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

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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

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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   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    quot SECONDARY propertyClass quot    Cannot assign to property   self  is immutable               varStruct    quot SECONDARY propertyStruct quot    Cannot assign to property   self  is immutable            mutating func mutatingFunction             varClass    quot SECONDARY propertyClass quot          varStruct    quot SECONDARY propertyStruct quot           Possible use cases func functionVarLetClassStruct              var varMyClass   MyClass  quot propertyClass quot    quot propertyStruct quot            varMyClass varClass    quot SECONDARY propertyClass quot      varMyClass varStruct    quot SECONDARY propertyStruct quot                 varMyClass letClass    quot SECONDARY propertyClass quot    Cannot assign to property   letClass  is a  let  constant           varMyClass letStruct    quot SECONDARY propertyStruct quot    Cannot assign to property   letStruct  is a  let  constant          let letMyClass   MyClass  quot propertyClass quot    quot propertyStruct quot            letMyClass varClass    quot SECONDARY propertyClass quot      letMyClass varStruct    quot SECONDARY propertyStruct quot                 letMyClass letClass    quot SECONDARY propertyClass quot    Cannot assign to property   letClass  is a  let  constant           letMyClass letStruct    quot SECONDARY propertyStruct quot    Cannot assign to property   letStruct  is a  let  constant          var varMyStruct   MyStruct  quot propertyClass quot    quot propertyStruct quot            varMyStruct varClass    quot SECONDARY propertyClass quot      varMyStruct varStruct    quot SECONDARY propertyStruct quot                 varMyStruct letClass    quot SECONDARY propertyClass quot    Cannot assign to property   letClass  is a  let  constant           varMyStruct letStruct    quot SECONDARY propertyStruct quot    Cannot assign to property   letStruct  is a  let  constant          let letMyStruct   MyStruct  quot propertyClass quot    quot propertyStruct quot                  letMyStruct varClass    quot SECONDARY propertyClass quot    Cannot assign to property   letMyStruct  is a  let  constant           letMyStruct varStruct    quot SECONDARY propertyStruct quot    Cannot assign to property   letMyStruct  is a  let  constant                letMyStruct letClass    quot SECONDARY propertyClass quot    Cannot assign to property   letClass  is a  let  constant           letMyStruct letStruct    quot SECONDARY propertyStruct quot    Cannot assign to property   letStruct  is a  let  constant         mutating - Mutating Struct s Functions You can mark a struct s method as mutating  Indicates that this function changes internal property values You are only able to call mutating function on var variable Result is visible when mutating function is finished  func testStructMutatingFunc           given     var varMyStruct   MyStruct  quot propertyClass quot    quot propertyStruct quot              when     varMyStruct mutatingFunction              than     XCTAssert varMyStruct varClass     quot SECONDARY propertyClass quot   amp  amp  varMyStruct varStruct     quot SECONDARY propertyStruct quot               It is not possible to call a mutating function on a let variable     let letMyStruct   MyStruct  quot propertyClass quot    quot propertyStruct quot             letMyStruct mutatingFunction     Cannot use mutating member on immutable value   letMyStruct  is a  let  constant    inout inside a function  inout allows you to reassign modify a passed original  value  You are only able to pass var variable inside inout parameter Result is visible when function is finished  inout has  a next flow   passed value is copied into copied value before a function called copied value is assign into passed value after the function finished    InOut func functionWithInOutParameter a  inout MyClass  s  inout MyStruct             a   MyClass  quot SECONDARY propertyClass quot    quot SECONDARY propertyStruct quot      lt -- assign     s   MyStruct  quot SECONDARY propertyClass quot    quot SECONDARY propertyStruct quot      lt -- assign     func testInOutParameter            given     var varMyClass   MyClass  quot PRIMARY propertyClass quot    quot PRIMARY propertyStruct quot       var varMyStruct   MyStruct  quot PRIMARY propertyClass quot    quot PRIMARY propertyStruct quot          when     functionWithInOutParameter a   amp varMyClass  s   amp varMyStruct         then     XCTAssert varMyClass varClass     quot SECONDARY propertyClass quot   amp  amp  varMyClass varStruct     quot SECONDARY propertyStruct quot       XCTAssert varMyStruct varClass     quot SECONDARY propertyClass quot   amp  amp  varMyStruct varStruct     quot SECONDARY propertyStruct quot                    It is not possible to pass let into inout parameter     let letMyClass   MyClass  quot PRIMARY propertyClass quot    quot PRIMARY propertyStruct quot       let letMyStruct   MyStruct  quot PRIMARY propertyClass quot    quot PRIMARY propertyStruct quot             functionWithInOutParameter a   amp letMyClass  s   amp 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    quot propertyClass quot      let letMyStruct   MyStruct str   quot propertyStruct quot              when     str append  quot  SECONDARY quot              then     XCTAssert letMyStruct letClass     quot propertyClass SECONDARY quot       Use let whenever you can  Use var when you must    Mutate structure

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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

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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

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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

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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

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

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SIMPLE DIFFERENCE  let    can not be changed   var    any time update

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let is used to define constants and var to define variables

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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

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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

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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  quot constant   variable quot  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      lt  aInt is not changeable  var aInt   1      lt  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                    lt  data of aPerson are changeable  not the reference  var aPerson   Person name Foo  first Bar                    lt  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    quot B quot     will compile

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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Found a good answer hope it can help

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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

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var value can be change  after initialize  But let value is not be change  when it is intilize 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

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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

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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

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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

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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

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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

[swift] What is the difference between `let` and `var` in swift?

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