Constants and Variables

A constant or variable gives a name to a value so you can use it later.

Real-world analogy: it's a labeled box. A constant is a sealed box — once you put something in, it stays. A variable is a box you can open and refill.

Syntax

let name = "Ada"   // constant: can't change
var score = 0      // variable: can change

Example

let city = "Lagos"
var temperature = 24

print(city)
print(temperature)

temperature = 27
print(temperature)

temperature += 1
print(temperature)

Expected output

Lagos
24
27
28

temperature += 1 is short for temperature = temperature + 1.

Prefer let

Use let by default, and var only when the value really needs to change. It makes code easier to understand — if something is a let, you know it never changes — and the compiler warns you when a var is never modified:

warning: variable 'city' was never mutated; consider changing to 'let' constant

Naming

• Use lowerCamelCase: firstName, totalPrice, isLoggedIn.
• Names can't start with a digit or contain spaces.
• Choose descriptive names — totalPrice beats tp.

Common mistakes

• Changing a constant:

let limit = 10
limit = 20

error: cannot assign to value: 'limit' is a 'let' constant

• Using a variable before giving it a value: error: variable 'total' used before being initialized.

Practice

Create a constant appName and a variable launchCount starting at 0. Increase launchCount by 1 twice, then print Notes has launched 2 times.

Hint: you can put a value inside a string with \(...) — the Strings lesson explains it: "\(appName) has launched ...".

let appName = "Notes"
var launchCount = 0
launchCount += 1
launchCount += 1
print("\(appName) has launched \(launchCount) times")