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6.x — Chapter 6 comprehensive quiz

Words of encouragement

Congratulations on reaching the end of the longest chapter in the tutorials! Unless you have previous programming experience, this chapter was probably the most challenging one so far. If you made it this far, you’re doing great!

The good news is that the next chapter is easy in comparison. And in the chapter beyond that, we reach the heart of the tutorials: Object-oriented programming!

Chapter summary

Arrays allow us to store and access many variables of the same type through a single identifier. Array elements can be accessed using the subscript operator ([]). Be careful not to index an array out of the array’s range. Arrays can be initialized using an initializer list or uniform initialization (in C++11).

Fixed arrays must have a length that is set at compile time. Fixed arrays will usually decay into a pointer when evaluated or passed to a function.

Loops can be used to iterate through an array. Beware of off-by-one errors, so you don’t iterate off the end of your array. For-each loops are useful when the array hasn’t decayed into a pointer.

Arrays can be made multidimensional by using multiple indices.

Arrays can be used to do C-style strings. You should generally avoid these and use std::string instead.

Pointers are variables that store the memory address of (point at) another variable. The address-of operator (&) can be used to get the address of a variable. The dereference operator (*) can be used to get the value that a pointer points at.

A null pointer is a pointer that is not pointing at anything. Pointers can be made null by initializing or assigning the value 0 (or in C++11, nullptr) to them. Avoid the NULL macro. Dereferencing a null pointer can cause bad things to happen. Deleting a null pointer is okay (it doesn’t do anything).

A pointer to an array doesn’t know how large the array they are pointing to is. This means sizeof() and for-each loops won’t work.

The new and delete operators can be used to dynamically allocate memory for a pointer variable or array. Although it’s unlikely to happen, operator new can fail if the operating system runs out of memory, so make sure to check whether new returned a null pointer.

Make sure to use the array delete (delete[]) when deleting an array. Pointers pointing to deallocated memory are called dangling pointers. Dereferencing a dangling pointer can cause bad things to happen.

Failing to delete dynamically allocated memory can result in memory leaks when the last pointer to that memory goes out of scope.

Normal variables are allocated from limited memory called the stack. Dynamically allocated variables are allocated from a general pool of memory called the heap.

A pointer to a const value treats the value it is pointing to as const.

A const pointer is a pointer whose value can not be changed after initialization.

A reference is an alias to another variable. References are declared using an ampersand, but this does not mean address-of in this context. References are implicitly const -- they must be initialized with a value, and a new value can not be assigned to them. References can be used to prevent copies from being made when passing data to or from a function.

The member selection operator (->) can be used to select a member from a pointer to a struct. It combines both a dereference and normal member access (.).

Void pointers are pointers that can point to any type of data. They can not be dereferenced directly. You can use static_cast to convert them back to their original pointer type. It’s up to you to remember what type they originally were.

Pointers to pointers allow us to create a pointer that points to another pointer.

std::array provides all of the functionality of C++ built-in arrays (and more) in a form that won’t decay into a pointer. These should generally be preferred over built-in fixed arrays.

std::vector provides dynamic array functionality that handles its own memory management and remember their size. These should generally be favored over built-in dynamic arrays.

Quiz time

1) Pretend you’re writing a game where the player can hold 3 types of items: health potions, torches, and arrows. Create an enum to identify the different types of items, and a fixed array to store the number of each item the player is carrying (use built-in fixed arrays, not std::array). The player should start with 2 health potions, 5 torches, and 10 arrows. Write a function called countTotalItems() that returns how many items the player has in total. Have your main() function print the output of countTotalItems().

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2) Write the following program: Create a struct that holds a student’s name and grade (on a scale of 0-100). Ask the user how many students they want to enter. Dynamically allocate an array to hold all of the students. Then prompt the user for each name and grade. Once the user has entered all the names and grades, sort the list by grade (highest first). Then print all the names and grades in sorted order.

For the following input:

Joe
82
Terry
73
Ralph
4
Alex
94
Mark
88

The output should look like this:

Alex got a grade of 94
Mark got a grade of 88
Joe got a grade of 82
Terry got a grade of 73
Ralph got a grade of 4

Hint: You can modify the selection sort algorithm from lesson 6.4 -- Sorting an array using selection sort to sort your dynamic array. If you put this inside its own function, the array should be passed by address (as a pointer).

