9.12. Multiple Choice Exercises¶

Which of the following are variables of type book?

struct book {
  string title, author;
  int year_published;
  double price;
};

int main() {
  book gatsby = { "The Great Gatsby", "F. Scott Fitzgerald", 1925, 4.75 };
  book frankenstein = { "Frankenstein", "Mary Shelley", 1823, 5.99 };
  string flies = "Lord of the Flies";
  int year = 1954;
}

gatsby

gatsby is a book.

frankenstein

frankenstein is a book.

flies

flies is a string.

year

year is an int.

Which of the following are instance variables of type book?

struct book {
  string title, author;
  int year_published;
  double price;
};

int main() {
  book gatsby = { "The Great Gatsby", "F. Scott Fitzgerald", 1925, 4.75 };
  book frankenstein = { "Frankenstein", "Mary Shelley", 1823, 5.99 };
  string flies = "Lord of the Flies";
  int year = 1954;
}

gatsby

gatsby is a book, not an instance variable of book.

title

title is an instance variable of book.

year

year is an int declared in main, not an instance variable of book.

price

price is an instance variable of book.

What is the output of the code below?

struct book {
  string title, author;
  int year_published;
  double price;
};

void print_book (book& b) {
  cout << '"' << b.title << "\" by " << b.author << " (" << b.year_published << "), $" << b.price << '\n';
}

int main() {
  book mockingbird = { "To Kill a Mockingbird", "Harper Lee", 1960, 9.25 };
  double discounted_price = 7.19;
  b.price = discounted_price;
  print_book (mockingbird);
}

To Kill a Mockingbird by Harper Lee (1960), $9.25

Take a closer look at main. Was the price of the book modified?

"To Kill a Mockingbird" by Harper Lee (1960), $9.25

Take a closer look at main. Was the price of the book modified?

"To Kill a Mockingbird" by Harper Lee (1960), $7.19

The price was changed from $9.25 to $7.19.

"To Kill a Mockingbird" by Harper Lee (1960), $7.19

The \ are escape characters. Used in this context, they allow us to print quotation marks.

What kind of function is print_book?

struct book {
  string title, author;
  int year_published;
  double price;
};

void print_book (book& b) {
  cout << '"' << b.title << "\" by " << b.author << " (" << b.year_published << "), $" << b.price << '\n';
}

int main() {
  book dracula = { "Dracula", "Bram Stoker", 1897, 3.95 };
  print_book (dracula);
}

Pure function

print_book takes a book as an object but it doesn't modify it.

Modifier function

Does print_book modify the book object?

Fill-in function

print_book takes one parameter, and its parameter is not an empty book object.

Fruitful function

print_book does not return anything.

What is wrong with the code below?

struct book {
  string title, author;
  int year_published;
  double price;
}

void print_book (book& b) {
  cout << '"' << b.title << "\" by " << b.author << " (" << b.year_published << "), $" << b.price << '\n';
}

void apply_discount (const book& b, double discount) {
  b.price -= discount;
}

int main() {
  book godfather = { "The Godfather", "Mario Puzo", 1969, 10.90 };
  apply_discount (godfather, 5.40);
  print_book (godfather);
}

The struct definition is missing a semicolon at the end.

It's a common mistake to forget the semicolon at the end of struct definitions.

We are not allowed to pass in a book object by reference in print_book.

We are allowed to do this. It's usually a good idea to pass structures by reference since it won't make copies of the structures, thus saving memory space.

The keyword const needs to be removed in the function definition for apply_discount.

Since the apply_discount function modifies the book passed into it, we don't it to be const.

There are no errors with the code.

There are a couple errors. Can you find them?

What kind of function is apply_discount?

struct book {
  string title, author;
  int year_published;
  double price;
};

void print_book (book& b) {
  cout << '"' << b.title << "\" by " << b.author << " (" << b.year_published << "), $" << b.price << '\n';
}

void apply_discount (book& b, double discount) {
  b.price -= discount;
}

int main() {
  book godfather = { "The Godfather", "Mario Puzo", 1969, 10.90 };
  apply_discount (godfather, 5.40);
  print_book (godfather);
}

Pure function

Does apply_discount modify the book object?

Modifier function

apply_discount modifies the book object by updating the price.

Fill-in function

apply_discount does not take an empty book object as a parameter.

Fruitful function

apply_discount does not return anything.

