Swift: Swift Sets and Dictionaries Tutorial

A Set is like a bag of colored marbles — one of each color, mixed together with no particular order. A Dictionary is like a phone book — look up a number quickly by name. This lesson covers both powerful data structures.

1. What You'll Learn

Create and manipulate Sets, including intersection, union, and other operations
Create and manipulate Dictionaries for key-value storage
Understand the Hashable protocol and custom types as keys
Use Sets for efficient deduplication and membership checks
Traverse and modify Dictionaries with common methods

2. A Backend Engineer's Real Story

(1) Pain: 500 lines of if-else for user tag deduplication — painfully slow

Bob is building a user profiling system. Each user has multiple tags (e.g. "VIP", "PromoSensitive", "HighSpender") and he needs extensive set operations: find users who are both VIP and high spenders, merge old and new tags, exclude blacklisted tags. He started with arrays and loop-based deduplication:

SWIFT

let oldTags = ["VIP", "HighSpender", "NewUser"]
let newTags = ["VIP", "PromoSensitive", "HighSpender"]
var merged: [String] = []
for tag in oldTags + newTags {
    if !merged.contains(tag) {
        merged.append(tag)
    }
}

200,000 users x O(n^2) algorithm = server CPU maxed out for 20 minutes. Bob got complaints from ops.

(2) The Set and Dictionary Solution

SWIFT

let oldTags: Set = ["VIP", "HighSpender", "NewUser"]
let newTags: Set = ["VIP", "PromoSensitive", "HighSpender"]
let merged = oldTags.union(newTags)
let common = oldTags.intersection(newTags)
print("Merged: \(merged)")
print("Common: \(common)")

(3) Result: 20 minutes → 0.5 seconds

Metric	Array Loop	Set/Dictionary
200K user dedup	20 minutes	0.5 seconds
Lines of code	500+	30
Memory usage	200 MB	45 MB
Union operation	Hand-written loop	`.union()` one line

3. Sets

A Set is an unordered collection of unique elements. Arrays care about "order and repetition"; Sets care about "uniqueness and membership."

graph TB
    A[Set A] --- B["{1, 2, 3}"]
    C[Set B] --- D["{2, 3, 4}"]
    E[Union] --- F["{1, 2, 3, 4}"]
    G[Intersection] --- H["{2, 3}"]
    I[Symmetric Diff] --- J["{1, 4}"]
    K[Subtracting] --- L["A - B = {1}"]

Set Operation	Swift Method	Result
Union	union(_:)	All elements from both sets
Intersection	intersection(_:)	Elements shared by both sets
Difference	subtracting(_:)	Elements in A but not in B
Symmetric difference	symmetricDifference(_:)	Elements in exactly one set
Is subset	isSubset(of:)	All of A's elements are in B
Contains	contains(_:)	O(1) membership check

(1) Creating and Basic Operations

SWIFT

var fruits: Set<String> = ["Apple", "Banana", "Orange"]
fruits.insert("Apple")
fruits.insert("Grape")
fruits.remove("Banana")
print(fruits.contains("Apple"))
print(fruits.count)

(2) Set Operations

SWIFT

let a: Set = [1, 2, 3, 4, 5]
let b: Set = [4, 5, 6, 7, 8]
print("Union: \(a.union(b).sorted())")
print("Intersection: \(a.intersection(b).sorted())")
print("A - B: \(a.subtracting(b).sorted())")
print("Symmetric Diff: \(a.symmetricDifference(b).sorted())")

▶ Example: User Tag Management System

SWIFT

// ============================================
// Managing user tags with Sets
// ============================================
var userTags: Set<String> = ["VIP", "NewUser", "HighSpender"]
let campaignTags: Set = ["VIP", "PromoSensitive"]
let excludeTags: Set = ["Inactive", "Fraud"]
userTags.insert("iOSUser")
userTags.insert("VIP")
let targetUsers = campaignTags.subtracting(excludeTags)
print("Target tags: \(targetUsers)")
let vipHighSpender = userTags.intersection(["VIP", "HighSpender"])
print("VIP high spenders: \(vipHighSpender)")
let allActive = userTags.union(campaignTags).subtracting(excludeTags)
print("All active tags: \(allActive.sorted())")

Output:

TEXT 📖 Display only

Target tags: ["VIP", "PromoSensitive"]
VIP high spenders: ["VIP", "HighSpender"]
All active tags: ["HighSpender", "iOSUser", "NewUser", "PromoSensitive", "VIP"]

4. Dictionaries

A Dictionary is an unordered collection of key-value pairs, each key mapping uniquely to a value. Ideal for "look up by name" scenarios.

graph TB
    A[Dictionary] --> B["Key: Apple -> Value: 3"]
    A --> C["Key: Banana -> Value: 5"]
    A --> D["Key: Orange -> Value: 2"]
    B --> E[O(1) lookup by key]

