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!

YOU MIGHT ALSO LIKE...

August 28, 2025
SwiftUI

MijickPopups Hero

Popups Alerts Resizable Sheets Banners

August 27, 2025
SwiftUI

SwiftUI Tooltip

This package provides you with an easy way to show tooltips over any SwiftUI view, since Apple does not provide ...

August 27, 2025
SwiftUI
Uncategorized

SimpleToast for SwiftUI

SimpleToast is a simple, lightweight, flexible and easy to use library to show toasts / popup notifications inside iOS or ...

August 27, 2025
SwiftUI

SSToastMessage

Create Toast Views with Minimal Effort in SwiftUI Using SSToastMessage. SSToastMessage enables you to effortlessly add toast notifications, alerts, and ...

August 27, 2025
SwiftUI

ToastUI

A simple way to show toast in SwiftUI Getting Started • Documentation • Change Log

1. Array<Element>

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

3. Dictionary<Key: Hashable, Value>

Quick chooser

Shared traits & tips

1. `Array<Element>`

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

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