Swift Variables, Constants and Literals

1. Swift Variables

In programming, a variable is a container (storage area) to hold data. For example,

var num = 10 

Here, num is a variable storing the value 10.

Declare Variables in Swift

In Swift, we use the var keyword to declare variables. For example,

var siteName:String
var id: Int


  • siteName is a variable of type String. Meaning, it can only store textual values.
  • id is a variable of Int type. Meaning, it can only store integer values.

Note: In Swift, we cannot change the type of a variable once it's declared.

Assign Values to Variables

You can assign values to variables using the = operator.

var siteName: String
siteName = "programiz.com"




You can also assign a variable directly without the type annotation as:

var siteName = "programiz.com"
print(siteName) // programiz.com

Here, the compiler automatically figures out that siteName is a variable of the String type.

Change Value of a Variable

You can change the value of an existing variable. Hence, the name variable. For example,

var siteName = "programiz.com"
// assigning a new value to siteName
siteName = "apple.com"



Here, the value of siteName is changed from "programiz.com" to "apple.com".

Rules for naming Swift Variables

The rules for naming variables are:

  1. Variables names must start with either a letter, an underscore _, or the dollar sign $. For example,
    // valid
    var a = "hello"
    var _a = "hello"
    var $a = "hello"
  2. Variable names cannot start with numbers. For example,
    // invalid
    var 1a = "hello" // throws error
  3. Swift is case-sensitive. So A and a are different variables. For example,
    var A = 5 
    var a = 55
    print(A) // 5
    print(a) // 55
  4. Avoid using Swift keywords like var, String, class, etc. as variable names.


  • It's a good practice to give a descriptive variable name. For example, numberofApples is a better variable name than a, apple, or n.
  • In Swift, variable names are generally written in camelCase if they have multiple words. For example, myVariable, addTwoNums, etc.

2. Swift Constants

A constant is a special type of variable whose value cannot be changed. For example,

let a = 5

Here, after a is initialized to 5, we cannot change its value.

Declare Constants in Swift

In Swift, we use the let keyword to declare constants. The value of a constant cannot be changed. For example,

let x = 5
x = 10      // Error


main.swift:4:1: error: cannot assign to value: 'x' is a 'let' constant

Also, you cannot declare a constant without initializing it. For example,

let siteName: String


main.swift:4:7: error: constant 'siteName' used before being initialized


  • If you are sure that the value of a variable won't change throughout the program, it's recommended to use let.
  • The rules for naming variables also apply to constants.

3. Swift Literals

Literals are representations of fixed values in a program. They can be numbers, characters, or strings, etc. For example, "Hello, World!", 12, 23.0, "C", etc.

Literals are often used to assign values to variables or constants.

For example:

let siteName = "Apple.com"

In the above expression, siteName is a variable, and "Apple.com" is a literal.

Integer Literals

Integer literals are those that do not have a fractional or an exponential part.

There are four types of integer literals in Swift:

Type Example Remarks
Decimal 5, 10, -68 Regular numbers.
Binary 0b101, 0b11 Start with 0b.
Octal 0o13 Start with 0o.
Hexadecimal 0x13 Start with 0x.

Floating-point Literals

Floating-point literals are numeric literals that have floating decimal points. For example,

let piValue: Float = 3.14

Here, 3.14 is a floating-point literal assigned to the piValue constant.

Boolean Literals

There are two boolean literals: true and false.

For example,

let pass: Bool = true  

Here, true is a boolean literal assigned to pass.

String and Character Literals

Character literals are Unicode characters enclosed in double-quotes. For example,

let someCharacter: Character = "S"

Here, S is a character literal assigned to someCharacter.

Similarly, String literals are sequences of characters enclosed in double quotes ".

For example,

let someString: String = "Swift is fun" 

Here, "Swift is fun" is a string literal assigned to someString.

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