9.7.1. Constant declaration #

Constant use keyword let to declare the syntax is as follows:

let constantName = <initial value>

The following is an example of using constants in a simple Swift program:

import Cocoa

let constA = 42
print(constA)

The execution results of the above procedures are as follows:

42

9.7.2. Type annotation #

When you declare a constant or variable, you can add a type annotation to indicate the type of value to be stored in the constant or variable. If you want to add a type annotation, you need to add a colon and space after the constant or variable name, and then add the type name.

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

The following is a simple example of the use of type annotations for constants in Swift. It is important to note that when a constant is defined,it must have an initial value:

import Cocoa

let constA = 42
print(constA)

let constB:Float = 3.14159

print(constB)

The execution results of the above procedures are as follows:

42
3.14159

9.7.3. Constant naming #

The naming of constants can consist of letters, numbers, and underscores.

Constants need to start with a letter or an underscore.

Swift is a case-sensitive language, so uppercase and lowercase letters are different.

Constant names can also be used with a simple Unicode characters, as anexample:

import Cocoa

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

let Hello="Hello World"
print(Hello)

The execution results of the above procedures are as follows:

Hello, Swift!
Hello World

9.7.4. Constant output #

Variables and constants can be used print (swift 2 replaces print with println) function to output.

You can insert constants in a string using parentheses and backslashes, as shown in the following example:

import Cocoa

let name = "Novice Tutorial"
let site = "http://www.runoob.com"

print("\(name) The official website address of is ：\(site)")

The execution results of the above procedures are as follows:

The official website address of Cainiao Tutorial is：http://www.runoob.com