Kotlin Ternary Conditional Operator

Question

What is the equivalent of this expression in Kotlin   a   b   c   This is not valid code in Kotlin

User · Answer

If you do not what to use the standard notation you can also create/simulate it using infix with something like this:

create a class to hold your target and result:

data class Ternary<T>(val target: T, val result: Boolean)

create some infix functions to simulate a ternary operation

infix fun <T> Boolean.then(target: T): Ternary<T> {
    return Ternary(target, this)
}

infix fun <T> Ternary<T>.or(target: T): T {
    return if (this.result) this.target else target
}

Then you will be able to use it like this:

val collection: List<Int> = mutableListOf(1, 2, 3, 4)

var exampleOne = collection.isEmpty() then "yes" or "no"
var exampleTwo = (collection.isNotEmpty() && collection.contains(2)) then "yes" or "no"
var exampleThree = collection.contains(1) then "yes" or "no"

User · Answer

Java s equivalent of ternary operator  a   b   c   is a simple IF in Kotlin in one line  if a  b else c      there is no ternary operator  condition   then   else   because   ordinary if works fine in this role    https   kotlinlang org docs reference control-flow html if-expression    Special case for Null comparison   you can use the Elvis operator  if   a    null   a else b    equivalent to a    b

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In Kotlin you can use ternary operation like this  val x   if a   add b  else  add c

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Kotlin doesn t have a ternary operator  You can use the regular if expression as shown below  if  condition  exp1 else exp2 Also in addition to the fact that if in Kotlin is not a statement but an expression  i e  it evaluates to a value   in the case where you have multiple statements inside the body of an if branch  or the body of else or else if   the last line of the block is the value of that branch  For example  if  a gt b        println  quot a is greater than b quot       a    value of this if   else       println  quot b is greater than a quot       b    value of else

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Java   int temp   a   b   c    Equivalent to Kotlin   var temp   if  a  b else c

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You can use var a  if  a  b else c in place of the ternary operator   Another good concept of kotlin is Elvis operater  You don t need to check null every time   val l   b  length    -1   This will return length if b is not null otherwise it executes right side statement

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Another short approach to use   val value   String    Kotlin   value         Here kotlin itself checks null value and if it is null then it passes empty string value

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In Kotlin  if statements are expressions  So the following code is equivalent   if  a  b else c   The distinction between expression and statement is important here  In Java C  JavaScript  if forms a statement  meaning that it does not resolve to a value  More concretely  you can t assign it to a variable      Valid Kotlin  but invalid Java C  JavaScript var v   if  a  b else c   If you re coming from a language where if is a statement  this might seem unnatural but that feeling should soon subside

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There is no ternary operation in Kotlin  but there are some fun ways to work around that   As others have pointed out  a direct translation into Kotlin would look like this   val x   if  condition  result1 else result2   But  personally  I think that can get a bit cluttered and hard to read   There are some other options built into the library   You can use takeIf    with an elvis operator   val x   result1 takeIf   condition      result2   What is happening there is that the takeIf     command returns either your result1 or null  and the elvis operator handles the null option   There are some additional options  takeUnless      for example   val x   result1 takeUnless   condition      result2   The language is clear  you know what that s doing     If it s a commonly used condition  you could also do something fun like use an inline extension method   Let s assume we want to track a game score as an Int  for example  and we want to always return 0 if a given condition is not met   inline fun Int zeroIfFalse func     - gt  Boolean    Int   if   func invoke    0 else this        Ok  that seems ugly   But consider how it looks when it is used   var score   0 val twoPointer   2 val threePointer   3  score    twoPointer zeroIfFalse   scoreCondition    score    threePointer zeroIfFalse   scoreCondition      As you can see  Kotlin offers a lot of flexibility in how you choose to express your code   There are countless variations of my examples and probably ways I haven t even discovered yet   I hope this helps

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For myself I use following extension functions   fun T  or lt T gt  default  T   T   if  this    null  default else this  fun T  or lt T gt  compute     - gt  T   T   if  this    null  compute   else this   First one will return provided default value in case object equals null  Second will evaluate expression provided in lambda in the same case   Usage   1  e  getMessage   or  unknown   2  obj  lastMessage  timestamp or   Date       Personally for me code above more readable than if construction inlining

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Why would one use something like this   when a      true - gt  b   false - gt  b     when you can actually use something like this  a is boolean in this case    when     a - gt  b   else - gt  b

