Collections

## Learning Objectives - Work with Arrays - Use Sets for unique values - Master Dictionaries - Understand collection mutability ## Arrays ### Creating Arrays ```swift // Empty array let empty: [Int] = [] let empty2 = Array() // With initial values let numbers = [1, 2, 3, 4, 5] let strings = ["one", "two", "three"] // Repeated value let zeros = [Int](repeating: 0, count: 5) // [0, 0, 0, 0, 0] ``` ### Type Annotation ```swift let ints: [Int] = [1, 2, 3] let doubles: [Double] = [1.0, 2.0, 3.0] let strings: [String] = ["a", "b", "c"] ``` ### Array Type Shorthand ```swift var names: [String] = ["Alice", "Bob"] // names is Array ``` ## Accessing Elements ### Index Access ```swift let fruits = ["apple", "banana", "cherry"] let first = fruits[0] // "apple" let last = fruits[2] // "cherry" // fruits[5] // Runtime error! ``` ### First and Last ```swift let first = fruits.first // Optional("apple") let last = fruits.last // Optional("cherry") if let first = fruits.first { print(first) } ``` ### Check if Empty ```swift let empty: [Int] = [] empty.isEmpty // true empty.count // 0 let numbers = [1, 2, 3] numbers.isEmpty // false numbers.count // 3 ``` ## Modifying Arrays ### var Arrays ```swift var array = [1, 2, 3] // Add element array.append(4) // [1, 2, 3, 4] // Append contents of another array array.append(contentsOf: [5, 6]) // [1, 2, 3, 4, 5, 6] // Insert at index array.insert(0, at: 0) // [0, 1, 2, 3, 4, 5, 6] // Remove array.remove(at: 0) // Returns 0, array is [1, 2, 3, 4, 5, 6] array.removeFirst() // Returns 1, array is [2, 3, 4, 5, 6] array.removeLast() // Returns 6, array is [2, 3, 4, 5] // Remove all array.removeAll() ``` ### Update Elements ```swift var scores = [90, 85, 88] scores[0] = 95 // [95, 85, 88] scores[1...2] = [100, 100] // [95, 100, 100] ``` ## Array Iteration ### For-In Loop ```swift let fruits = ["apple", "banana", "cherry"] for fruit in fruits { print(fruit) } // With index for (index, fruit) in fruits.enumerated() { print("\(index): \(fruit)") } ``` ## Array Methods ### Sorting ```swift var numbers = [3, 1, 4, 1, 5, 9, 2, 6] numbers.sort() // In-place: [1, 1, 2, 3, 4, 5, 6, 9] let sorted = numbers.sorted() // Returns new array // Custom sort let sortedDesc = numbers.sorted(by: >) // Descending ``` ### Reversing ```swift let reversed = numbers.reversed() // ReversedCollection let reversedArray = Array(numbers.reversed()) // [6, 2, 9, 5, 1, 4, 1, 3] ``` ### Contains and Find ```swift let numbers = [1, 2, 3, 4, 5] numbers.contains(3) // true numbers.contains { $0 > 3 } // true if let index = numbers.firstIndex(of: 3) { print("Found at \(index)") } if let first = numbers.first(where: { $0 > 3 }) { print("First > 3 is \(first)") // 4 } ``` ### Filtering ```swift let numbers = [1, 2, 3, 4, 5, 6] let evens = numbers.filter { $0 % 2 == 0 } // [2, 4, 6] let greaterThan3 = numbers.filter { $0 > 3 } // [4, 5, 6] ``` ### Mapping ```swift let numbers = [1, 2, 3] let doubled = numbers.map { $0 * 2 } // [2, 4, 6] let strings = numbers.map { String($0) } // ["1", "2", "3"] ``` ### Reducing ```swift let numbers = [1, 2, 3, 4, 5] let sum = numbers.reduce(0) { $0 + $1 } // 15 let product = numbers.reduce(1, *) // 120 ``` ## Sets ### Creating Sets ```swift // Empty set let empty: Set = Set() let empty2 = Set() // With values var fruits: Set = ["apple", "banana", "cherry"] let numbers: Set = [1, 2, 3, 4, 5] ``` ### Key Properties ```swift let colors = Set(["red", "green", "blue"]) colors.count // 3 colors.isEmpty // false colors.first // Optional value (no order) ``` ## Set Operations ### Insert and Remove ```swift var fruits: Set = ["apple", "banana"] fruits.insert("cherry") // ("cherry", true) if new fruits.remove("apple") // Optional("apple") if existed fruits.remove("grape") // nil (not in set) // Check membership if fruits.contains("banana") { print("Has banana") } ``` ### Union ```swift let setA: Set = [1, 2, 3] let setB: Set = [3, 4, 5] let union = setA.union(setB) // [1, 2, 3, 4, 5] let unionA = setA | setB // [1, 2, 3, 4, 5] ``` ### Intersection ```swift let common = setA.intersection(setB) // [3] let common2 = setA & setB // [3] ``` ### Subtraction ```swift let onlyA = setA.subtracting(setB) // [1, 2] let onlyA2 = setA - setB // [1, 2] ``` ### Symmetric Difference ```swift let diff = setA.symmetricDifference(setB) // [1, 2, 4, 5] let diff2 = setA ^ setB // [1, 2, 4, 5] ``` ### Subset and Superset ```swift let a: Set = [1, 2] let b: Set = [1, 2, 3] let c: Set = [1, 2, 3] a.isSubset(of: b) // true b.isSuperset(of: a) // true b.isStrictSubset(of: c) // false b.isStrictSuperset(of: a) // true ``` ## Dictionaries ### Creating Dictionaries ```swift // Empty dictionary let empty: [String: Int] = [:] let empty2 = Dictionary() // With values var ages = ["Alice": 30, "Bob": 25, "Charlie": 35] ``` ### Dictionary Type Annotation ```swift var names: [Int: String] = [ 1: "One", 2: "Two", 3: "Three" ] ``` ## Accessing Values ### Subscript Access ```swift let ages = ["Alice": 30, "Bob": 25] let age = ages["Alice"] // Optional(30) let unknown = ages["Eve"] // nil // Provide default let name = ages["Alice", default: 0] // 30 let missing = ages["Eve", default: 0] // 0 ``` ### Key Existence ```swift if ages["Alice"] != nil { print("Alice exists") } // Alternative if let aliceAge = ages["Alice"] { print("Alice is \(aliceAge)") } ``` ## Modifying Dictionaries ### Add/Update ```swift var ages = ["Alice": 30, "Bob": 25] // Add new ages["Charlie"] = 35 // Update existing ages["Alice"] = 31 // Update with closure ages.updateValue(40, forKey: "Bob") // Returns old value (25) ``` ### Remove ```swift var ages = ["Alice": 30, "Bob": 25, "Charlie": 35] ages.removeValue(forKey: "Charlie") // Returns removed value ages.removeValue(forKey: "Eve") // Returns nil ages["Bob"] = nil // Also removes ages.removeAll() ``` ## Dictionary Iteration ### Dictionary For-In Loop ```swift let ages = ["Alice": 30, "Bob": 25, "Charlie": 35] for (name, age) in ages { print("\(name): \(age)") } // Keys only for name in ages.keys { print(name) } // Values only for age in ages.values { print(age) } ``` ## Dictionary Methods ### Properties ```swift let ages = ["Alice": 30, "Bob": 25] ages.count // 2 ages.isEmpty // false ages.keys // ["Alice", "Bob"] ages.values // [30, 25] ``` ### Dictionary Filtering ```swift let ages = ["Alice": 30, "Bob": 25, "Charlie": 35, "Diana": 40] let adults = ages.filter { $0.value >= 30 } // ["Alice": 30, "Charlie": 35, "Diana": 40] ``` ### Dictionary Mapping ```swift let ages = ["Alice": 30, "Bob": 25] let descriptions = ages.mapValues { "\($0) years old" } // ["Alice": "30 years old", "Bob": "25 years old"] ``` ## Summary - Arrays: ordered, indexed, allow duplicates - Sets: unordered, unique values, mathematical operations - Dictionaries: key-value pairs, fast lookup - Use `var` for mutable collections - Access with subscripts or methods - Higher-order functions: map, filter, reduce - `first`, `last` return optionals - Use `isEmpty` before accessing