Learn

Dictionary<Key, Value> (or [Key: Value] shorthand) is Swift's hash map:

var ages: [String: Int] = [
    "Alice": 30,
    "Bob": 25,
]

print(ages["Alice"])           // Optional(30) — subscript returns V?
print(ages["Alice"] ?? 0)      // 30
ages["Carol"] = 40              // add
ages.removeValue(forKey: "Bob")

Looking up a missing key returns nil — the subscript type is V?. Use ["key", default: 0] for a default:

let count = ages["Carol", default: 0]

Iterate with destructuring:

for (name, age) in ages {
    print("\(name): \(age)")
}

Iteration order is not guaranteed. If you need ordered key-value pairs, use OrderedDictionary from the Swift Collections package.