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You could define your own Boolean extension function that returns null when the Boolean is false to provide a structure similar to the ternary operator   infix fun  lt T gt  Boolean then param  T   T    if  this  param else null   This would make an a   b   c expression translate to a then b    c  like so   println condition then  yes      no     Update  But to do some more Java-like conditional switch you will need something like that  infix fun  lt T gt  Boolean then param     - gt  T   T    if  this  param   else null  println condition then    yes        no   pay attention on the lambda  its content calculation should be postponed until we make sure condition is true  This one looks clumsy  that is why there is high demanded request exist to port Java ternary operator into Kotlin

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Remember Ternary operator and Elvis operator hold separate meanings in Kotlin unlike in many popular languages  Doing expression  value1  value2 would give you bad words by the Kotlin compiler  unlike any other language as there is no ternary operator in Kotlin as mentioned in the official docs  The reason is that the if  when and try-catch statements themselves return values   So  doing expression  value1  value2 can be replaced by     val max   if  a   b         print  Choose a      else       print  Choose b         The Elvis operator that Kotlin has  works only in the case of nullable variables ex         If I do something like value3   value1    value2 then if value1 is null then value2 would be returned otherwise value1 would be returned      A more clear understanding can be achieved from these answers

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You can use if expression for this in Kotlin  In Kotlin if is an expression with a result value  So in Kotlin we can write  fun max a  Int  b  Int    if  a  gt  b  a else b   and in Java we can achieve the same but with larger code  int max int a  int b    return a  gt  b   a   b

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Take a look at the docs      In Kotlin  if is an expression  i e  it returns a value  Therefore there   is no ternary operator  condition   then   else     because ordinary if works fine in this role

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There is no ternary operator in Kotlin  the most closed are the below two cases    If else as expression statement      val a   true if a  print  A is true   else print  A is false      Elvis operator      If the expression to the left of    is not null  the elvis operator   returns it  otherwise it returns the expression to the right  Note   that the right-hand side expression is evaluated only if the left-hand   side is null     val name   node getName      throw IllegalArgumentException  name expected     Reference docs

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In case someone needs ternary operator with implicit casting        t   for ternary operator    Suppress  quot FunctionName quot    OptIn ExperimentalContracts  class  fun  lt T gt   t bool  Boolean  onTrue  T  onFalse  T   T       contract   returns   implies bool       return if  bool  onTrue else onFalse    Although not sure how useful this is

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as Drew Noakes quoted  kotlin use if statement as expression   so Ternary Conditional Operator is not necessary anymore   but with the extension function and infix overloading  you could implement that yourself  here is an example  infix fun  lt T gt  Boolean then value  T     TernaryExpression this  value   class TernaryExpression lt out T gt  val flag  Boolean  val truly  T         infix fun  lt T gt  or falsy  T     if  flag  truly else falsy     then use it like this  val grade   90 val clazz    grade  gt  80  then  A  or  B

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example    var energy  Int   data  get position   energy  toInt      0   In kotlin if you are using      it will work like if the statement will return null then     0 it will take 0 or whatever you have write this side

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Another interesting approach would be to use when  when a      true - gt  b   false - gt  c    Can be quite handy in some more complex scenarios  And honestly  it s more readable for me than if     else

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You can do something like this  val ans    exp1    exp2  then  quot yes quot      quot no quot   by using this extension  infix fun lt T gt  Boolean then first  T   T    if  this  first else null  P S  Dont modify above infix function to accept first  T   the expression will become logically incorrect  Eg  If you modify it to accept nullable first  T   then val ans    true    true  then null     quot abcd quot   ans will be  quot abcd quot   which would not be correct

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There is no ternary operator in kotlin  as the if else block returns value  so  you can do  val max   if  a  gt  b  a else b instead of java s max    a  gt  b    b   c  We can also use when construction  it also return value   val max   when a  gt  b        true - gt  a     false - gt  b     Here is link for kotlin documentation   Control Flow  if  when  for  while

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You can do it many way in Kotlin   Using if  if a  b else c  Using when  when  a         true - gt  print  value b        false - gt  print  value c        else - gt              print  default return in any other case              Null Safety  val a   b    c

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In Kotlin  if is an expression  i e  it returns a value  Therefore   there is no ternary operator  condition   then   else   because   ordinary if works fine in this role   manual source from here      Traditional usage  var max   a  if  a  lt  b  max   b     With else  var max  Int if  a  gt  b        max   a   else       max   b       As expression  val max   if  a  gt  b  a else b

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When working with apply    let seems very handy when dealing with ternary operations  as it is more elegant and give you room  val columns  List lt String gt        val band   Band   apply       name   columns 0      album   columns 1      year   columns 2  takeIf   it isNotEmpty      let   it toInt        0

