Swift Variables and Data Types

Variables and constants store values so your program can use them later.

let and var

let name = "Ali"
var score = 70
score = 85
print("\(name) scored \(score)")

Ali scored 85

Writing name = "Bilal" would give an error, because name is a constant.

Type Inference and Annotations

Swift works out the type from the value. This is called type inference. You can also write the type yourself with a colon. This is a type annotation.

let city = "Karachi"          // inferred as String
let population: Int = 20000000
let rate: Double = 278.5
var isOpen: Bool = true
isOpen = false
print(city, population, rate, isOpen)

Karachi 20000000 278.5 false

Once a variable has a type, it cannot hold a different type. A String variable can never store a number.

Data Types

A Character also uses double quotes, but you must write the type: let grade: Character = "A".

Type Conversion

Swift never mixes types by itself. Int plus Double is an error. Convert one of them first:

let pens = 3
let pricePerPen = 25.5
let total = Double(pens) * pricePerPen
print(total)

let rounded = Int(total)
print(rounded)

let label = "Pens: " + String(pens)
print(label)

76.5
76
Pens: 3

Int(76.5) cuts off the decimal part. It does not round.

Converting Text to Numbers

Turning a string into a number may fail, for example Int("hello"). So Swift gives back an optional, which is either a number or nil (no value). You will learn optionals properly in the conditions lesson. For now, ?? gives a default value:

let input = "42"
let number = Int(input) ?? 0
print(number + 8)

let bad = Int("abc") ?? 0
print(bad)

50
0

Naming Rules

• Names can contain letters, numbers and underscores, but cannot start with a number
• Use camelCase: studentName, totalPrice
• Names cannot be Swift keywords such as let or class

Exercise