also equal, return false
  return false;

Then when we invoke it, it is obvious from the syntax which of the two possible questions we are asking:

Card card1 (2, 10);
Card card2 (2, 4);

if (card1.isGreater (card2)) {
  card1.print ();
  cout << "is greater than" << endl;
  card2.print ();

You can almost read it like English: “If card1 isGreater card2 …” The output of this program is

10 of Hearts
is greater than
4 of Hearts

According to isGreater, aces are less than deuces (2s). As an exercise, fix it so that aces are ranked higher than Kings, as they are in most card games.

Take a look at the active code below, which uses the isGreater function. Feel free to change the values of the cards.

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