12.3. The print_card function

When you create a new type, the first step is usually to declare the instance variables and write constructors. The second step is often to write a function that prints the object in human-readable form.

In the case of playing_card objects, “human-readable” means that we have to map the internal representation of the rank and suit onto words. A natural way to do that is with a vector of strings. You can create a vector of strings the same way you create an vector of other types:

std::vector<std::string> suits (4);

Of course, in order to use vectors and strings, you will have to include the header files for both.

To initialize the elements of the vector, we can use a series of assignment statements.

suits[0] = "Clubs";
suits[1] = "Diamonds";
suits[2] = "Hearts";
suits[3] = "Spades";

A state diagram for this vector looks like this:

We can build a similar vector to decode the ranks. Then we can select the appropriate elements using the suit and rank as indices. Finally, we can write a function called print that outputs the card on which it is invoked:

void playing_card::print () const {
  std::vector<std::string> suits (4);
  suits[0] = "Clubs";
  suits[1] = "Diamonds";
  suits[2] = "Hearts";
  suits[3] = "Spades";

  std::vector<std::string> ranks (14);
  ranks[1] = "Ace";
  ranks[2] = "2";
  ranks[3] = "3";
  ranks[4] = "4";
  ranks[5] = "5";
  ranks[6] = "6";
  ranks[7] = "7";
  ranks[8] = "8";
  ranks[9] = "9";
  ranks[10] = "10";
  ranks[11] = "Jack";
  ranks[12] = "Queen";
  ranks[13] = "King";

  std::cout << ranks[rank] << " of " << suits[suit] << std::endl;
}

The expression suits[suit] means “use the instance variable suit from the current object as an index into the vector named suits, and select the appropriate string.”

Because print is a playing_card member function, it can refer to the instance variables of the current object implicitly (without having to use dot notation to specify the object). The output of this code

playing_card card (1, 11);
card.print ();

is Jack of Diamonds.

The active code below uses the playing_card::print() function. Feel free to modify the values that card is being initialized to in the constructor: this will change the output from the playing_card::print() function.

Example c192_12_3
 1#include <iostream>
 2#include <string>
 3#include <vector>
 4
 5struct playing_card {
 6    int suit, rank;
 7
 8    playing_card ();
 9    playing_card (int s, int r);
10    void print () const;
11};
12
13int main() {
14    playing_card card (1,11);
15    card.print ();
16}

You might notice that we are not using the zeroeth element of the ranks vector. That’s because the only valid ranks are 1–13. By leaving an unused element at the beginning of the vector, we get an encoding where 2 maps to “2”, 3 maps to “3”, etc. From the point of view of the user, it doesn’t matter what the encoding is, since all input and output uses human-readable formats. On the other hand, it is often helpful for the programmer if the mappings are easy to remember.

How would we select the appropriate string for the instance variable rank?

 
playing_card card (3, 1);
card.print ();

What is printed by card.print()? Type your answer exactly as it would appear in the terminal.

Does it matter how we encode a mapping?