Swift - Constants

Constants refer to fixed values that a program may not alter during its execution. Constants can be of any of the basic data types like an integer constant, a floating constant, a character constant, or a string literal. There are enumeration constants as well.

Constants are treated just like regular variables except the fact that their values cannot be modified after their definition.

Constants Declaration

Before you use constants, you must declare them using let keyword as follows −

let constantName = <initial value>

Following is a simple example to show how to declare a constant in Swift 4 −

let constA = 42
print(constA)

When we run the above program using playground, we get the following result −

42

Type Annotations

You can provide a type annotation when you declare a constant, to be clear about the kind of values the constant can store. Following is the syntax −

var constantName:<data type> = <optional initial value>

The following example shows how to declare a constant in Swift 4 using Annotation. Here it is important to note that it is mandatory to provide an initial value while creating a constant −

let constA = 42
print(constA)

let constB:Float = 3.14159
print(constB)

When we run the above program using playground, we get the following result.

42
3.1415901184082

Naming Constants

The name of a constant can be composed of letters, digits, and the underscore character. It must begin with either a letter or an underscore. Upper and lowercase letters are distinct because Swift 4 is a case-sensitive programming language.

You can use simple or Unicode characters to name your variables. Following are valid examples −

let _const = "Hello, Swift 4!"
print(_const)

let 你好 = "你好世界"
print(你好)

When we run the above program using playground, we get the following result −

Hello, Swift 4!
你好世界

Printing Constants

You can print the current value of a constant or variable using print function. You can interpolate a variable value by wrapping the name in parentheses and escape it with a backslash before the opening parenthesis: Following are valid examples −

let constA = "Godzilla"
let constB = 1000.00

print("Value of \(constA) is more than \(constB) millions")

When we run the above program using playground, we get the following result −

Value of Godzilla is more than 1000.0 millions