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3) Write your own function to swap the value of two integer variables. Write a main() function to test it.

Hint: Use reference parameters

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4) Write a function to print a C-style string character by character. Use a pointer to step through each character of the string and print that character. Stop when you hit a null terminator. Write a main function that tests the function with the string literal “Hello, world!”.

Hint: Use operator++ to advance the pointer to the next character

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5) What’s wrong with each of these snippets, and how would you fix it?

a)

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b)

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c)

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d)

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e)

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6) Let’s pretend we’re writing a card game.

6a) A deck of cards has 52 unique cards (13 card ranks of 4 suits). Create enumerations for the card ranks (2, 3, 4, 5, 6, 7, 8, 9, 10, Jack, Queen, King, Ace) and suits (clubs, diamonds, hearts, spades).

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6b) Each card will be represented by a struct named Card that contains a rank and a suit. Create the struct.

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6c) Create a printCard() function that takes a const Card reference as a parameter and prints the card rank and value as a 2-letter code (e.g. the jack of spades would print as JS).

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6d) A deck of cards has 52 cards. Create an array (using std::array) to represent the deck of cards, and initialize it with one of each card.

Hint: Use static_cast if you need to convert an integer into an enumerated type.

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6e) Write a function named printDeck() that takes the deck as a const reference parameter and prints the values in the deck. Use a for-each loop.

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6f) Write a swapCard function that takes two Cards and swaps their values.

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6g) Write a function to shuffle the deck of cards called shuffleDeck(). To do this, use a for loop to step through each element of your array. Pick a random number between 1 and 52, and call swapCard with the current card and the card picked at random. Update your main function to shuffle the deck and print out the shuffled deck.

Hint: Review lesson 5.9 -- Random number generation for help with random numbers.
Hint: Don’t forget to call srand() at the top of your main function.
Hint: If you’re using Visual Studio, don’t forget to call rand() once before using rand.

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6h) Write a function named getCardValue() that returns the value of a Card (e.g. a 2 is worth 2, a ten, jack, queen, or king is worth 10. Assume an Ace is worth 11).

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7) Alright, challenge time! Let’s write a simplified version of Blackjack. If you’re not already familiar with Blackjack, the Wikipedia article for Blackjack has a summary.

Here are the rules for our version of Blackjack:
* The dealer gets one card to start (in real life, the dealer gets two, but one is face down so it doesn’t matter at this point).
* The player gets two cards to start.
* The player goes first.
* A player can repeatedly “hit” or “stand”.
* If the player “stands”, their turn is over, and their score is calculated based on the cards they have been dealt.
* If the player “hits”, they get another card and the value of that card is added to their total score.
* An ace normally counts as a 1 or an 11 (whichever is better for the total score). For simplicity, we’ll count it as an 11 here.
* If the player goes over a score of 21, they bust and lose immediately.
* The dealer goes after the player.
* The dealer repeatedly draws until they reach a score of 17 or more, at which point they stand.
* If the dealer goes over a score of 21, they bust and the player wins immediately.
* Otherwise, if the player has a higher score than the dealer, the player wins. Otherwise, the player loses (we’ll consider ties as dealer wins for simplicity).

In our simplified version of Blackjack, we’re not going to keep track of which specific cards the player and the dealer have been dealt. We’ll only track the sum of the values of the cards they have been dealt for the player and dealer. This keeps things simpler.

Start with the code you wrote in quiz #6. Create a function named playBlackjack() that returns true if the player wins, and false if they lose. This function should:
* Accept a shuffled deck of cards as a parameter.
* Initialize a pointer to the first Card named cardPtr. This will be used to deal out cards from the deck (see the hint below).
* Create two integers to hold the player’s and dealer’s total score so far.
* Implement Blackjack as defined above.

Hint: The easiest way to deal cards from the deck is to keep a pointer to the next card in the deck that will be dealt out. Whenever we need to deal a card, we get the value of the current card, and then advance the pointer to point at the next card. This can be done in one operation:

This returns the current card’s value (which can then be added to the player or dealer’s total), and advances cardPtr to the next card.

Also write a main() function that plays a single game of Blackjack.

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7a) Extra credit: Critical thinking time: Describe how you could modify the above program to handle the case where aces can be equal to 1 or 11.