Operation	Syntax	Notes
Create	KeyType: ValueType	Empty dictionary
Literal	["a": 1, "b": 2]	With initial values
Read	dict["key"]	Returns Optional
Assign	dict["key"] = value	Add or update
Delete	dict["key"] = nil	Remove the key-value pair
Traverse	`for (k, v) in dict`	Iterate all

(1) Creating and Modifying

SWIFT

var scores: [String: Int] = [:]
var populations = [
    "New York": 8_336_817,
    "Los Angeles": 3_979_576,
    "Chicago": 2_693_976
]
populations["Houston"] = 2_320_268
populations["New York"] = 8_400_000
populations["Chicago"] = nil
if let nyPopulation = populations["New York"] {
    print("NY population: \(nyPopulation)")
}

(2) Traversing Dictionaries

SWIFT

let capitals = [
    "USA": "Washington DC",
    "UK": "London",
    "Japan": "Tokyo",
    "France": "Paris"
]
for (country, city) in capitals {
    print("\(country): \(city)")
}
print("Countries: \(capitals.keys.sorted())")
print("Capitals: \(capitals.values.sorted())")

▶ Example: Inventory Management

SWIFT

// ============================================
// Managing inventory with Dictionary
// ============================================
var inventory: [String: Int] = [
    "Laptop": 15,
    "Mouse": 50,
    "Keyboard": 30
]
inventory["Monitor"] = 10
inventory["Mouse"]! += 20
if let laptopStock = inventory["Laptop"], laptopStock > 0 {
    inventory["Laptop"] = laptopStock - 1
    print("Shipped 1 Laptop")
}
let product = "Tablet"
if let stock = inventory[product] {
    print("\(product): \(stock) units")
} else {
    print("\(product) not found in inventory")
}
print("\n=== Inventory Report ===")
for (product, quantity) in inventory {
    let status = quantity < 20 ? "Low stock" : "In stock"
    print("\(product): \(quantity) units -- \(status)")
}

Output:

TEXT 📖 Display only

Shipped 1 Laptop
Tablet not found in inventory

=== Inventory Report ===
Laptop: 14 units -- Low stock
Mouse: 70 units -- In stock
Keyboard: 30 units -- In stock
Monitor: 10 units -- Low stock

5. Hashable Protocol and Nested Collections

Dictionary keys and Set elements must conform to the Hashable protocol — Swift uses hash values for fast lookup.

Built-in Hashable Types	When Manual Conformance Is Needed
String, Int, Double, Bool	Custom structs/classes
Array (with hashable elements)	Enums with associated values
Set, Dictionary (as values)	Types containing non-hashable properties

(1) Custom Types as Keys

SWIFT

struct Product: Hashable {
    let id: Int
    let name: String
}
var cart: [Product: Int] = [:]
let laptop = Product(id: 1001, name: "Laptop")
cart[laptop] = 2
print("Cart items: \(cart.count)")

(2) Nested Collections: Dictionary of Sets

SWIFT

var cityTags: [String: Set<String>] = [
    "Paris": ["Eiffel Tower", "Louvre"],
    "Tokyo": ["Shibuya", "Sensoji"]
]
cityTags["Paris"]?.insert("Arc de Triomphe")
cityTags["London"] = ["Big Ben", "Tower Bridge"]
for (city, landmarks) in cityTags {
    print("\(city): \(landmarks.sorted().joined(separator: ", "))")
}

▶ Example: User Group Statistics

SWIFT

// ============================================
// User group stats with Dictionary and Set
// ============================================
let userLanguages: [String: Set<String>] = [
    "Alice": ["Swift", "Python", "JavaScript"],
    "Bob": ["Python", "Java", "Go"],
    "Charlie": ["Swift", "Kotlin", "JavaScript"],
    "Diana": ["Java", "C#", "Python"]
]
let swiftUsers = userLanguages.filter { $0.value.contains("Swift") }
print("Swift developers: \(swiftUsers.count)")
let allLanguages = userLanguages.values.reduce([]) { $0.union($1) }
print("All languages: \(allLanguages.sorted())")
let fullStack = userLanguages.filter { $0.value.count >= 3 }
for (name, langs) in fullStack {
    print("Full stack: \(name) -- \(langs.sorted().joined(separator: ", "))")
}

Output:

TEXT 📖 Display only

Swift developers: 2
All languages: ["C#", "Go", "Java", "JavaScript", "Kotlin", "Python", "Swift"]
Full stack: Alice -- JavaScript, Python, Swift
Full stack: Charlie -- JavaScript, Kotlin, Swift
Full stack: Diana -- C#, Java, Python

