What is the difference between `let` and `var` in Swift?

Question

What is the difference between let and var in Apple's Swift language?

In my understanding, it is a compiled language but it does not check the type at compile time. It makes me confused. How does the compiler know about the type error? If the compiler doesn't check the type, isn't it a problem with production environment?

This error is given when I try to assign a value to a let:

Cannot assign to property: 'variableName' is a 'let' constant
Change 'let' to 'var' to make it mutable

Answer 1

The let keyword defines a constant:

let theAnswer = 42

The theAnswer cannot be changed afterwards. This is why anything weak can't be written using let. They need to change during runtime and you must be using var instead.

The var defines an ordinary variable.

What is interesting:

The value of a constant doesn’t need to be known at compile time, but you must assign the value exactly once.

Another strange feature:

You can use almost any character you like for constant and variable names, including Unicode characters:

let 🐶🐮 = "dogcow"

Excerpts From: Apple Inc. “The Swift Programming Language.” iBooks. https://itunes.apple.com/WebObjects/MZStore.woa/wa/viewBook?id=881256329

---

Community Wiki

Because comments are asking for adding other facts to the answer, converting this to community wiki answer. Feel free edit the answer to make it better.

Answer 2

According to The Swift Programming Language Book

Like C, Swift uses variables to store and refer to values by an identifying name. Swift also makes extensive use of variables whose values cannot be changed. These are known as constants, and are much more powerful than constants in C.

Both var and let are references, therefore let is a const reference. Using fundamental types doesn't really show how let is different than const. The difference comes when using it with class instances (reference types):

class CTest
{
    var str : String = ""
}

let letTest = CTest()
letTest.str = "test" // OK

letTest.str = "another test" // Still OK

//letTest = CTest() // Error

var varTest1 = CTest()
var varTest2 = CTest()
var varTest3 = CTest()

varTest1.str = "var 1"
varTest2.str = "var 2"
varTest3 = varTest1
varTest1.str = "var 3"

varTest3.str // "var 3"

Answer 3

Swift let vs var

let - constant
var - variable

[Constant vs variable]
[Struct vs Class]

Official doc docs.swift.org says

The value of a constant can’t be changed once it’s set, whereas a variable can be set to a different value in the future.

This terminology actually describes a reassign mechanism

Mutability

Mutability - changeable - object's state can be changed after creation^[About]

Value and Reference Type^[About]

Reference Type(Class)

Swift's classes are mutable a-priory

var + class
It can be reassigned or changed

let + class = constant of address
It can not be reassigned and can be changed

Value(Struct, Enum)

Swift's struct can change their mutability status:

var + struct = mutable
It can be reassigned or changed

let + struct = *immutable or rather unmodifiable^{[About] [Example] [Example]} = constant of value
It can not be reassigned or changed

*immutable - check testStructMutability test

Experiments:

class MyClass {
    var varClass: NSMutableString
    var varStruct: String
    
    let letClass: NSMutableString
    let letStruct: String
    
    init(_ c: NSMutableString, _ s: String) {
        varClass = c
        varStruct = s
        
        letClass = c
        letStruct = s
    }
}

struct MyStruct {
    var varClass: NSMutableString
    var varStruct: String
    
    let letClass: NSMutableString
    let letStruct: String
    
    init(_ c: NSMutableString, _ s: String) {
        varClass = c
        varStruct = s
        
        letClass = c
        letStruct = s
    }
    
    
    //mutating function block
    func function() {
//            varClass = "SECONDARY propertyClass" //Cannot assign to property: 'self' is immutable
//            varStruct = "SECONDARY propertyStruct" //Cannot assign to property: 'self' is immutable
    }

    mutating func mutatingFunction() {
        varClass = "SECONDARY propertyClass"
        varStruct = "SECONDARY propertyStruct"
    }
}

Possible use cases

func functionVarLetClassStruct() {
    
    var varMyClass = MyClass("propertyClass", "propertyStruct")
    
    varMyClass.varClass = "SECONDARY propertyClass"
    varMyClass.varStruct = "SECONDARY propertyStruct"
    
