Welcome to today’s lesson! We’ll explore how to use Ruby’s Set class in a practical context by creating a student enrollment system for courses. Imagine you’re managing enrollments for an online course platform, where students enroll in courses, check their enrollments, or get a list of classmates. Using Set is ideal here since it automatically prevents duplicates, ensuring each student can only enroll in a course once.

By the end of this lesson, you’ll have a solid understanding of how to use sets and hashes for handling unique collections. Let’s get started!

In this system, we’ll build four methods for managing student enrollments:

• enroll(student, course): Adds a student to a course. If the student is already enrolled, it does nothing.
• unenroll(student, course): Removes a student from a course. Returns true if the student was enrolled and has now been removed, otherwise returns false. If the course becomes empty after unenrolling the student, it should also be removed from the system.
• is_enrolled(student, course): Checks if a student is enrolled in a course, returning true if they are and false otherwise.
• list_students(course): Returns an array of all students enrolled in a given course. If no students are enrolled, it returns an empty array.

Let’s implement each method one step at a time.

We’ll start by defining our EnrollmentSystem class and initializing it with a hash to store enrollments.

Ruby
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1require 'set'
2
3class EnrollmentSystem
4  def initialize
5    @courses_enrollment = {}  # Hash to store each course with its enrolled students
6  end
7end

This code sets up an empty courses_enrollment hash, where each course will map to a set of students. Now that we have our class structure, let’s move to the methods.

The enroll method adds a student to a course. If the course doesn’t exist yet in @courses_enrollment, we’ll create a new set for it.

Ruby
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1def enroll(student, course)
2  @courses_enrollment[course] ||= Set.new  # Initialize set if course doesn't exist
3  @courses_enrollment[course].add(student) # Add student to the course's set
4end

This method first checks if the course exists in @courses_enrollment. If it doesn’t, @courses_enrollment[course] ||= Set.new creates a new set for that course. Then, the student is added to the course’s set, ensuring the student is only enrolled once.

Ruby
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1# Example usage
2es = EnrollmentSystem.new
3es.enroll("Alice", "Math101")
4puts es.inspect  # Output: {"Math101"=>#<Set: {"Alice"}>}

We see here that after enrolling Alice in "Math101," the course is listed in @courses_enrollment with Alice as the only student.

The unenroll method removes a student from a course. If the course or student isn’t found, it returns false. If the student is successfully removed, it also checks if the course is now empty and, if so, removes it.

Ruby
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1def unenroll(student, course)
2  if @courses_enrollment[course] && @courses_enrollment[course].include?(student)
3    @courses_enrollment[course].delete(student)  # Remove student from the course
4    @courses_enrollment.delete(course) if @courses_enrollment[course].empty?  # Remove course if empty
5    return true
6  end
7  false
8end

Here, we first verify that both the course and the student exist in @courses_enrollment. If they do, @courses_enrollment[course].delete(student) removes the student from the course. If the course set becomes empty, @courses_enrollment.delete(course) removes the course entry altogether. This method returns true if the student was successfully unenrolled and false otherwise.

Ruby
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1# Example usage
2es = EnrollmentSystem.new
3es.enroll("Alice", "Math101")
4es.unenroll("Alice", "Math101")
5puts es.inspect  # Output: {}

In this example, Alice is enrolled and then successfully unenrolled. After Alice is removed, "Math101" no longer appears in @courses_enrollment since it has no enrolled students.

The is_enrolled method checks if a student is enrolled in a course, returning true if they are and false otherwise.

Ruby
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1def is_enrolled(student, course)
2  @courses_enrollment[course] && @courses_enrollment[course].include?(student)
3end

This method checks if both the course and the student exist in @courses_enrollment. If they do, it returns true; otherwise, it returns false.

Ruby
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1# Example usage
2es = EnrollmentSystem.new
3es.enroll("Alice", "Math101")
4puts es.is_enrolled("Alice", "Math101")  # Output: true
5puts es.is_enrolled("Bob", "Math101")    # Output: false

Here, the output confirms that Alice is enrolled in "Math101" but Bob is not.

Finally, the list_students method returns an array of all students enrolled in a course. If no students are enrolled, it returns an empty array.

Ruby
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1def list_students(course)
2  return @courses_enrollment[course].to_a if @courses_enrollment[course]
3  []
4end

In this method, @courses_enrollment[course].to_a converts the set of students to an array. If the course is not in @courses_enrollment, it simply returns an empty array.

Ruby
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1# Example usage
2es = EnrollmentSystem.new
3es.enroll("Alice", "Math101")
4es.enroll("Bob", "Math101")
5puts es.list_students("Math101").inspect  # Output: ["Alice", "Bob"]
6puts es.list_students("Physics101").inspect  # Output: []

This example shows that both Alice and Bob are listed in "Math101," while an empty array is returned for "Physics101," as no one is enrolled in it.

Below is the complete implementation of the EnrollmentSystem class combining all the methods we discussed:

Ruby
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1require 'set'
2
3class EnrollmentSystem
4  def initialize
5    @courses_enrollment = {}  # Hash to store each course and enrolled students
6  end
7
8  def enroll(student, course)
9    @courses_enrollment[course] ||= Set.new
10    @courses_enrollment[course].add(student)
11  end
12
13  def unenroll(student, course)
14    if @courses_enrollment[course] && @courses_enrollment[course].include?(student)
15      @courses_enrollment[course].delete(student)
16      @courses_enrollment.delete(course) if @courses_enrollment[course].empty?
17      return true
18    end
19    false
20  end
21
22  def is_enrolled(student, course)
23    @courses_enrollment[course] && @courses_enrollment[course].include?(student)
24  end
25
26  def list_students(course)
27    return @courses_enrollment[course].to_a if @courses_enrollment[course]
28    []
29  end
30end

This consolidated code provides a ready-to-use EnrollmentSystem class complete with methods to manage student enrollments effectively.

Below are examples demonstrating how to use the methods in the EnrollmentSystem class:

Ruby
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1# Creating a new enrollment system
2es = EnrollmentSystem.new
3
4# Enrolling students
5es.enroll("Alice", "Math101")
6es.enroll("Bob", "Math101")
7puts es.inspect  # Output: {"Math101"=>#<Set: {"Alice", "Bob"}>}
8
9# Checking enrollment
10puts es.is_enrolled("Alice", "Math101")  # Output: true
11puts es.is_enrolled("Charlie", "Math101")  # Output: false
12
13# Listing students in a course
14puts es.list_students("Math101").inspect  # Output: ["Alice", "Bob"]
15puts es.list_students("Physics101").inspect  # Output: []
16
17# Unenrolling students
18es.unenroll("Alice", "Math101")
19puts es.list_students("Math101").inspect  # Output: ["Bob"]
20
21# Removing an empty course
22es.unenroll("Bob", "Math101")
23puts es.inspect  # Output: {}

These examples demonstrate how to use the EnrollmentSystem methods to enroll, check enrollment, list students, and unenroll students while maintaining the system's integrity.

In today’s lesson, we created an EnrollmentSystem class using Ruby’s Set and Hash classes to manage student enrollments. We implemented methods to enroll and unenroll students, check if a student is enrolled, and list students in a course. This exercise demonstrates how to efficiently manage unique collections in Ruby using sets, especially in scenarios requiring efficient lookups and duplicate prevention.

Try extending this class with additional features or tackle similar challenges to deepen your understanding. Happy coding!