Array comparison and set operations in Ruby - Mini Project: Build & Apply

Array comparison and set operations

📖 Scenario: You are managing two lists of attendees for two different events. You want to find out who attended both events, who attended only one, and who attended either event.

🎯 Goal: Build a Ruby program that compares two arrays of names using set operations to find common attendees, unique attendees, and all attendees combined.

📋 What You'll Learn

Create two arrays with exact names as given

Create a variable to hold the common attendees

Create a variable to hold attendees unique to the first event

Create a variable to hold all attendees from both events

Print the results exactly as specified

💡 Why This Matters

🌍 Real World

Comparing lists of participants, customers, or items is common in event planning, marketing, and inventory management.

💼 Career

Understanding array comparison and set operations helps in data analysis, filtering, and reporting tasks in many programming jobs.

1

Create two arrays of attendees

Create an array called event1_attendees with these exact names: "Alice", "Bob", "Charlie". Also create an array called event2_attendees with these exact names: "Bob", "Diana", "Edward".

Ruby

# Create the arrays event1_attendees and event2_attendees with the exact names
# Your code here

Hint

Use square brackets [] to create arrays and separate names with commas.

2

Create variables for set operations

Create a variable called common_attendees that holds the names present in both event1_attendees and event2_attendees. Create a variable called unique_to_event1 that holds names in event1_attendees but not in event2_attendees. Create a variable called all_attendees that holds all unique names from both arrays combined.

Ruby

event1_attendees = ["Alice", "Bob", "Charlie"]
event2_attendees = ["Bob", "Diana", "Edward"]
# Create common_attendees, unique_to_event1, and all_attendees variables using set operations
# Your code here

Hint

Use & for intersection, - for difference, and | for union of arrays.

3

Prepare output strings

Create a variable called common_str that joins the names in common_attendees with commas. Create a variable called unique_str that joins the names in unique_to_event1 with commas. Create a variable called all_str that joins the names in all_attendees with commas.

Ruby

event1_attendees = ["Alice", "Bob", "Charlie"]
event2_attendees = ["Bob", "Diana", "Edward"]

common_attendees = event1_attendees & event2_attendees
unique_to_event1 = event1_attendees - event2_attendees
all_attendees = event1_attendees | event2_attendees
# Create common_str, unique_str, and all_str by joining array elements with commas
# Your code here

Hint

Use the join method with ", " to combine array elements into a string.

4

Print the results

Print the following lines exactly using puts:
1. "Common attendees: " followed by common_str
2. "Unique to event 1: " followed by unique_str
3. "All attendees: " followed by all_str

Ruby

event1_attendees = ["Alice", "Bob", "Charlie"]
event2_attendees = ["Bob", "Diana", "Edward"]

common_attendees = event1_attendees & event2_attendees
unique_to_event1 = event1_attendees - event2_attendees
all_attendees = event1_attendees | event2_attendees

common_str = common_attendees.join(", ")
unique_str = unique_to_event1.join(", ")
all_str = all_attendees.join(", ")
# Print the results as specified
# Your code here

Hint

Use puts and string interpolation with #{} to print variables inside strings.

Practice

1.

Which operator in Ruby returns the common elements between two arrays?

[1, 2, 3] ? [2, 3, 4]

easy

A. & (intersection)

B. | (union)

C. - (difference)

D. == (equality)

Solution

1. Step 1: Understand the operators

   The & operator returns elements common to both arrays.

2. Step 2: Apply to example arrays

   For arrays [1, 2, 3] and [2, 3, 4], common elements are 2 and 3.

3. Final Answer:

   & (intersection) -> Option A

4. Quick Check:

   Common elements = & [OK]

Hint: Use & for common items between arrays [OK]

Common Mistakes:

• Confusing union (|) with intersection (&)
• Using - for common elements
• Thinking == compares elements regardless of order

2.

Which of the following is the correct syntax to find the difference between two arrays a and b in Ruby?

easy

A. a == b

B. a - b

C. a | b

D. a & b

Solution

1. Step 1: Recall difference operator

   The - operator returns elements in a not in b.

2. Step 2: Confirm syntax correctness

   Using a - b is the correct Ruby syntax for difference.

3. Final Answer:

   a - b -> Option B

4. Quick Check:

   Difference = - [OK]

Hint: Use a - b to get items in a but not in b [OK]

Common Mistakes:

• Using & instead of - for difference
• Using | which is union
• Using == which checks equality

3.

What is the output of the following Ruby code?

a = [1, 2, 3, 4]
b = [3, 4, 5, 6]
result = a | b
puts result.inspect

medium

A. [1, 2, 3, 4, 5, 6]

B. [3, 4]

C. [1, 2]

D. [1, 2, 3, 4]

Solution

1. Step 1: Understand the union operator

   The | operator combines all unique elements from both arrays.

2. Step 2: Apply union to arrays

   Arrays a and b combined uniquely are [1, 2, 3, 4, 5, 6].

3. Final Answer:

   [1, 2, 3, 4, 5, 6] -> Option A

4. Quick Check:

   Union = | [OK]

Hint: Use | to combine unique elements from both arrays [OK]

Common Mistakes:

• Confusing union with intersection
• Expecting duplicates in output
• Using - which removes elements

4.

Identify the error in the following Ruby code snippet:

a = [1, 2, 3]
b = [2, 3, 4]
result = a & b
puts result

What is the problem?

medium

A. Syntax error due to missing parentheses

B. Outputs all elements from both arrays

C. Runtime error because & is not defined for arrays

D. No error; code runs and outputs common elements

Solution

1. Step 1: Check syntax and operators

   The & operator is valid for arrays to find intersection.

2. Step 2: Verify output behavior

   The code outputs common elements [2, 3] without error.

3. Final Answer:

   No error; code runs and outputs common elements -> Option D

4. Quick Check:

   Intersection & works fine [OK]

Hint: & works for arrays to find common items without syntax errors [OK]

Common Mistakes:

• Thinking & causes syntax error
• Expecting union instead of intersection
• Confusing output with difference

5.

You have two arrays representing user IDs:

active_users = [101, 102, 103, 104, 105]
inactive_users = [103, 105, 107]

Which Ruby expression gives you the list of users who are active but not inactive?

hard

A. active_users | inactive_users

B. active_users & inactive_users

C. active_users - inactive_users

D. active_users == inactive_users

Solution

1. Step 1: Understand the problem

   We want users in active_users but not in inactive_users.

2. Step 2: Use difference operator

   The - operator returns elements in first array not in second.

3. Step 3: Apply to given arrays

   active_users - inactive_users = [101, 102, 104]

4. Final Answer:

   active_users - inactive_users -> Option C

5. Quick Check:

   Difference = - [OK]

Hint: Use - to exclude inactive from active users [OK]

Common Mistakes:

• Using & which gives only common users
• Using | which combines all users
• Using == which checks equality