//        varMyClass.letClass = "SECONDARY propertyClass" //Cannot assign to property: 'letClass' is a 'let' constant
//        varMyClass.letStruct = "SECONDARY propertyStruct" //Cannot assign to property: 'letStruct' is a 'let' constant
    
    let letMyClass = MyClass("propertyClass", "propertyStruct")
    
    letMyClass.varClass = "SECONDARY propertyClass"
    letMyClass.varStruct = "SECONDARY propertyStruct"
    
//        letMyClass.letClass = "SECONDARY propertyClass" //Cannot assign to property: 'letClass' is a 'let' constant
//        letMyClass.letStruct = "SECONDARY propertyStruct" //Cannot assign to property: 'letStruct' is a 'let' constant
    
    var varMyStruct = MyStruct("propertyClass", "propertyStruct")
    
    varMyStruct.varClass = "SECONDARY propertyClass"
    varMyStruct.varStruct = "SECONDARY propertyStruct"
    
//        varMyStruct.letClass = "SECONDARY propertyClass" //Cannot assign to property: 'letClass' is a 'let' constant
//        varMyStruct.letStruct = "SECONDARY propertyStruct" //Cannot assign to property: 'letStruct' is a 'let' constant
    
    let letMyStruct = MyStruct("propertyClass", "propertyStruct")
    
//        letMyStruct.varClass = "SECONDARY propertyClass" //Cannot assign to property: 'letMyStruct' is a 'let' constant
//        letMyStruct.varStruct = "SECONDARY propertyStruct" //Cannot assign to property: 'letMyStruct' is a 'let' constant
    
//        letMyStruct.letClass = "SECONDARY propertyClass" //Cannot assign to property: 'letClass' is a 'let' constant
//        letMyStruct.letStruct = "SECONDARY propertyStruct" //Cannot assign to property: 'letStruct' is a 'let' constant
    
}

mutating - Mutating Struct's Functions

You can mark a struct's method as mutating

1. Indicates that this function changes internal property values
2. You are only able to call mutating function on var variable
3. Result is visible when mutating function is finished

func testStructMutatingFunc() {
    //given
    var varMyStruct = MyStruct("propertyClass", "propertyStruct")
    
    //when
    varMyStruct.mutatingFunction()
    
    //than
    XCTAssert(varMyStruct.varClass == "SECONDARY propertyClass" && varMyStruct.varStruct == "SECONDARY propertyStruct")
    
    // It is not possible to call a mutating function on a let variable
    let letMyStruct = MyStruct("propertyClass", "propertyStruct")
//        letMyStruct.mutatingFunction() //Cannot use mutating member on immutable value: 'letMyStruct' is a 'let' constant
}

inout inside a function

1. inout allows you to reassign/modify a passed(original) value.
2. You are only able to pass var variable inside inout parameter
3. Result is visible when function is finished

inout has a next flow:

1. passed value is copied into copied value before a function called
2. copied value is assign into passed value after the function finished

//InOut
func functionWithInOutParameter(a: inout MyClass, s: inout MyStruct) {
    
    a = MyClass("SECONDARY propertyClass", "SECONDARY propertyStruct") //<-- assign
    s = MyStruct("SECONDARY propertyClass", "SECONDARY propertyStruct") //<-- assign
}


func testInOutParameter() {

    //given
    var varMyClass = MyClass("PRIMARY propertyClass", "PRIMARY propertyStruct")
    var varMyStruct = MyStruct("PRIMARY propertyClass", "PRIMARY propertyStruct")

    //when
    functionWithInOutParameter(a: &varMyClass, s: &varMyStruct)

    //then
    XCTAssert(varMyClass.varClass == "SECONDARY propertyClass" && varMyClass.varStruct == "SECONDARY propertyStruct")
    XCTAssert(varMyStruct.varClass == "SECONDARY propertyClass" && varMyStruct.varStruct == "SECONDARY propertyStruct")
    
