Swift Collections Beyond Arrays¶

Arrays are the most common collection, but Swift provides Sets and Dictionaries for different access patterns. Choosing the right collection type directly impacts performance and correctness.

Sets - Unordered Unique Elements¶

A Set stores unique elements with no defined order. Lookup, insertion, and deletion are O(1) average.

// Declaration
var numbers: Set<Int> = [1, 2, 3, 4, 5]

// Duplicates silently ignored
var withDupes: Set<Int> = [1, 2, 2, 3, 3, 3]
print(withDupes.count)  // 3

// Order is NOT guaranteed
print(numbers)  // could be {3, 1, 5, 2, 4} or any permutation

Set Operations¶

let setA: Set<Int> = [1, 2, 3, 4]
let setB: Set<Int> = [3, 4, 5, 6]

setA.union(setB)           // {1, 2, 3, 4, 5, 6}
setA.intersection(setB)    // {3, 4}
setA.subtracting(setB)     // {1, 2}
setA.symmetricDifference(setB)  // {1, 2, 5, 6}

setA.isSubset(of: [1, 2, 3, 4, 5])   // true
setA.isDisjoint(with: [7, 8, 9])      // true (no common elements)

Set API¶

var fruits: Set<String> = ["apple", "banana"]

fruits.insert("cherry")     // inserts, returns (inserted: true, ...)
fruits.insert("apple")      // no-op, returns (inserted: false, ...)
fruits.remove("banana")     // removes by VALUE (not index - sets have no indices)
fruits.contains("apple")    // true - O(1) lookup
fruits.count                // element count
fruits.isEmpty              // boolean check

When to Use Sets¶

Use Case	Array	Set
Ordered data	Yes	No
Index-based access	Yes	No
Fast membership test	O(n)	O(1)
Guaranteed uniqueness	Manual	Automatic
Duplicates allowed	Yes	No
Mathematical operations	Manual	Built-in

Dictionaries - Key-Value Pairs¶

var user: [String: Any] = [
    "name": "Dmitry",
    "age": 25,
    "isPremium": true
]

// Access (returns Optional because key may not exist)
let name = user["name"]           // Optional("Dmitry")
let email = user["email"]         // nil

// Default value for missing keys
let email = user["email", default: "none"]  // "none"

// Modification
user["email"] = "[email protected]"  // add
user["age"] = 26                      // update
user.removeValue(forKey: "isPremium") // remove

Iterating Dictionaries¶

for (key, value) in user {
    print("\(key): \(value)")
}

// Keys and values as separate collections
let keys = Array(user.keys)
let values = Array(user.values)

Higher-Order Functions on Collections¶

All Swift collections support map, filter, reduce, compactMap, flatMap:

map - Transform Each Element¶

let numbers = [1, 2, 3, 4, 5]
let doubled = numbers.map { $0 * 2 }        // [2, 4, 6, 8, 10]
let strings = numbers.map { String($0) }     // ["1", "2", "3", "4", "5"]

// map returns same-sized collection with potentially different type
let isEven = numbers.map { $0 % 2 == 0 }    // [false, true, false, true, false]

filter - Select Elements¶

let evens = numbers.filter { $0 % 2 == 0 }  // [2, 4]

reduce - Aggregate to Single Value¶

let sum = numbers.reduce(0, +)               // 15
let product = numbers.reduce(1, *)           // 120

compactMap - Transform + Remove Nils¶

let strings = ["1", "two", "3", "four", "5"]
let ints = strings.compactMap { Int($0) }    // [1, 3, 5]

String Interpolation¶

Swift uses \() syntax for embedding values in strings:

let name = "Anna"
let greeting = "Hello, \(name)!"          // "Hello, Anna!"
let calc = "2 * 5 = \(2 * 5)"            // "2 * 5 = 10"

// Works with any expression
let price = 9.99
let formatted = "Price: $\(price)"        // "Price: $9.99"

This is distinct from concatenation and avoids the spacing issues of passing multiple arguments to print().

Gotchas¶

Set elements must be Hashable. Custom types need to conform to Hashable to be stored in a Set or used as Dictionary keys.
Dictionary access returns Optional. Always unwrap safely: if let value = dict["key"] or use default values: dict["key", default: 0].
Set order is random. Do not rely on the iteration order of a Set - it may change between runs. If order matters, use an Array and manually ensure uniqueness.
map on Dictionary returns Array of tuples, not a new Dictionary. To create a new dictionary from a transformation, use Dictionary(uniqueKeysWithValues: dict.map { ... }) or mapValues.

Cross-References¶

swift fundamentals - basic types, variables, constants
swift enums and optionals - Optional handling for dictionary access
swift generics - generic collection types
swiftui lists and grids - displaying collections in UI