This tutorial belongs to the Swift series

Variables let us assign a value to a label, and are defined using the var keyword:

var name = "Roger"
var age = 8

Once a variable is defined, we can change its value:

age = 9

Variables that you do not want to change can be defined as constants, using the let keyword:

let name = "Roger"
let age = 8

Changing the value of a constant is forbidden.

When you define a variable and you assign it a value, Swift implicitly infers the type of it.

8 is an Int value.

"Roger" is a String value.

A decimal number like 3.14 is a Double value.

You can also specify the type at initialization time:

let age: Int = 8

but it’s usual to let Swift infer it, and it’s mostly done when you declare a variable without initializing it.

You can declare a constant, and initialize it later:

let age : Int

age = 8

Once a variable is defined, it is bound to that type, and you cannot assign to it a different type, unless you explicitly convert it.

You can’t do this:

var age = 8
age = "nine"

Int and String are just two of the built-in data types provided by Swift.