Real-world modeling in C++ - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a class named Car.

C++

class [1] {
public:
    int speed;
};

Drag options to blanks, or click blank then click option'

ACar

BVehicle

CEngine

DDrive

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different class name that doesn't match the problem description.

Forgetting to write the class name after the 'class' keyword.

The class is named Car, so the correct keyword after 'class' is 'Car'.

2fill in blank

medium

Complete the code to add a constructor that sets the speed of the Car.

C++

class Car {
public:
    int speed;
    Car([1] s) {
        speed = s;
    }
};

Drag options to blanks, or click blank then click option'

Abool

Bfloat

Cstring

Dint

Attempts:

3 left

💡 Hint

Common Mistakes

Using float or string instead of int for speed.

Forgetting to specify the type of the constructor parameter.

The speed is an integer, so the constructor parameter should be of type int.

3fill in blank

hard

Fix the error in the method that increases the speed by a given amount.

C++

class Car {
public:
    int speed;
    void accelerate(int amount) {
        speed [1] amount;
    }
};

Drag options to blanks, or click blank then click option'

A+=

B-=

C*=

D/=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-=' which decreases speed instead of increasing.

Using '*=' or '/=' which multiply or divide speed.

To increase speed by amount, use the '+=' operator.

4fill in blank

hard

Fill both blanks to create a method that returns true if the car is moving.

C++

class Car {
public:
    int speed;
    bool isMoving() {
        return speed [1] [2];
    }
};

Drag options to blanks, or click blank then click option'

A>

B0

C==

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' instead of '>' which only checks if speed equals zero.

Using '<' which checks if speed is less than zero, which is not correct here.

The car is moving if speed is greater than 0, so use 'speed > 0'.

5fill in blank

hard

Fill all three blanks to create a method that sets the speed only if the new speed is valid (non-negative).

C++

class Car {
public:
    int speed;
    void setSpeed(int newSpeed) {
        if (newSpeed [1] [2]) {
            speed [3] newSpeed;
        }
    }
};

Drag options to blanks, or click blank then click option'

A>=

B0

C=

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' instead of '>=' which excludes zero as valid speed.

Using '==' which checks equality instead of comparison.

Using '+=' instead of '=' which would add instead of set.

We check if newSpeed is greater or equal to 0, then assign it to speed using '='.

Practice

1. What is the main purpose of using classes in real-world modeling in C++?

easy

A. To write faster code without any structure

B. To represent real-world objects with attributes and behaviors

C. To avoid using variables in the program

D. To make the program run without errors

Solution

1. Step 1: Understand the role of classes

   Classes group data (attributes) and functions (methods) to model real-world things.

2. Step 2: Identify the purpose in modeling

   Classes help represent objects with their properties and actions clearly.

3. Final Answer:

   To represent real-world objects with attributes and behaviors -> Option B

4. Quick Check:

   Classes model objects = A [OK]

Hint: Classes group data and actions like real objects [OK]

Common Mistakes:

• Thinking classes only speed up code
• Confusing classes with simple variables
• Believing classes remove all errors

2. Which of the following is the correct way to declare a class named Car in C++?

easy

A. Car class { };

B. class = Car { };

C. class Car { };

D. class Car();

Solution

1. Step 1: Recall C++ class syntax

   The correct syntax starts with the keyword class, followed by the class name and braces.

2. Step 2: Check each option

   class Car { }; uses class Car { }; which is correct. Others have syntax errors.

3. Final Answer:

   class Car { }; -> Option C

4. Quick Check:

   Correct class syntax = B [OK]

Hint: Use 'class Name { };' to declare classes [OK]

Common Mistakes:

• Writing 'Car class' instead of 'class Car'
• Using parentheses after class name
• Missing semicolon after class definition

3. What will be the output of this C++ code?

class Dog {
public:
  int age;
  void bark() { std::cout << "Woof!"; }
};

int main() {
  Dog myDog;
  myDog.age = 5;
  std::cout << myDog.age << " ";
  myDog.bark();
  return 0;
}

medium

A. Woof!5

B. Woof! 5

C. 5Woof!

D. 5 Woof!

Solution

1. Step 1: Analyze output statements

   The code prints myDog.age followed by a space, then calls bark() which prints "Woof!".

2. Step 2: Combine outputs

   Output is 5 plus Woof!, so result is "5 Woof!".

3. Final Answer:

   5 Woof! -> Option D

4. Quick Check:

   Print age + space + bark = 5 Woof! [OK]

Hint: Check order and spaces in output statements [OK]

Common Mistakes:

• Assuming space after bark output
• Mixing order of printed values
• Ignoring no newline after prints

4. Identify the error in this C++ class definition:

class Book {
  string title;
  int pages;
public:
  void setTitle(string t) { title = t; }
  void setPages(int p) { pages = p; }
};

medium

A. Missing std:: before string

B. Missing semicolon after class definition

C. Methods should be declared outside the class

D. Private members must be declared after public

Solution

1. Step 1: Check usage of string type

   The code uses string without std:: or using namespace std;.

2. Step 2: Identify missing namespace

   In C++, string is in the std namespace, so std::string is needed unless using directive is present.

3. Final Answer:

   Missing std:: before string -> Option A

4. Quick Check:

   Use std::string or using namespace std [OK]

Hint: Always prefix string with std:: or use namespace std [OK]

Common Mistakes:

• Forgetting std:: prefix for string
• Thinking methods must be outside class
• Believing private must come after public

5. You want to model a Library that holds many Book objects. Which design best represents this real-world relationship in C++?

hard

A. class Library { std::vector<Book> books; };

B. class Library { Book book; };

C. class Library { int bookCount; };

D. class Library { void addBook(Book b); };

Solution

1. Step 1: Understand the relationship

   A library contains many books, so it needs a collection (like a vector) of Book objects.

2. Step 2: Evaluate options

   class Library { std::vector<Book> books; }; uses std::vector<Book> to hold multiple books, matching the real-world model.

3. Final Answer:

   class Library { std::vector<Book> books; }; -> Option A

4. Quick Check:

   Use collection for many objects = D [OK]

Hint: Use vector to hold many objects in a class [OK]

Common Mistakes:

• Using single object instead of collection
• Only storing count without objects
• Defining methods without data members