Ternary operator in kotlin

Question

I can write in java

int i = 10;
String s = i==10 ? "Ten" : "Empty";

Even I can pass it in method parameter.

callSomeMethod(i==10 ? "Ten" : "Empty");

How do I convert it to kotlin? Lint shows error when writing same thing in kotlin.

score 7 · Answer 1 · answered Aug 10 '18 at 05:14

7

callSomeMethod( if (i==10) "Ten" else "Empty")

Discussion about ternary operator: https://discuss.kotlinlang.org/t/ternary-operator/2116/3

answered Aug 10 '18 at 05:14

Ilya E

700
7
13

Kotlin provide many shortcuts than Java, but now it fails. – Khemraj Sharma Aug 10 '18 at 05:17
3

This is actually the standard syntax in FP languages, where flow-control constructs are expressions that can be evaluated. If you think Kotlin chases opportunities to provide "shortcuts" that replace four characters with one, no, that has never been Kotlin's goal. – Marko Topolnik Aug 10 '18 at 05:43

score 5 · Answer 2 · answered Aug 10 '18 at 08:28

Instead of

String s = i==10 ? "Ten" : "Empty";

Technically you can do

val s = if(i == 10) "Ten" else "Empty"

val s = when {
    i == 10 -> "Ten"
    else -> "Empty"
}

val s = i.takeIf { it == 10 }?.let { "Ten" } ?: "Empty"

// not really recommended, just writing code at this point
val s = choose("Ten", "Empty") { i == 10 } 
inline fun <T> choose(valueIfTrue: T, valueIfFalse: T, predicate: () -> Boolean) = 
    if(predicate()) valueIfTrue else valueIfFalse

score 3 · Answer 3 · answered Aug 10 '18 at 05:32

3

You can create an extension function with generic for boolean value

 fun <T> Boolean.elvis( a :T, b :T ): T{
     if(this) // this here refers to the boolean result
         return a
     else
         return b
}

and now you can use it for any boolean value (cool Kotlin)

//                                         output
System.out.print((9>6).elvis("foo","bar")) foo
System.out.print((5>6).elvis("foo","bar")) bar

Extensions in kotlin

answered Aug 10 '18 at 05:32

Pavneet_Singh

36,884
5
53
68

1

This is a nice answer. But I will use `if else` way. Cause `char counts` are almost **same**, and I will not add extension to do same char count code. – Khemraj Sharma Aug 10 '18 at 05:41
@Khemraj thanks and thanks to you, I got the chance to play a little, I totally respect and agree with your decision , happy coding :) – Pavneet_Singh Aug 10 '18 at 05:43
1

By the way, thanks for sharing extension info. I am new to kotlin & appreciate your way. – Khemraj Sharma Aug 10 '18 at 05:46

score 1 · Answer 4 · answered Aug 10 '18 at 06:18

As the original question was using the term 'Elvis operator' it may be a good idea to provide a short comparison with the ternary operator.

The main difference between ternary operator and Elvis operator is that ternary is used for a short if/else replacement while the Elvis operator is used for null safety, e.g.:

port = (config.port != null) ? config.port : 80; - a shortcut for an if/then/else statement
port = config.port ?: 80 - provides a default value to be used in case the original one is null

The examples look very similar but the Ternary operator can be used with any type of boolean check while the Elvis operator is a shorthand for use config.port if it's not null else use 80 so it only checks for null.

Now, I think Kotlin doesn't have a ternary operator and you should use an if statement like so - s = if (i==10) "Ten" else "Empty"

Ternary operator in kotlin

4 Answers4

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