Swift Basics: Variables, Types and Optionals

let and var

Swift has two ways to name a value: let for constants (can't change) and var for variables. Prefer let – it makes your intent clear and your code safer.

let name = "Ada"      // constant
var score = 0         // variable
score = 10            // fine
// name = "Grace"     // error: name is a constant

Type safety and inference

Every value has a type (Int, Double, String, Bool…). Swift usually infers it, but you can be explicit:

let count: Int = 42
let price = 9.99          // inferred Double
let greeting = "Hello, " + name

Optionals: Swift's safety net

An optional is a value that might be missing, written with a ?. This forces you to handle "no value" instead of crashing on a null. Safely unwrap with if let or guard let:

var middleName: String? = nil   // may or may not exist

if let mn = middleName {
    print("Middle name is (mn)")
} else {
    print("No middle name")
}

Optionals are why Swift apps crash far less than apps in languages that allow silent nulls.

Key points

• Use let by default; var only when a value must change.
• Swift is strongly typed but infers types for you.
• An optional (?) may hold a value or be nil.
• Unwrap optionals with if let / guard let – never force-unwrap blindly.