    
    // It is not possible to pass let into inout parameter
    let letMyClass = MyClass("PRIMARY propertyClass", "PRIMARY propertyStruct")
    let letMyStruct = MyStruct("PRIMARY propertyClass", "PRIMARY propertyStruct")
//        functionWithInOutParameter(a: &letMyClass, s: &letMyStruct) //Cannot pass immutable value as inout argument: 'letMyClass', 'letMyStruct' are 'let' constants
}     

*You steal are able to mutate let + struct

func testStructMutability()  {
    //given
    let str: NSMutableString = "propertyClass"
    let letMyStruct = MyStruct(str, "propertyStruct")
    
    //when
    str.append(" SECONDARY")
    
    //then
    XCTAssert(letMyStruct.letClass == "propertyClass SECONDARY")
}

Use let whenever you can. Use var when you must.

[Mutate structure]

Answer 4

let is used to define constants and var to define variables.

Like C, Swift uses variables to store and refer to values by an identifying name. Swift also makes extensive use of variables whose values can’t be changed. These are known as constants, and are much more powerful than constants in C. Constants are used throughout Swift to make code safer and clearer in intent when you work with values that don’t need to change.

https://docs.swift.org/swift-book/LanguageGuide/TheBasics.html

Answer 5

It's maybe better to state this difference by the Mutability / Immutability notion that is the correct paradigm of values and instances changeability in Objects space which is larger than the only "constant / variable" usual notions. And furthermore this is closer to Objective C approach.

2 data types: value type and reference type.

In the context of Value Types:

'let' defines a constant value (immutable). 'var' defines a changeable value (mutable).

let aInt = 1   //< aInt is not changeable

var aInt = 1   //< aInt can be changed

In the context of Reference Types:

The label of a data is not the value but the reference to a value.

if aPerson = Person(name:Foo, first:Bar)

aPerson doesn't contain the Data of this person but the reference to the data of this Person.

let aPerson = Person(name:Foo, first:Bar)
               //< data of aPerson are changeable, not the reference

var aPerson = Person(name:Foo, first:Bar)
               //< both reference and data are changeable.

eg:

var aPersonA = Person(name:A, first: a)
var aPersonB = Person(name:B, first: b)

aPersonA = aPersonB

aPersonA now refers to Person(name:B, first: b)

and

let aPersonA = Person(name:A, first: a)
let aPersonB = Person(name:B, first: b)

let aPersonA = aPersonB // won't compile

but

let aPersonA = Person(name:A, first: a)

aPersonA.name = "B" // will compile

Answer 6

Very simple:

• let is constant.
• var is dynamic.

Bit of description:

let creates a constant. (sort of like an NSString). You can't change its value once you have set it. You can still add it to other things and create new variables though.

var creates a variable. (sort of like NSMutableString) so you can change the value of it. But this has been answered several times.

Answer 7

The

Declaring Constants and Variables section of The Swift Programming Language documentation specifies the following:

You declare constants with the let keyword and variables with the var keyword.

Make sure to understand how this works for Reference types. Unlike Value Types, the object's underlying properties can change despite an instance of a reference type being declared as a constant. See the Classes are Reference Types section of the documentation, and look at the example where they change the frameRate property.

Answer 8

let defines a "constant". Its value is set once and only once, though not necessarily when you declare it. For example, you use let to define a property in a class that must be set during initialization:

class Person {

    let firstName: String
    let lastName: String

    init(first: String, last: String) {
         firstName = first
         lastName = last
         super.init()
    }
}

With this setup, it's invalid to assign to firstName or lastName after calling (e.g.) Person(first:"Malcolm", last:"Reynolds") to create a Person instance.

You must define a type for all variables (let or var) at compile time, and any code that attempts to set a variable may only use that type (or a subtype). You can assign a value at run time, but its type must be known at compile time.

Answer 9

let is used to declare a constant value - you won't change it after giving it an initial value.
var is used to declare a variable value - you could change its value as you wish.

Answer 10

One more difference, which I've encountered in other languages for Constants is : can't initialise the constant(let) for later , should initialise as you're about to declare the constant.

