Variables & Data Types

Swift has two ways to declare a value: let for a constant that can never be reassigned, and var for a variable that can. Reaching for let by default, and only using var when you genuinely need to reassign, is idiomatic Swift.

let vs var

Swift main.swift

let name = "Priya"
var score = 10
score += 5

print(name, score)

Output

Priya 15

Trying to reassign name after its first assignment is a compile error, not a warning — Swift enforces immutability by default rather than trusting you to respect it.

Type inference and explicit types

Swift infers a value's type from what you assign it, but you can also state the type explicitly with a colon:

Swift main.swift

let age: Int = 30
let price: Double = 19.99
let isAvailable: Bool = true
let label: String = "In stock"

print(age, price, isAvailable, label)

Output

30 19.99 true In stock

Type safety

Swift main.swift

var count = 5
count = "five"  // compile error

Compiler output

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

Note: once Swift infers count is an Int from its first assignment, that type is locked in for the rest of the variable's life — there's no implicit conversion between types like Int and String, even for something as reasonable-looking as reassigning a number-holding variable to a different type. You'd need an explicit conversion like String(count) to go the other way intentionally.

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