Quick Answer

Unordered unique collections with fast membership testing

Understanding the Issue

Sets store unique values in no particular order. They provide O(1) complexity for membership tests and efficient set operations like union and intersection.

The Problem

This code demonstrates the issue:

Swift Error 
// Problem: Checking array membership
let names = ["Alice", "Bob", "Charlie"]
if names.contains("Bob") { // O(n) operation
    print("Found")
}

The Solution

Here's the corrected code:

Swift Fixed 
// Solution: Using a set
let nameSet: Set = ["Alice", "Bob", "Charlie"]
if nameSet.contains("Bob") { // O(1) operation
    print("Found")
}

// Set operations
let admins: Set = ["Alice", "Bob"]
let users: Set = ["Bob", "Charlie"]
let both = admins.intersection(users) // ["Bob"]

Key Takeaways

Use sets when you need fast membership testing or unique elements.