8.13. Multiple Choice Exercises¶

Which of the following are compound values?

struct student {
  string first_name, last_name;
  int year;
  double gpa;
};

struct professor {
  string first_name, last_name;
  string department;
  int class;
};

int main() {
  student x = { "John", "Doe", 2, 3.46 };
  student y = { "Jane", "Doe", 3, 3.68 };
  professor z = { "Richard", "Roe", "Computer Science", 101 };
  string college = "University of College";
  int student_pop = 3400;
  double avg_gpa = 3.2;
}

x

x is a student which is a struct.

y

y is a student which is a struct.

z

z is a professor which is a struct.

college

college is a string which is made up of characters.

student_pop

An int is not a compound value.

avg_gpa

A double is not a compound value.

What is wrong with the following struct definition?

struct chicken {
  string name;
  int num_legs;
  int eggs;
  bool eggs;
}

The word "struct" needs to be capitalized.

"struct" shouldn't be capitalized in a struct definition.

There needs to be a semicolon after the end curly brace.

It is a common error to forgot the semicolon at the end of struct definitions.

The struct cannot have two instance variables that are both named eggs.

Two symbol names in the same scope is a compile error

There is nothing wrong with the struct definition.

There is an error with the definition. Can you find it?

How do we assign the value of 4 to the instance variable num_legs of the dog object?

 struct dog {
   string name;
   int num_legs;
   bool is_panting;
 };

int main() {
  dog fido = { "Fido", 0, true };
}

dog.num_legs = 4;

The dog object is fido. We can use the dot notation on an object.

fido.legs = 4;

Check the name of the instance variable in the struct definition.

fido[legs] = 4;

We assign values to the instance variables of a struct using dot notation.

fido.num_legs = 4;

Using the member access operator on fido, we can set the value of num_legs to 4.

What is the output of the code below?

struct cube {
  int edge_length;
  int volume;
  int mass;
};

int main() {
  cube c;
  c.edge_length = 4;
  c.volume = 64;
  c.mass = 128;
  cout << c.edge_length << ", " << c.volume << ", " << c.mass << ", ";
  int density = c.mass / c.volume;
  cout << density;
}

4, 2, 64, 128

Check the ordering of the output statements.

4, 64, 128

Take a closer look at the output statements.

4, 64, 128, 2

The code outputs all instance variables and the density in the proper order.

edge_length, volume, mass, density

Dot notation accesses the values of the instance variables, not the names.

What is the output of the code below?

struct cube {
  int edge_length;
  int volume;
  int mass;
};

int calculate_density (cube c) {
  return c.mass / c.volume;
}

int main() {
  cube c = { 2, 8, 4 };
  int density = calculate_density (c);
  cout << density;
}

Because of integer division, density is 0 and thus the output is 0.

Density is mass divided by volume.

0.5

Take a closer look at what kind of division we are doing.

Integer division truncates the extra digits.

What is the value of s.coffee_cup_full when the code is done running?

struct student {
  string name;
  bool is_sleepy;
  bool coffee_cup_full;
};

void pour_coffee (student s) {
  s.coffee_cup_full = true;
}

int main() {
  student s = { "Thor Odinson", true, false };
  if (s.is_sleepy) {
    pour_coffee (s);
  }
}

true

Take a closer look at the function definition of pour_coffee.

false

Since we pass a student object by value to pour_coffee, the function makes a copy of the object and does not modify the original. If you wanted the original value to change, pass it by reference!

The type of coffe_cup_full is bool.

The type of coffe_cup_full is bool.

What is the value of r.battery_level_percentage when the code is done running?

struct robot {
  string name;
  int battery_level_percentage;
  bool is_fully_charged;
};

void charge_robot (robot& r) {
  if (r.battery_level_percentage + 50 > 100) {
    r.battery_level_percentage = 100;
    r.is_fully_charged = true;
  }
  else {
    r.battery_level_percentage = r.battery_level_percentage + 50;
  }
}

int main() {
  robot r = { "Rob", 60, false };
  charge_robot (r);
}

100

The robot object is passed by reference to charge_robot, which caps the battery_level_percentage at 100.

110

Take a closer look at the charge_robot function.

Is the robot object passed by value or by reference to charge_robot?

That is the final value of r.is_fully_charged.

What is the output of the code below?

void foo (int& x, int y) {
  x = x + 4;
  y = 2 * x + 3 * y;
}

void bar (int x, int y) {
  y = 2 * x;
  x = x - 1;
  foo (x, x);
}

void func (int &x, int& y) {
  x = x + 3;
  bar (y, x);
}

int main() {
  int x = 4;
  int y = 7;
  func (y, x);
  cout << x << ", " << y;
}

4, 7

Take a closer look at func and its parameters. Are they passed by value, passed by reference, or both?

4, 10

Since bar doesn't pass either parameter by reference, neither bar nor foo affect the values of x and y.

7, 7

Check the order of the arguments passed into func.

35, 8

Take a closer look at the three functions. Are they all passed by reference?

If the user inputted the string "R2-D2", what is the output of the code below?

int main() {
  string name;
  cin >> name;
  cout << "Hello, " << name << '!';
}

R2-D2

Take another look at the cout statement.

Hello name!

name is not in quotes so the value stored in name will be printed.

Hello, R2-D2!

"R2-D2" is stored in name and is then outputted in the cout statement.

name

cin reads input from the user.

Q10

If the user inputted the string "C-3PO", what is the output of the code below?

int main() {
  char name;
  cin >> name;
  cout << "Hello, " << name << '!';
}

Hello, CPO!

cin reads the first char in from user input.

Hello, C!

Since 'C' is the first char in the input, this is the correct output. The program will ignore everything that comes after the first char.

Hello, C-3PO!

Check the data type of name.

Error, we cannot read a character from user input.

We can read characters from user input.