Collections

Swift has three built-in collection types: Array for an ordered list, Dictionary for key-value pairs, and Set for a collection of unique, unordered values. All three are structs, so everything from the previous lesson about copy-on-assignment applies to them too.

Array

var scores = [88, 92, 79]
scores.append(95)

print(scores)
print("First: \(scores[0]), Count: \(scores.count)")

[88, 92, 79, 95]
First: 88, Count: 4

Dictionary

var ages: [String: Int] = ["Priya": 30, "Sam": 25]
ages["Jordan"] = 40

if let age = ages["Priya"] {
    print("Priya is \(age)")
}
print(ages["Nobody"] as Any)

Priya is 30
nil

Looking up a Dictionary key always returns an optional — ages["Priya"] is a String?, not a plain Int, precisely because the key might not exist. That's exactly the pattern from the optionals lesson showing up in a built-in type.

Set

var tags: Set<String> = ["swift", "ios", "swift"]

print(tags.count)
print(tags.contains("ios"))

2
true

Adding "swift" twice in the literal still leaves only 2 elements — a Set silently drops duplicates and doesn't guarantee any particular order, which is exactly why it's the right tool when uniqueness and fast membership checks matter more than sequence.

Iterating a Dictionary

let prices = ["apple": 1.5, "bread": 3.0]

for (item, price) in prices.sorted(by: { $0.key < $1.key }) {
    print("\(item): \(price)")
}

apple: 1.5
bread: 3.0