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Unit 4 • Lesson 8

Sets and Unique Collections

Overview

Sets automatically remove duplicates and allow for fast membership testing, making them perfect for unique collections. You'll learn how to perform set operations such as union, intersection, and difference for comparing data, enabling you to solve problems involving unique values and set mathematics.

Intermediate 20–25 min
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What You Will Learn in This Lesson

By the end of this lesson, you will know:

  • What sets are: Understand how sets store unique, unordered items.
  • Creating sets: Learn how to create and modify sets.
  • Set operations: Discover union, intersection, and difference operations.
  • When to use sets: Understand when sets are better than lists.

Why This Matters

Sets automatically ensure all items are unique - no duplicates allowed! This makes them perfect for situations where you need to eliminate duplicates from a list, check if something exists (membership testing), or perform mathematical set operations. Sets are also optimized for fast lookups, making them very efficient for checking membership!

Step 1: What Is a Set?

A set is an unordered collection of unique items. Unlike lists, sets don't allow duplicates and don't maintain order:

Creating Sets
# Create a set with curly braces
fruits = {"apple", "banana", "orange"}

# Create set from a list (removes duplicates)
numbers = set([1, 2, 2, 3, 3, 3])
print(numbers)  # Output: {1, 2, 3} (duplicates removed!)

# Empty set (must use set(), not {})
empty = set()
1

Curly Braces

Sets use curly braces {}, but unlike dictionaries, they contain individual values, not key-value pairs. To create an empty set, you must use set() because {} creates an empty dictionary!

2

No Duplicates

Sets automatically remove duplicates. If you try to add the same item twice, it only appears once in the set!

3

Unordered

Sets don't maintain the order of items. When you print a set, items might appear in a different order than you added them!

Key Concept: Sets are perfect for storing unique values. If you have a list with duplicates and need to remove them, convert it to a set! Sets also provide fast membership testing - checking if an item is in a set is very fast, even with large sets!

Mini Practice #1: Creating Sets

Try It Yourself

Try creating sets and see how duplicates are removed:

Press Run to see output

What happened? Sets automatically removed duplicates! Notice how the list [1, 2, 2, 3, 3, 3, 4] became {1, 2, 3, 4} - all duplicates were removed. This is one of the most useful features of sets - converting a list to a set is the easiest way to remove duplicates!

Step 2: Adding and Removing Items

Use add() to add items and remove() or discard() to remove them:

Modifying Sets
fruits = {"apple", "banana"}

# Add an item
fruits.add("orange")
print(fruits)  # Output: {'apple', 'banana', 'orange'}

# Remove an item
fruits.remove("banana")
print(fruits)  # Output: {'apple', 'orange'}

# Discard (doesn't error if item doesn't exist)
fruits.discard("grape")  # Safe - no error if "grape" doesn't exist

How It Works

The add() method adds a single item to the set. remove() removes an item and raises an error if it doesn't exist. discard() also removes an item but doesn't raise an error if the item doesn't exist - it's safer to use when you're not sure if the item is in the set!

Step 3: Set Operations

Sets support mathematical operations like union, intersection, and difference:

Set Operations
set1 = {1, 2, 3, 4}
set2 = {3, 4, 5, 6}

# Union (all items from both sets)
union = set1 | set2  # or set1.union(set2)
print(union)  # Output: {1, 2, 3, 4, 5, 6}

# Intersection (items in both sets)
intersection = set1 & set2  # or set1.intersection(set2)
print(intersection)  # Output: {3, 4}

# Difference (items in set1 but not in set2)
difference = set1 - set2  # or set1.difference(set2)
print(difference)  # Output: {1, 2}
1

Union

Union combines all items from both sets, removing duplicates. Use | or union() method.

2

Intersection

Intersection gives you items that appear in both sets. Use & or intersection() method.

3

Difference

Difference gives you items in the first set but not in the second. Use - or difference() method.

Mini Practice #2: Set Operations

Try It Yourself

Try performing set operations:

Press Run to see output

What happened? You used set operations to find common elements (intersection) and combine sets (union). The intersection found "Bob" who is in both sets. The union combined all students from both sets, automatically removing duplicates. Set operations are powerful for comparing and combining collections!

Step 4: Checking Membership

Sets are optimized for fast membership testing:

Membership Testing
fruits = {"apple", "banana", "orange"}

# Check if item is in set
if "apple" in fruits:
    print("Apple is in the set!")

# Fast membership test
print("banana" in fruits)  # Output: True
print("grape" in fruits)   # Output: False

Remember: Checking if an item is in a set is very fast, even with large sets. This makes sets perfect for membership testing. Use the in keyword to check membership!

End-of-Lesson Exercises

Exercise 1: Create and Modify a Set

Create a set called colors with values {"red", "green", "blue"}. Add "yellow" to the set, then remove "green". Print the final set.

Use add() to add items and remove() to remove items.

Write your code above and click "Check Answer" to verify it's correct.

Exercise 2: Set Operations

Create two sets: set1 = {1, 2, 3, 4} and set2 = {3, 4, 5, 6}. Find and print the intersection (common items) and the union (all items) of both sets.

Use & for intersection and | for union.

Write your code above and click "Check Answer" to verify it's correct.
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