For instance :

let constantValue : Int // Compile error - let declarations require an initialiser expression

Variable

var variableValue : Int // No issues 

Answer 11

let is used to define constants and var to define variables. You define the string using var then particular String can be modified (or mutated) by assigning it to a variable (in which case it can be modified), and if you define the string using let its a constant (in which case it cannot be modified):

var variableString = "Apple"
variableString += " and Banana"
// variableString is now "Apple and Banana"

let constantString = "Apple"
constantString += " and another Banana"
// this reports a compile-time error - a constant string cannot be modified

Answer 12

in swift language let is a constant means is can not reassign but var can be reassigned

let question = "what is the difference between let and var?"

question = "another question" // this line cause syntax error

var answer = "let is constant and var is simple variable"

answer = "let can't be reassigned var can be reassigned" // this line will be excecuted

Answer 13

let keyword defines a constant

let myNum = 7

so myNum can't be changed afterwards;

But var defines an ordinary variable.

The value of a constant doesn’t need to be known at compile time, but you must assign it a value exactly once.

You can use almost any character you like for constant and variable names, including Unicode characters;

e.g.

var x = 7 // here x is instantiated with 7 
x = 99 // now x is 99 it means it has been changed.

But if we take let then...

let x = 7 // here also x is instantiated with 7 
x = 99 // this will a compile time error

Answer 14

Like Luc-Oliver, NullData, and a few others have said here, let defines immutable data while var defines mutable data. Any func that can be called on the variable that is marked mutating can only be called if it is a var variable (compiler will throw error). This also applies to func's that take in an inout variable.

However, let and var also mean that the variable cannot be reassigned. It has two meanings, both with very similar purposes

Answer 15

Everyone has pretty much answered this but here's a way you can remember what's what

Let will always say the same think of "let" as let this work for once and always as for "var" variable's can always change hence them being called variable's

Answer 16

The keyword var is used to define a variable whose value you can easily change like this:

var no1 = 1 // declaring the variable 
no1 = 2 // changing the value since it is defined as a variable not a constant

However, the let keyword is only to create a constant used when you do not want to change the value of the constant again. If you were to try changing the value of the constant, you will get an error:

let no2 = 5 // declaring no2 as a constant
no2 = 8 // this will give an error as you cannot change the value of a constant 

Answer 17

Let is an immutable variable, meaning that it cannot be changed, other languages call this a constant. In C++ it you can define it as const.

Var is a mutable variable, meaning that it can be changed. In C++ (2011 version update), it is the same as using auto, though swift allows for more flexibility in the usage. This is the more well-known variable type to beginners.

Answer 18

let is a constant value, so it can never be changed.

let number = 5  
number = 6               //This will not compile.

Var is a variable, and can change (but after it is defined not to a different data type.)

var number = 5
number = 6               //This will compile.

If you try changing the variable to a different dataType, it will not work

var number = 5
number = "Hello World"   //This will not compile.

Answer 19

The main difference is that var variable value can change, and let can't. If you want to have a user input data, you would use var so the value can be changed and use let datatype variable so the value can not be changed.

var str      = "dog"  // str value is "dog"
str          = "cat"  // str value is now "cat"

let strAnimal = "dog" // strAnimal value is "dog"
strAnimal     = "cat" // Error !

Answer 20

A value can be reassigned in case of var

 //Variables
 var age = 42
 println(age) //Will print 42
 age = 90
 println(age) //Will Print 90

** the newAge constant cannot be reassigned to a new value. Trying to do so will give a compile time error**

//Constants
let newAge = 92 //Declaring a constant using let
println(newAge) //Will print 92.

Answer 21

“Use let to make a constant and var to make a variable”

Excerpt From: Apple Inc. “The Swift Programming Language.” iBooks. https://itun.es/us/jEUH0.l

Answer 22

Even though you have already got many difference between let and var but one main difference is:

let is compiled fast in comparison to var.

Answer 23

var value can be changed, after initialization. But let value cannot be changed when it is initialized once.

