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.
The active code below 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.
- 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".
Q-2: How would we select the appropriate string for the instance variable rank?
- 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.
Q-4: Does it matter how we encode a mapping?