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

Here,

• 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"

print(siteName) 

Output

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"
print(siteName)

Output

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.

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
print(x)

Output

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
print(siteName)

Output

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

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:

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.