Swift Data Types

The Core Data Types

Every value in Swift has a type, and the four you will reach for constantly are Int for whole numbers, Double for decimal numbers, String for text, and Bool for true/false values. Swift is a strongly typed language, meaning a value's type is fixed once it's created and cannot silently change into something else.

let age: Int = 29
let price: Double = 19.99
let name: String = "Ada"
let isMember: Bool = true

print(age, price, name, isMember)

Type Inference

You rarely need to write type annotations in Swift because the compiler infers the type from the value on the right-hand side of the assignment. Writing let age = 29 is exactly equivalent to writing let age: Int = 29 — the compiler chooses Int because 29 has no decimal point. This keeps code concise while remaining fully type-safe.

let inferredInt = 42          // Int
let inferredDouble = 3.14     // Double
let inferredString = "hello"  // String
let inferredBool = false      // Bool

// Mixing Int and Double directly is not allowed:
// let total = inferredInt + inferredDouble // error

Int and Double in Practice

Int represents whole numbers and is the default choice for counting, indexing, and any value that shouldn't have a fractional part. Double represents 64-bit floating-point numbers and is the default choice for decimal math — even things like 2.0 are Double unless you say otherwise. Swift also has Float for lower-precision decimals, but Double is preferred unless memory is extremely tight.

var itemCount = 5          // Int
let unitPrice = 2.5        // Double
let total = Double(itemCount) * unitPrice
print(total) // 12.5

• Int — whole numbers, positive or negative, no decimal point.
• Double — decimal numbers with high precision, the default floating-point type.
• String — text, always enclosed in double quotes.
• Bool — exactly two values: true or false, used for conditions and flags.

Type Overview