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How To Think Like a Computer Scientist C++ Edition The Pretext Interactive Version

Section 12.3 The printCard 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 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:
vector<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 Card::print() const {
  vector<string> suits(4);
  suits[0] = "Clubs";
  suits[1] = "Diamonds";
  suits[2] = "Hearts";
  suits[3] = "Spades";

  vector<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";

  cout << ranks[rank] << " of " << suits[suit] << 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 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
Card card (1, 11);
card.print();
is Jack of Diamonds.

Checkpoint 12.3.1.

This active code uses the 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 Card::print() function.
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.

Checkpoint 12.3.2.

How would we select the appropriate string for the instance variable rank?
  • rank.ranks
  • Incorrect! Remember, ranks is a vector!
  • ranks.rank
  • Incorrect! Remember, ranks is a vector!
  • ranks[rank]
  • Correct! This is an example of how we use mapping!
  • rank[ranks]
  • Incorrect! This is using the vector "ranks" as an index to a single "rank".

Checkpoint 12.3.3.

Checkpoint 12.3.4.

Does it matter how we encode a mapping?
  • Yes, because the mappings should be easy for the programmer to remember.
  • Correct! The programmer should uses mappings that are easy to remember (even if this means we don’t use the zeroeth element of the ranks vector).
  • Yes, because the mappings should be easy for the user to remember.
  • Incorrect! The user doesn’t need to know how things are mapped.
  • No! All input and output uses human-readable formats, so the programmer doesn’t need to understand what is going on behind the scenes.
  • Incorrect! The programmer should always know what is going on with their code.
  • No! All input and output uses human-readable formats, so the user doesn’t need to understand what is going on behind the scenes.
  • Correct! The user doesn’t need to know how the programmer coded things.
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