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

Barbara Ericson, Allen B. Downey, Jason L. Wright (Editor)

Exercises 8.13 Multiple Choice Exercises

1.

Which of the following are compound values?
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struct Student {
  string firstName, lastName;
  int year;
  double gpa;
};

struct Professor {
  string firstName, lastName;
  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 studentPop = 3400;
  double avgGPA = 3.2;
}
  • x
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  • x is a Student which is a struct.
  • y
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  • y is a Student which is a struct.
  • z
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  • z is a Professor which is a struct.
  • college
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  • college is a string which is made up of characters.
  • studentPop
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  • An int is not a compound value.
  • avgGPA
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  • A double is not a compound value.
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2.

What is wrong with the following struct definition?
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struct Chicken {
  string name;
  int numLegs;
  int 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.
  • A struct cannot have two instance variables of both string and int
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  • Instance variables of different types in a single struct is fine.
  • There is nothing wrong with the struct definition.
    🔗
  • There is an error with the definition. Can you find it?
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3.

How do we assign the value of 4 to the instance variable numLegs of the Dog object?
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struct Dog {
  string name;
  int numLegs;
  bool isPanting;
};

int main() {
  Dog doug = { "Doug", 0, true };
}
  • Dog.numLegs = 4;
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  • The Dog object is doug. We can use the dot notation on an object.
  • doug.legs = 4;
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  • Check the name of the instance variable in the struct definition.
  • doug[legs] = 4;
    🔗
  • We can assign values to the instance variables of a struct using dot notation.
  • doug.numLegs = 4;
    🔗
  • Using dot notation on doug, we can set the value of numLegs to 4.
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4.

What is the output of the code below?
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struct Cube {
  int edgeLength;
  int volume;
  int mass;
};

int main() {
  Cube c;
  c.edgeLength = 4;
  c.volume = 64;
  c.mass = 128;
  cout << c.edgeLength << ", " << c.mass << ", " << c.volume << ", ";
  int density = c.mass / c.volume;
  cout << density;
}
  • 4, 64, 128, 2
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  • Check the ordering of the output statements.
  • 4, 128, 64
    🔗
  • Take a closer look at the output statements.
  • 4, 128, 64, 2
    🔗
  • The code outputs all instance variables and the density in the proper order.
  • edgeLength, volume, mass, density
    🔗
  • Dot notation accesses the values of the instance variables, not the names.
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5.

What is the output of the code below?
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struct Cube {
  int edgeLength;
  int volume;
  int mass;
};

int calculateDensity(Cube c) {
  return c.mass / c.volume;
}

int main() {
  Cube c;
  c = (Cube) { 2, 8, 4 };
  int density = calculateDensity(c);
  cout << density;
}
  • Because of integer division, density is 0 and thus the output is 0.
  • Density is mass divided by volume.
  • 0.5
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  • Take a closer look at what kind of division we are doing.
  • Integer division truncates the extra digits.
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6.

What is the value of s.coffeeCupFull when the code is done running?
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struct Student {
  string name;
  bool isSleepy;
  bool coffeeCupFull;
};

void pourCoffee(Student s) {
  s.coffeeCupFull = true;
}

int main() {
  Student s = { "Thor Odinson", true, false };
  if (s.isSleepy) {
    pourCoffee (s);
  }
}
  • true
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  • C++ outputs boolean values as either a 0 or 1.
  • false
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  • C++ outputs boolean values as either a 0 or 1.
  • Take a closer look at the function definition of pourCoffee.
  • Since we pass a Student object by value to pourCoffee, 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!
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7.

What is the value of r.batteryLevelPercentage when the code is done running?
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struct Robot {
  string name;
  int batteryLevelPercentage;
  bool isFullyCharged;
};

void chargeRobot(Robot& r) {
  if (r.batteryLevelPercentage + 50 > 100) {
    r.batteryLevelPercentage = 100;
    r.isFullyCharged = true;
  }
  else {
    r.batteryLevelPercentage = r.batteryLevelPercentage + 50;
  }
}

int main() {
  Robot r = { "Rob", 60, false };
  chargeRobot(r);
}
  • 100
    🔗
  • The Robot object is passed by reference to chargeRobot, which caps the batteryLevelPercentage at 100.
  • 110
    🔗
  • Take a closer look at the chargeRobot function.
  • 60
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  • Is the Robot object passed by value or by reference to chargeRobot?
  • That is the final value of r.isFullyCharged.
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8.

What is the output of the code below?
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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
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  • Take a closer look at func and its parameters. Are they passed by value, passed by reference, or both?
  • 4, 10
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  • 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
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  • Take a closer look at the three functions. Are they all passed by reference?
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9.

If the user inputted the string “R2-D2”, what is the output of the code below?
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int main() {
  string name;
  cin >> name;
  cout << "Hello, " << name << "!";
}
  • R2-D2
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  • Take another look at the cout statement.
  • Hello name!
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  • name is not in quotes so the value stored in name will be printed.
  • Hello, R2-D2!
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  • “R2-D2” is stored in name and is then outputted in the cout statement.
  • name
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  • cin reads input from the user.
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10.

If the user inputted the string “C-3PO”, what is the output of the code below?
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int main() {
  char name;
  cin >> name;
  cout << "Hello, " << name << "!";
}
  • Hello, CPO!
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  • cin reads the first char in from user input.
  • Hello, C!
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  • 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!
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  • Check the data type of name.
  • Error, we cannot read a character from user input.
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  • We can read characters from user input.
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11.

If the user inputted the string “Darth Vader”, what is the output of the code below?
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int main() {
  string quote;
  getline(cin, quote);
  cout << quote << " is the epitome of Star Wars!";
}
  • quote is the epitome of Star Wars!
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  • quote is not in quotes so the value stored in quote will be printed.
  • Darth Vader is the epitome of Star Wars!
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  • getline reads the entire line until the user hits Return or Enter.
  • Darth is the epitome of Star Wars!
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  • Check the manner in which the user input is acquired.
  • D is the epitome of Star Wars!
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  • Try Again! Pay attention to the way in which user input is recieved.
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