In case of var

  function variable() {
     var number = 5, number = 6;
     console.log(number); // return console value is 6
  }
  variable();

In case of let

   function abc() {
      let number = 5, number = 6;
      console.log(number); // TypeError: redeclaration of let number
   }
   abc();

Answer 24

var is the only way to create a variables in swift. var doesn't mean dynamic variable as in the case of interpreted languages like javascript. For example,

var name = "Bob"

In this case, the type of variable name is inferred that name is of type String, we can also create variables by explicitly defining type, for example

var age:Int = 20

Now if you assign a string to age, then the compiler gives the error.

let is used to declare constants. For example

let city = "Kathmandu"

Or we can also do,

let city:String = "Kathmandu"

If you try to change the value of city, it gives error at compile time.

Answer 25

var is variable which can been changed as many times you want and whenever

for example

var changeit:Int=1
changeit=2
 //changeit has changed to 2 

---

let is constant which cannot been changed

for example

let changeit:Int=1
changeit=2
 //Error becuase constant cannot be changed

Answer 26

Though currently I am still reading the manual, but I think this is very close to C/C++ const pointer. In other words, something like difference between char const* and char*. Compiler also refuses to update content, not only reference reassignment (pointer).

For example, let's say you have this struct. Take care that this is a struct, not a class. AFAIK, classes don't have a concept of immutable state.

import Foundation


struct
AAA
{
    var inner_value1    =   111

    mutating func
    mutatingMethod1()
    {
        inner_value1    =   222
    }
}


let aaa1    =   AAA()
aaa1.mutatingMethod1()      // compile error
aaa1.inner_value1 = 444     // compile error

var aaa2    =   AAA()
aaa2.mutatingMethod1()      // OK
aaa2.inner_value1 = 444     // OK

Because the structs are immutable by default, you need to mark a mutator method with mutating. And because the name aaa1 is constant, you can't call any mutator method on it. This is exactly what we expected on C/C++ pointers.

I believe this is a mechanism to support a kind of const-correctness stuff.

Answer 27

Declare constants with the let keyword and variables with the var keyword.

let maximumNumberOfLoginAttempts = 10 var currentLoginAttempt = 0   
let maximumNumberOfLoginAttempts = 10
var currentLoginAttempt = 0

Declare multiple constants or multiple variables on a single line, separated by commas:

var x = 0.0, y = 0.0, z = 0.0

Printing Constants and Variables

You can print the current value of a constant or variable with the println function:

println(friendlyWelcome)

Swift uses string interpolation to include the name of a constant or variable as a placeholder in a longer string

Wrap the name in parentheses and escape it with a backslash before the opening parenthesis:

println("The current value of friendlyWelcome is \(friendlyWelcome)")

Reference : http://iosswift.com.au/?p=17

Answer 28

let is used for constants that can’t be modified while var is an ordinary variable

Example:

let name = “Bob” Something like name = “Jim” will throw an error since a constant can’t be modified.

Answer 29

SIMPLE DIFFERENCE

let = (can not be changed)

var = (any time update)

Answer 30

Source: https://thenucleargeeks.com/2019/04/10/swift-let-vs-var/

When you declare a variable with var, it means it can be updated, it is variable, it’s value can be modified.

When you declare a variable with let, it means it cannot be updated, it is non variable, it’s value cannot be modified.

var a = 1 
print (a) // output 1
a = 2
print (a) // output 2

let b = 4
print (b) // output 4
b = 5 // error "Cannot assign to value: 'b' is a 'let' constant"

Let us understand above example: We have created a new variable “a” with “var keyword” and assigned the value “1”. When I print “a” I get output as 1. Then I assign 2 to “var a” i.e I’m modifying value of variable “a”. I can do it without getting compiler error because I declared it as var.

In the second scenario I created a new variable “b” with “let keyword” and assigned the value “4”. When I print “b” I got 4 as output. Then I try to assign 5 to “let b” i.e. I’m trying to modify the “let” variable and I get compile time error “Cannot assign to value: ‘b’ is a ‘let’ constant”.