Arrays, Dictionaries, Sets and Functional Operations

Collections as values

Swift's main collections are Array (ordered), Dictionary (key-value, unordered) and Set (unique, unordered). They are value types: assigning an array to another variable creates an independent copy, implemented efficiently with copy-on-write, so the actual copying happens only when one copy is modified. Collections declared with let are fully immutable. Accessing an array index out of range traps at run time; use indices.contains(i) or safe helpers when the index may be invalid. Dictionary subscripts return optionals, and dict[key, default: 0] += 1 makes counting easy. Higher-order functions express transformations clearly: map, filter, reduce, compactMap (drops nils), flatMap, sorted(by:), first(where:), contains(where:), allSatisfy and Dictionary(grouping:by:). Sets offer union, intersection, subtracting and O(1) average membership tests. Elements of sets and dictionary keys must be Hashable.

Collections in everyday code

Grouping, filtering, counting and set operations.

struct Order { let id: String; let city: String; let total: Double }

let orders = [
    Order(id: "o1", city: "Pune",  total: 1200),
    Order(id: "o2", city: "Delhi", total: 300),
    Order(id: "o3", city: "Pune",  total: 800),
]

let bigOrders = orders.filter { $0.total >= 500 }.sorted { $0.total > $1.total }
let revenue = orders.reduce(0) { $0 + $1.total }
let byCity = Dictionary(grouping: orders, by: \.city)          // [String: [Order]]
let cityTotals = byCity.mapValues { $0.reduce(0) { $0 + $1.total } }

var wordCounts: [String: Int] = [:]
for word in "to be or not to be".split(separator: " ") {
    wordCounts[String(word), default: 0] += 1
}

let quantities = ["3", "x", "5"].compactMap { Int($0) }        // [3, 5]

let morning: Set = ["Asha", "Ravi", "Meera"]
let evening: Set = ["Ravi", "Kabir"]
let both = morning.intersection(evening)                      // ["Ravi"]

var copy = orders          // value semantics: an independent copy (copy-on-write)
copy.removeAll()           // orders is unchanged

Out-of-range indices trap

array[5] on a three-element array crashes immediately. When an index comes from outside your control, check array.indices.contains(i) first or use first, last and first(where:), which return optionals.

Quick check: What does copy-on-write mean for Swift arrays?

• Copies share storage until one of them is modified, then the data is actually copied
• Arrays cannot be copied
• Every assignment copies all elements immediately
• Arrays are reference types