What is the output of the code below?

struct point_3d {
  double x, y, z;
};

void print_point_3d (const point_3d& p) {
  cout << '(' << p.x << ", " << p.y << ", " << p.z << ')' << '\n';
}

void midpoint (const point_3d& p1, const point_3d& p2, point_3d p3) {
  p3.x = (p1.x + p2.x) / 2;
  p3.y = (p1.y + p2.y) / 2;
  p3.z = (p1.z + p2.z) / 2;
}

int main() {
  point_3d p1 = { 3.0, 5.0, 2.0 };
  point_3d p2 = { 6.0, 3.5, 9.3 };
  point_3d p3 = { 0.0, 0.0, 0.0 };
  midpoint (p1, p2, p3);
  print_point_3d (p3);
}

(4.5, 4.25, 5.65)

Look at the function declaration of midpoint carefully.

(3.0, 5.0, 2.0)

Look at the function declaration of midpoint carefully.

(9.0, 8.5, 11.3)

Look at the function declaration of midpoint carefully.

(0, 0, 0)

The last parameter in midpoint is not passed by reference, so a copy is made and changes are made to the copy, not the original.

What kind of function is midpoint?

struct point_3d {
  double x, y, z;
};

void print_point_3d (const point_3d& p) {
  cout << '(' << p.x << ", " << p.y << ", " << p.z << ')' << '\n';
}

void midpoint (const point_3d& p1, const point_3d& p2, point_3d& p3) {
  p3.x = (p1.x + p2.x) / 2;
  p3.y = (p1.y + p2.y) / 2;
  p3.z = (p1.z + p2.z) / 2;
}

int main() {
  point_3d p1 = { 3.0, 5.0, 2.0 };
  point_3d p2 = { 6.0, 3.5, 9.3 };
  point_3d p3 = { 0.0, 0.0, 0.0 };
  midpoint (p1, p2, p3);
  print_point_3d (p3);
}

Pure function

Does midpoint modify a point_3d object?

Modifier function

midpoint modifies the last point_3d object.

Fill-in function

midpoint takes an "empty" third point_3d and fills it with the average of the other two point_3d objects.

Fruitful function

midpoint does not return anything.

What is the output of the code below?

struct point_3d {
  double x, y, z;
};

void print_point_3d (const point_3d& p) {
  cout << '(' << p.x << ", " << p.y << ", " << p.z << ')' << '\n';
}

void midpoint (const point_3d& p1, const point_3d& p2, point_3d& p3) {
  p3.x = (p1.x + p2.x) / 2;
  p3.y = (p1.y + p2.y) / 2;
  p3.z = (p1.z + p2.z) / 2;
}

point_3d reflect_xy_plane(const point_3d& p) {
  point_3d flipped = p;
  flipped.z = -flipped.z;
  return flipped;
}

int main() {
  point_3d p = { 11.3, 4.5, 2.9 };
  point_3d p_reflected = reflect_xy_plane (p);
  print_point_3d (p_reflected);
}

(11.3, 4.5, 2.9)

Take a closer look at the implementation of reflect_xy_plane.

(11.3, 4.5, -2.9)

The point is reflected across the XY plane, so the z value is inverted.

(-11.3, -4.5, 2.9)

Take a closer look at the implementation of reflect_xy_plane.

(5.65, 2.25, 1.45)

Take a closer look at the implementation of reflect_xy_plane.

Q10

What is the output of the code below?

struct point_3d {
  double x, y, z;
};

void print_point_3d (const point_3d& p) {
  cout << '(' << p.x << ", " << p.y << ", " << p.z << ')' << '\n';
}

void midpoint (const point_3d& p1, const point_3d& p2, point_3d& p3) {
  p3.x = (p1.x + p2.x) / 2;
  p3.y = (p1.y + p2.y) / 2;
  p3.z = (p1.z + p2.z) / 2;
}

point_3d reflect_xy_plane(const point_3d& p) {
  point_3d flipped = p;
  flipped.z = -flipped.z;
  return flipped;
}

int main() {
  point_3d p1 = { 7.0, 3.5, 6.7 };
  point_3d p2 = { 2.0, 1.0, 0.0 };
  point_3d p3 = { 3.9, 4.5, 10.0 };
  point_3d p4 = reflect_xy_plane (p1);
  midpoint (p4, p3, p2);
  print_point_3d (p2);
}

(5.45, 4, 1.65)

Take a closer look at the implementation of reflect_xy_plane.

(5.45, 4, 8.35)

Check the arguments passed into midpoint.

(7.0, 3.5, -6.7)

Take a closer look at which point is being printed.

(4.5, 2.25, 3.35)

Check the arguments passed into midpoint.