Quick Answer

Dictionaries provide fast lookups by key

Understanding the Issue

Swift dictionaries are collections of key-value pairs with O(1) access time. Keys must be Hashable. Dictionaries are value types (copied on assignment).

The Problem

This code demonstrates the issue:

Swift Error 
// Problem: Accessing missing key
let dict = ["a": 1]
let value = dict["b"] // nil

The Solution

Here's the corrected code:

Swift Fixed 
// Solution: Safe access
if let value = dict["a"] {
    print(value)
}

// Default values:
let value = dict["b", default: 0]

Key Takeaways

Use dictionaries for efficient key-based lookups with proper nil handling.