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fun max x Int y Int    String   if  x gt y   quot max    x quot  else  quot max    y quot   inline funcation

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Some corner cases not mentioned in other answers   Since appearance of takeIf in Kotlin 1 1 the ternary operator a   b   c can also be expressed like this   b takeIf   a      c   This becomes even shorter in case c is null   b takeIf   a     Also note that typical in Java world null checks like value    null   value   defaultValue translate in ideomatic Kotlin to just value    defaultValue   Similar a    null   b   c can be translated to a  let   b      c

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With the following infix functions I can cover many common use cases pretty much the same way it can be done in Python     class TestKotlinTernaryConditionalOperator         Test     fun testAndOrInfixFunctions             Assertions assertThat true and  yes  or  no   isEqualTo  yes           Assertions assertThat false and  yes  or  no   isEqualTo  no            Assertions assertThat  A  and  yes  or  no   isEqualTo  yes           Assertions assertThat    and  yes  or  no   isEqualTo  no            Assertions assertThat 1 and  yes  or  no   isEqualTo  yes           Assertions assertThat 0 and  yes  or  no   isEqualTo  no            Assertions assertThat Date   and  yes  or  no   isEqualTo  yes            Suppress  CAST NEVER SUCCEEDS           Assertions assertThat null as Date  and  yes  or  no   isEqualTo  no            infix fun  lt E gt  Boolean  and other  E    E    if  this    true  other else null infix fun  lt E gt  CharSequence  and other  E    E    if    this        isEmpty    other else null infix fun  lt E gt  Number  and other  E    E    if  this  toInt      0    0  other else null infix fun  lt E gt  Any  and other  E    E    if  this    null  other else null infix fun  lt E gt  E  or other  E    E    this    other

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There is no ternary operator in Kotlin  It seems problematic at the first glance  But think we can do it with inline if else statement because this is expression here  Simply we have to do -  var number   if n gt 0   Positive  else  Negetive    Here we can else if block too as many as we need  Like-  var number   if n gt 0   Positive  else if n lt 0   Negative  else  Zero    So this line is so simple and much readable than ternary operator  when we use more than one ternary operator in java it seems horrible  But here we have a clear syntax  even we can write it in multiple line too

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TASK  Let s consider the following example  if   answer isSuccessful          result    quot wrong quot    else       result   answer body   string     return result  We need the following equivalent in Kotlin   return    answer isSuccessful        quot wrong quot    answer body   string     SOLUTION 1 a  You can use if-expression in Kotlin  return if   answer isSuccessful     quot wrong quot  else answer body   string    SOLUTION 1 b  It can be much better if you flip this if-expression  let s do it without not   return if  answer isSuccessful    answer body   string   else  quot wrong quot    SOLUTION 2  Kotlin   s Elvis operator    can do a job even better  return answer body    string       quot wrong quot    SOLUTION 3  Or use an Extension function for the corresponding Answer class  fun Answer bodyOrNull    Body    if  isSuccessful    body   else null   SOLUTION 4  Using the Extension function you can reduce a code thanks to Elvis operator  return answer bodyOrNull    string       quot wrong quot    SOLUTION 5  Or just use when operator  when   answer isSuccessful          parseInt str  - gt  result    quot wrong quot      else - gt  result   answer body   string

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TL DR if  a  b else c    is what you can use instead of the ternary operator expression a   b   c which Kotlin syntax does not allow   In Kotlin  many control statements  such as if  when  and even try  can be used as expressions  As a result  these statements can have a result which may be assigned to a variable  be returned from a function  etc  Syntactically  there s no need for ternary operator As a result of Kotlin s expressions  the language does not really need the ternary operator  if  a  b else c  is what you can use instead of the ternary operator expression a   b   c  I think the idea is that the former expression is more readable since everybody knows what ifelse does  whereas     is rather unclear if you re not familiar with the syntax already  Nevertheless  I have to admit that I often miss the more convenient ternary operator   Other Alternatives when You might also see when constructs used in Kotlin when conditions are checked  It s also a way to express if-else cascades in an alternative way  The following corresponds to the OTs example  when a        true - gt  b     false - gt  c    Extensions As many good examples  Kotlin Ternary Conditional Operator  in the other answers show  extensions can also help with solving your use case

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when replaces the switch operator of C-like languages  In the simplest form it looks like this  when  x        1 - gt  print  x    1       2 - gt  print  x    2       else - gt            print  x is neither 1 nor 2

[kotlin] Kotlin Ternary Conditional Operator

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