Array, Set, and Dictionary

Swift Learning

July 23, 2025

In Swift (on iOS or anywhere else), Array, Set, and Dictionary are all value-type collections (structs) with different guarantees:

1. `Array<Element>`

Ordered, indexed, allows duplicates.
Random access by integer index (O(1)).
Literal syntax: let nums = [1, 2, 3]

Common ops:

var a = [1, 2, 3]
a.append(4)
let first = a[0]
a.remove(at: 1)

Use when order matters or you need positions (e.g., table view data).

2. `Set<Element>` (where `Element: Hashable`)

Unordered, unique elements (no duplicates).
Fast membership tests/inserts/removals (O(1) average).
Literal via initializer: let primes: Set<Int> = [2, 3, 5, 7]

Useful set algebra:

let a: Set = [1, 2, 3]
let b: Set = [3, 4, 5]
a.union(b)            // {1,2,3,4,5}
a.intersection(b)     // {3}
a.subtracting(b)      // {1,2}
a.isSuperset(of: [1]) // true

Use when you only care whether something is present, not its position or duplicates.

3. `Dictionary<Key: Hashable, Value>`

Key–value lookup table. Keys are unique and hashable.
Average O(1) lookup/insert/remove by key.
Literal syntax: let ages = ["Alice": 30, "Bob": 27]

Common ops:

var dict = ["score": 10]
dict["score"] = 20           // update
dict["level"] = 3            // insert
let s = dict["score"]        // optional Int?
dict.removeValue(forKey: "score")
dict.updateValue(99, forKey: "bonus") // returns old value if any

Use when you need to associate values with identifiers (IDs, names, enums, etc.).

Quick chooser

Need…	Pick
Ordered list, duplicates, index access	`Array`
Fast “is this here?” checks, no duplicates	`Set`
Map keys to values	`Dictionary`

Shared traits & tips

Value semantics: Assigning or passing them copies the value. Swift uses copy-on-write, so it’s cheap until you mutate.
Generics & type inference: Compiler usually infers Element, Key, Value.
Mutability: let makes the whole collection immutable; use var to mutate.
Iteration order: Arrays keep order. Sets are conceptually unordered. Dictionaries maintain insertion order in current Swift, but don’t rely on it for logic.

Transforming:

let arr = [1, 2, 3]
let set = Set(arr)
let dict = Dictionary(uniqueKeysWithValues: arr.map { ($0, "\($0)") })

That’s the core difference. Want examples for custom types, performance, or bridging with Objective-C? Just ask!

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