Hint: It’s important to note that we’re only keeping track of the sum of the cards, not which specific cards the user has.

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7b) In actual blackjack, if the player and dealer have the same score, the result is a tie and neither wins. Describe how you’d modify the above program to account for this.

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7.1 -- Function parameters and arguments
Index
6.16 -- An introduction to std::vector

280 comments to 6.x — Chapter 6 comprehensive quiz

  • Kyle S

    5b solution - you don’t have information on how to fix it.  In general, I find your material very clear, but I’ve found this quiz to be very difficult to interpret the expectations and the answers you provide.

  • Michael

    Hi Alex,
    In 5) d)

    Suppose temp is not destroyed(memory freed up) when out of the block. Then temp is still considered in scope right? Since we have a pointer that points to temp which grants us access to it, and according to the definition, scope means where a variable is accessible.

    So is the duration of a variable related to whether its memory is freed up and the scope of a variable related to whether we still have access to that memory address?

    Thanks!

    • Alex

      See this lesson for a reminder of the definitions of scope and duration.

      A better example would be this:

      Variable array is a local variable with local scope and automatic duration. Hence, it gets destroyed at the end of the function. However, the allocated memory itself has dynamic duration and isn’t destroyed until it’s manually deleted by the programmer.

  • liuchen

    Hi,Alex! In the following program,I don’t understand why the result of "sizeof(string)" is 4 in the printCh function.
    void printCh(char string[])
    {
        std::cout << sizeof(string) << "\n";
        std::cout << sizeof(string[0]) << "\n";
    }

    int main()
    {
        char string[] = "hello world";
        printCh(string);
        std::cout << sizeof(string) << "\n";
        std::cout << sizeof(string[0]) << "\n";

        unsigned int x = 2;
        std::cout << sizeof(x) << "\n";

        return 0;
    }
    The result is:
    4
    1
    12
    1
    4

  • Kyo

    Hi Alex,
    First time i’m posting here, so wanted to thank you guys for this amazing website that teaches me much more than my school teachers do (french school ftw).

    Question is about the code i wrote for 7 a), modification for the ace value (getCardValue is the same function as in the solution) :

    This goes into my do..while that draws the cards when player hits, and issues the following warning : "value computed is not used [-Wunused-value]" on *cardPtr++.
    However, this snippet still seem to do what I want it to do… Should I ignore this warning ?

    Thank you again.

    • Kyo

      Sorry for the useless message (except for the thanks), I found the solution. I should have tried Stack Overflow before asking here.
      But hey, at least I inaugurated the 5th page.

      For anyone encountering the same warning :

      *cardPtr++ is the same as *(cardPtr++), it evaluates the increment first then dereferences it, so it gets the value the pointer points to (after being incremented) but does not use it. Hence the warning.

      TL;DR : cardPtr++ ftw.

      • Kyo

        Actually I need to edit what I said because it caused my own confusion and may cause confusion to other people trying hard to understand what’s happening.

        "so it gets the value the pointer points to (after being incremented)" <- WRONG, because apparently the post-increment operator, despite having a higher precedence than the dereference operator, "binds more tightly" (don’t ask me, Stack Overflow said so), which means dereference will happen BEFORE cardPtr is incremented. But it is still the pointer that gets incremented, NOT the value it’s pointing to.

        So that is why getCardValue(*cardPtr++) works and actually DOES read the current card’s value and not the next’s.

        Would definitely like to have your insight on this, Alex.

        (Since that’s way too many messages, please feel free to delete any or all of them if necessary…)

        • Alex

          If you look at http://www.learncpp.com/cpp-tutorial/31-precedence-and-associativity/, post-increment does take priority over the dereference. So the post-increment gets processed first.

          So with getCardValue(*cardPtr++), cardPtr++ gets executed first, which increments cardPtr, BUT because this is a post-increment evaluates to the pre-incremented version of cardPtr. This pre-incremented version gets dereferenced, so we get the current card value.

          Similarly, *cardPtr++ post-increments cardPtr and evaluates to the pre-increment version of cardPtr, which then gets dereferenced. Because the dereferenced value is not used, the compiler generates a warning. So, because the dereferenced value is not used, this expression is equivalent to cardPtr++.

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