Swift constant

Constant Once set, the program is running you can not change its value.

Constants can be of any type of data, such as: integer constant, floating-point constants, character constants or string constants. There are also enumerated type constants:

Constants are like variables except that the value of a constant can not be changed once set, and the value of a variable can be altered.

Constant declaration

Constants are declared using the keywordlet the following syntax:

let constantName = <initial value>

The following is a simple instance of Swift program constants:

import Cocoa

let constA = 42
print(constA)

The above program execution results:

42

Type annotation

When you declare a variable or constant can add type annotations (type annotation), illustrate the type of constant or variable to store the value. If you want to add a type annotation, you need to add a colon and space after constant or variable name, and then add the type name.

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

The following is a simple example to demonstrate the Swift is in constant use type annotations. Note that the initial value must be constant defines:

import Cocoa

let constA = 42
print(constA)

let constB:Float = 3.14159

print(constB)

The above program execution results:

42
3.14159

Named Constant

Named constants can consist of letters, numbers and underscores.

The constant need to start with a letter or an underscore.

Swift is a case-sensitive language, so uppercase and lowercase letters are not the same.

You can also use a simple constant name Unicode characters following examples:

import Cocoa

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

let 你好 = "你好世界"
print(你好)

The above program execution results:

Hello, Swift!
你好世界

Constant output

Variables and constants can use print (swift 2 replaces the print println) function to output.

Parentheses can be used in the string with a backslash to insert constants as examples:

import Cocoa

let name = "本教程"
let site = "http://www.w3big.com"

print("\(name)的官网地址为：\(site)")

The above program execution results:

本教程的官网地址为：http://www.w3big.com