How to do multiple variable assignments in one line in Kotlin like C,C++?

Question

I had to swap 2 numbers in one line expression using no other variable except x and y. So I wrote the following .c program to swapp two numbers with the given conditions and it works like charm.


int main() {
    
    int x =5, y =2;
    x = y-x+(y=x);
    printf("X=%d, y=%d", x, y);

    return 0;
}

But when i try to do the same in kotlin it gives me an error that

Assignments are not expressions, and only expressions are allowed in this context,

I can resolve this issue by introducing a third variable just like this. But I'm not allowed to have any other variable except x and y which are already given. So is there any other way I can do this in one line using any kotlin property?

Below is the kotlin program

fun main() {
    
    var x = 5
    var y = 10
    x = y-x+(y=x)   
    println("X = $x, Y = $y")

}

are you required to use one line, or is the only restriction that you can't use a 3rd variable? — nigh_anxiety
– nigh_anxiety, Commented Sep 10, 2022 at 3:30
Kotlin doesn’t support that, but you can do a = b.also { b = a } — Tenfour04
– Tenfour04, Commented Sep 10, 2022 at 3:30
@nigh_anxiety in one line expression without any 3rd variable, just like i did in c. — Shoaib Kakal
– Shoaib Kakal, Commented Sep 10, 2022 at 3:32
if the requirement is a single line, then x = x+y; y = x-y; x = x-y does the trick. If it has to be a single statement then TenFour's suggestion of x = y.also{y = x} appears to work when testing in a Kotlin playground — nigh_anxiety
– nigh_anxiety, Commented Sep 10, 2022 at 5:16
Why do you want to do this though? Kotlin isn't C, and although you can technically do this stuff on a single written line, the bytecode it gets compiled to will create extra variables and generally be more verbose (nigh_anxiety's example aside!). If it's about neat, efficient memory tricks, it's not really that kind of language. If it's about conciseness, just do the also thing, or write a reusable swap function that takes property references so you can go swap(::a, ::b) or something like that — cactustictacs
– cactustictacs, Commented Sep 10, 2022 at 19:23

aSemy · Accepted Answer · 2022-12-10 14:37:32Z

4

While I have two suggestions below, I want to start with a recommendation against either of them, at least in this simple example.

It's usually a lot more clear to optimise code for developers to read in the following ways:

create an extra variable with a descriptive name
prefer val over var to avoid accidental mutations
and try to make the code 'linear', so the operations can be read from top-to-bottom without jumping between functions
avoid code that needs an IDE to see what the type-hints are

And I'll trust that the compiler will make make the code performant.

fun main() {
  val x = 5
  val y = 10

  val newX = y
  val newY = x

  println("X = $newX, Y = $newY")
}

Local function

You could use a local function to perform the swap, as the function will still be able to access the original values.

fun main() {
  var x = 5
  var y = 10

  fun swap(originalX: Int, originalY: Int) {
    y = originalX
    x = originalY
  }

  swap(x, y)

  println("X = $x, Y = $y")
}

Scope function

This could be code-golfed into one line

use to to create a Pair<Int, Int>,
and a scope function to use the result.

fun main() {
  var x = 5
  var y = 10

  (x to y).apply { x = second; y = first }

  println("X = $x, Y = $y")
}

One line? Yes. More difficult to read? I think so.

edited Dec 10, 2022 at 14:37

answered Sep 10, 2022 at 6:48

aSemy

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1 Comment

gidds Over a year ago

“Programs must be written for people to read, and only incidentally for machines to execute.” — Abelson & Sussman, Structure And Interpretation Of Computer Programs, preface to the 1st edition

Matthew Read · Accepted Answer · 2023-12-23 23:16:56Z

0

I had to swap 2 numbers in one line expression using no other variable except x and y.

Using only two variables can sometimes be reasonable, given extreme constraints (like working solely with a small set of CPU registers, etc.), although that wouldn't typically apply to the use cases for a high-level language like Kotlin.

However, writing it all in one line is definitely not reasonable; it just makes it pointlessly harder to read. Don't do that.

The common way to swap two variables without a temporary is this (as written in Kotlin):

x = x xor y
y = x xor y
x = x xor y

answered Dec 23, 2023 at 23:16

Matthew Read

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