6. Full Example: User Profile Tagging System

SWIFT

// ============================================
// User profile tagging system
// Combining Set and Dictionary concepts
// ============================================
import Foundation
struct UserProfile: Hashable {
    let id: Int
    let name: String
    var tags: Set<String>
}
var users: [Int: UserProfile] = [
    1: UserProfile(id: 1, name: "Alice", tags: ["VIP", "HighSpender", "iOS"]),
    2: UserProfile(id: 2, name: "Bob", tags: ["NewUser", "Android"]),
    3: UserProfile(id: 3, name: "Charlie", tags: ["VIP", "Android", "HighSpender"]),
    4: UserProfile(id: 4, name: "Diana", tags: ["iOS", "NewUser"])
]
let campaignTags: Set = ["VIP", "iOS"]
let excludeTags: Set = ["Fraud", "Inactive"]
var targetUserIds: Set<Int> = []
for (id, profile) in users {
    let effectiveTags = profile.tags.subtracting(excludeTags)
    if !effectiveTags.intersection(campaignTags).isEmpty {
        targetUserIds.insert(id)
    }
}
print("=== Campaign Target Users ===")
for id in targetUserIds.sorted() {
    if let user = users[id] {
        print("\(user.name) -- tags: \(user.tags.sorted().joined(separator: ", "))")
    }
}
print("\n=== Adding Tag: BetaTester ===")
for id in users.keys {
    users[id]?.tags.insert("BetaTester")
}
print("\n=== Tag Distribution ===")
var tagCounts: [String: Int] = [:]
for (_, profile) in users {
    for tag in profile.tags {
        tagCounts[tag, default: 0] += 1
    }
}
for (tag, count) in tagCounts.sorted(by: { $0.value > $1.value }) {
    print("\(tag): \(count) users")
}

Output:

TEXT 📖 Display only

=== Campaign Target Users ===
Alice -- tags: HighSpender, iOS, VIP
Charlie -- tags: Android, HighSpender, VIP
Diana -- tags: iOS, NewUser

=== Adding Tag: BetaTester ===

=== Tag Distribution ===
BetaTester: 4 users
Android: 2 users
HighSpender: 2 users
iOS: 2 users
NewUser: 2 users
VIP: 2 users

❓ FAQ

Q How do I choose between Set and Array?

A Use Set when you need uniqueness, fast lookup (O(1)), and don't care about order. Use Array when you need ordering, allow duplicates, and index-based access.

Q What does a Dictionary return when a key isn't found?

A It returns nil (Optional). Always handle dictionary lookups with if-let or the ?? operator.

Q What is Hashable, and why do Set and Dictionary keys need it?

A Hashable means the type can be hashed. Swift uses hash values to locate elements quickly (O(1) complexity). Int, String, and other basic types conform to Hashable by default.

Q What should I watch out for when using a custom struct as a dictionary key?

A It must conform to Hashable. If all stored properties are Hashable, Swift auto-synthesizes the hash method. Otherwise, implement hash(into:) and == manually.

Q What do the keys and values properties of Dictionary return?

A They return collection types — keys is Dictionary.Keys (usable like a Set), values is Dictionary.Values (usable like an Array). Both can be traversed directly.

📖 Summary

Set is an unordered, unique-element collection ideal for deduplication and membership checks
Set supports union, intersection, subtracting, and symmetricDifference operations
Dictionary is an unordered key-value pair collection; reading returns Optional
Dictionary keys and Set elements must conform to Hashable
Add dictionary entries by assigning a value; remove by setting to nil
Nested collections (Dictionary of Sets) enable complex data modeling

📝 Exercises

Beginner: Create a Set of your 5 favorite books. Check if "Swift Programming" is in it, add 2 new books, and print the final set.
Intermediate: Implement a simple English-Chinese dictionary using Dictionary. Add 5 words with translations, implement a lookup function (input English, return Chinese), and handle cases where a word is not found.
Challenge: Analyze a dataset: ["Alice": ["Swift", "Python"], "Bob": ["Java", "Swift"], "Charlie": ["Python", "Go"], "Diana": ["Swift", "Go"]]. Find users who know Swift but not Go, users who know Python or Java, the set of all languages (deduplicated), and the user count per language.

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About Us

Swift: Swift Sets and Dictionaries Tutorial

1. What You'll Learn

2. A Backend Engineer's Real Story

(1) Pain: 500 lines of if-else for user tag deduplication — painfully slow

(2) The Set and Dictionary Solution

(3) Result: 20 minutes → 0.5 seconds

3. Sets

(1) Creating and Basic Operations

(2) Set Operations

▶ Example: User Tag Management System

4. Dictionaries

(1) Creating and Modifying

(2) Traversing Dictionaries

▶ Example: Inventory Management

5. Hashable Protocol and Nested Collections

(1) Custom Types as Keys

(2) Nested Collections: Dictionary of Sets

▶ Example: User Group Statistics

6. Full Example: User Profile Tagging System

❓ FAQ

📖 Summary

📝 Exercises

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