Collections: arrays, dictionaries, and sets

Collections store multiple values. Swift’s everyday collection types are arrays, dictionaries, and sets.

Arrays

An array stores ordered values of the same type:

let tasks = ["Write tests", "Fix layout", "Ship build"]
let firstTask = tasks[0]

Use arrays when order matters or when duplicates are allowed.

Prefer safe access when an index may be out of range:

if tasks.indices.contains(2) {
    print(tasks[2])
}

Dictionaries

A dictionary stores values by key:

let scores = [
    "Maya": 92,
    "Noah": 88,
]

let mayaScore = scores["Maya"]

Dictionary lookup returns an optional because the key might not exist.

Use dictionaries when you need fast lookup by an identifier, name, or other stable key.

Sets

A set stores unique values with no guaranteed order:

var selectedIDs: Set<String> = []
selectedIDs.insert("article-1")
selectedIDs.insert("article-1")

The value appears only once. Use sets when uniqueness matters more than order.

Transforming collections

Swift collections support useful transformations:

let prices = [12.0, 18.5, 7.25]

let discounted = prices.map { $0 * 0.9 }
let affordable = prices.filter { $0 < 15 }
let total = prices.reduce(0, +)

These methods are helpful when they make intent clearer. Use a loop when step-by-step logic is easier to read.

What to carry forward

• arrays store ordered values
• dictionaries store values by key
• sets store unique values
• dictionary lookups return optionals
• collection transforms are useful when they express intent clearly
• choose collection types by access pattern, not habit

Next, you will learn how closures make transformations and callbacks possible.