Strings, numbers, and booleans | Swift

Swift programs use small built-in types constantly. The most common are String, Int, Double, and Bool.

Strings

A String stores text:

let firstName = "Maya"
let message = "Welcome, \(firstName)"

The \(firstName) syntax is string interpolation. Swift evaluates the expression inside the parentheses and inserts it into the string.

Strings are values, not loose character bags. Swift handles Unicode correctly, which matters for names, emoji, accented characters, and text from real users.

Numbers

Use Int for whole numbers:

let unreadMessages = 12
let page = 1

Use Double for decimal values:

let price = 19.99
let progress = 0.75

Swift does not automatically mix numeric types:

let count = 3
let price = 9.99

let total = Double(count) * price

The conversion is explicit because it can change meaning. Swift prefers visible conversions over hidden surprises.

Booleans

A Bool is either true or false:

let isLoggedIn = true
let hasUnreadMessages = false

Booleans are useful when they answer a clear yes-or-no question. Name them that way: isEnabled, hasPermission, canRetry.

Avoid stringly typed state

Do not use strings for values that have a limited set of valid states:

let status = "loading"

This is easy to mistype and hard for the compiler to protect. Later, you will use enums for this:

enum LoadingState {
    case idle
    case loading
    case failed
}

This is a core Swift habit: use the type system to model what is valid.

What to carry forward

String stores text
Int stores whole numbers
Double stores decimal numbers
Bool stores true-or-false state
Swift requires explicit numeric conversions
prefer meaningful types over magic strings

Next, you will use these values in control flow.