Lesson 07 of 20

9 mins read

Arrays, Sets & Higher-Order Collection Operations

In Plain English

Master Swift primary sequence collections. Learn ordered, type-safe Arrays ([Element]), unordered unique-element Sets (Set<Element>), set algebra (union, intersection), and functional pipelines with map, filter, reduce, and sorted.

The Mental Model

"An Array is a sequential playlist where song order matters and songs can repeat. A Set is a collector bag of unique badges: adding a duplicate badge does nothing, and membership lookup is instant O(1)."

Deep Dive: How It Works

Array ([T]): Ordered collection with zero-indexed access (e.g. var fruits = ["Apple", "Banana"]).

Set (Set<T>): Unordered collection of unique values requiring elements to conform to the Hashable protocol.

Higher-Order Transformers: .map() transforms elements, .filter() keeps matching elements, .reduce() aggregates to single value.

Copy-on-Write: Collections are value types copied lazily on modification for maximum performance.

Syntax Blueprint

var list = [1, 2, 3]
list.append(4)
let evens = list.filter { $0 % 2 == 0 }
let unique = Set(list)

Declare arrays with [], filter with closure shorthand $0, and deduplicate with Set.

Core Rules to Remember

Type Safety: Arrays and Sets only accept elements matching their declared type (e.g. [Int] cannot store strings).

Set Algebra: Perform instant math operations: setA.intersection(setB), setA.union(setB), setA.subtracting(setB).

Shorthand Closures: Use $0, $1 for inline parameters inside map, filter, and reduce.

Common Beginner Traps & How to Fix Them

Mistake:Accessing an index >= array.count.

Why it happens: Swift array subscripting bounds-checks and traps with a fatal error on out-of-bounds.

How to fix: Check index < array.count or use array.indices.contains(index).

Pro Tip: Use array.first and array.last to safely retrieve optionals instead of subscripting when collections might be empty.

Live Interactive Example

Hit Run Code to see it live

Functional Array Pipeline: Filter, Map and Reduce

Swift 5.10 / LLVM

Output Console

Click "Run Code" to view the rendered output.

How it works: Filtered to [20, 25, 30], doubled to [40, 50, 60], and reduced with + to sum 150.

Your Turn: Micro Challenge

No pressure! Edit the starter code below and test your solution with instant feedback.

Micro Exercise

Deduplicate and Sort Numbers

Declare let rawNumbers = [5, 2, 8, 2, 5, 1, 9].
Convert to a Set to eliminate duplicates.
Sort the set into ascending order using .sorted().
Print "Sorted Unique: " followed by the sorted array.

Sandbox Output

Click "Run & Check" to test your solution.

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