Dictionaries & Sets

Swift provides Dictionaries for key-value pairs and Sets for unique values.

Dictionaries

A dictionary stores associations between keys of the same type and values of the same type in a collection with no defined ordering.

Creating Dictionaries

var airPorts = ["YYZ": "Toronto", "DUB": "Dublin"]
var emptyDict: [String: String] = [:]

Accessing and Modifying

print(airPorts["YYZ"] ?? "Unknown") // Subscripting returns an Optional

airPorts["LHR"] = "London" // Adding/Updating
airPorts["LHR"] = nil      // Removing a key

Iterating over Dictionaries

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

You can iterate over only the keys or only the values of the dictionaries as well, as shown below:

for key in airPorts.keys {
    print(key)
}

for value in airPorts.values {
    print(value)
}

Sets

A set stores unique values of the same type in a collection with no defined ordering.

Creating Sets

var genres: Set<String> = ["Rock", "Classical", "Jazz"]

// Empty Sets
var emptySet = Set<Int>()
var anotherEmptySet: Set<String> = [] // Type must be explicit to use []

Common Operations

Sets are highly efficient for membership tests.

genres.insert("Hip Hop")
genres.remove("Rock")

if genres.contains("Jazz") {
    print("I love Jazz!")
}

Set Operations

Swift sets support standard mathematical operations:

• .intersection(_:): Values in both sets.
• .union(_:): All values from both sets.
• .subtracting(_:): Values in one set but not the other.
• .symmetricDifference(_:): Values in either set, but not both.

let oddDigits: Set = [1, 3, 5, 7, 9]
let evenDigits: Set = [0, 2, 4, 6, 8]
let singleDigitPrimeNumbers: Set = [2, 3, 5, 7]

// Union: All values from both (1, 3, 5, 7, 9, 0, 2, 4, 6, 8)
oddDigits.union(evenDigits).sorted()

// Intersection: Values common to both (3, 5, 7)
oddDigits.intersection(singleDigitPrimeNumbers).sorted()

// Subtracting: Values in first but not in second (1, 9)
oddDigits.subtracting(singleDigitPrimeNumbers).sorted()

// Symmetric Difference: Values in either but not both (1, 2, 9)
oddDigits.symmetricDifference(singleDigitPrimeNumbers).sorted()