Variables & Data Types

Introduction

Every Swift program is built out of values stored in variables and constants. Swift makes an unusually strong distinction between values that can change and values that can't — and pushes you toward the latter by default, which prevents a surprising number of bugs before they happen.

This lesson covers declaring variables and constants, letting Swift infer types automatically, Swift's basic data types, string interpolation, tuples, and why Swift never silently converts between types for you.

var vs let

Swift has two keywords for declaring storage: var for a variable whose value can change, and let for a constant whose value is set once and can never change afterward.

var score = 10        // variable — can change
let maxScore = 100    // constant — cannot change

score = 15             // OK
// maxScore = 200      // Compile-time error: cannot assign to a constant

The Swift compiler will actually warn you if you declare something with var but never change it, nudging you toward let. This isn't just style — immutable values are easier to reason about and are automatically safe to share across concurrent code.

Type Inference

Swift is statically typed, but you rarely have to write types out by hand. The compiler infers a variable's type from its initial value at the point of declaration.

let city = "Bengaluru"   // inferred as String
let temperature = 28.5   // inferred as Double
let isRaining = false     // inferred as Bool

Even though you didn't write the types explicitly, they are fixed the moment the variable is created — city can never later be assigned an Int, for example.

Type Annotations

You can also state a type explicitly using a colon, which is useful when you want a specific type (like Double instead of the inferred Int) or when there's no initial value yet.

var age: Int = 25
var price: Double = 19.99
var name: String = "Priya"
var isActive: Bool = true
var rating: Double  // declared, no value yet — must be assigned before use

Basic Types

Swift's standard library defines the core value types nearly every program uses.

String Interpolation

Swift lets you embed the value of any expression directly inside a string literal using \(...), called string interpolation.

let user = "Aisha"
let points = 42
print("\(user) scored \(points) points")

Tuples

A tuple groups multiple values into one compound value, optionally with named elements. Tuples are useful for quickly bundling a few related values without defining a whole new type.

let httpResponse = (statusCode: 200, message: "OK")

print(httpResponse.statusCode)
print(httpResponse.message)

Type Safety

Swift never silently converts between types, even numeric ones. Mixing an Int and a Double in an expression is a compile-time error unless you convert explicitly.

let wholeApples = 5
let sharedAmong = 2

// let average = wholeApples / sharedAmong        // Int division: 2, likely not what you want
let average = Double(wholeApples) / Double(sharedAmong)
print(average)

Dividing two Int values in Swift performs integer division and silently truncates the decimal portion, which is a very common source of subtle bugs — explicitly converting to Double first avoids it.

Type Aliases

A typealias gives an existing type a new, often more descriptive, name — purely for readability, it does not create a distinct type.

typealias Milliseconds = Int

func wait(for duration: Milliseconds) {
    print("Waiting for \(duration)ms")
}
wait(for: 500)

Common Mistakes

Best Practices

Frequently Asked Questions

Key Takeaways

Summary

You now know how to declare variables and constants, let Swift infer types or state them explicitly, and work with the language's core data types safely. Next, you'll learn how to control the flow of your programs with conditionals, switch statements, and loops.