Swift Constants + Variables

You’ll be hard pressed to write any code without running into constants and variables. They are so integral to code that they often go unnoticed. Swift variables and constants have a few nuances that we’ll go over here.

Constants are declared with the keyword `let`, and their value cannot be changed. Because you cannot change the value of a constant, you need to initialize it when you define it.

let aConstant: String = "This is a constant"
aConstant = "try a new value"  //Error- Cannot assign to value: 'aConstant' is a 'let' constant

Variables are declared with the keyword `var`, and their value can be changed as they are used in an application. Because variables can be changed, you’re free to declare a variable and initialize it later.

var aVariable: String = "This is a variable"
var anotherVariable: Int
anotherVariable = 7

Though a variable’s value can change, its type cannot. Once declared a string, it must always be a string.

aVariable = 4 // Error — Cannot convert value of type ‘String’ to specified type ‘Int’

Because Swift is a typed language when you declare a constant or variable, the type needs to be clear. Above we saw examples of explicit type declaration where we named the variable — aVariable, declared a type — String, and set its value — “This is a variable”.

This is all great, but there’s another way, which is where type inference comes into play. This means that if you declare a variable and initialize it with string value and no type declaration, the variable will be typed as a string.

Explicit Type Declaration: 
var myVar: Bool
Implicit Type Declaration:
var myNewVar = false

The caveat to type inference is that you must initialize the variable with a value in order to take advantage of type inference. The compiler needs some information in order to understand what type you are trying to use. Give it either a value or define a type.

While declaring the type can be unnecessary when you get the hang of type inference, there are times that it can be useful. When deciding whether or not to declare a type, I consider my future self — will I have to work to understand what’s happening if I do not declare a type? Oftentimes, the answer is no, so I make use of type inference and move on with my life. I am, however, an advocate for making sure my code is clear and self-documenting. Keep this in mind as you write Swift and become more familiar with the language.

Thanks for reading! Find more articles about Swift and programming here.