Data Types and Type Inference

Every value in Swift has a type, which decides what you can do with it. The core types:

Type inference

Swift works out the type from the value you assign. This is called type inference:

let count = 10          // Int
let price = 9.99        // Double
let title = "Swift"     // String
let isReady = true      // Bool

Type annotations

You can also write the type explicitly after a colon. This is a type annotation:

let score: Int = 100
let ratio: Double = 3     // stored as 3.0
let initial: Character = "A"

Annotations are needed when the value alone would give a different type (3 would otherwise be an Int), or when you declare a constant before assigning it.

Types are fixed

Once a variable has a type, it keeps it. Swift never quietly converts between types:

var level = 1
level = "two"

error: cannot assign value of type 'String' to type 'Int'

Checking type(of:) shows a value's type:

print(type(of: 42))
print(type(of: 4.2))
print(type(of: "42"))

Int
Double
String

Common mistakes

• Mixing types in arithmetic. let total = 5 + 2.5 works, because both are literal numbers, but let a = 5; let b = 2.5; a + b fails — a is already an Int. Convert explicitly: Double(a) + b.