using val in kotlin lambda

Question

i'm playing around with map in kotlin, and currently I would like to see whether it would be possible to map a list of integers to a list of functions (Int) -> Int, while doing a bunch of operations inside the lambda that would require me to save values to vals

So I have this simple code:

val num = arrayOf(1, 2, 3, 4)

  val funcs = num.map <Int, (Int) -> Int > { x  -> {
        y -> y + x
    }}

This is a bit of a condensed example, the thing i'm really trying to do is a bit more convoluted. But I need to quite a bit of operations inside of of the 'inner' lambda, so I need to write codeblocks where I can use val. Like this:

val fs = num.map <Int, (Int) -> Int > { x  -> {
        y -> {
        val tmp = y *2
        val tmp1 = x / 2
        tmp + tmp1

        }
}}

But this part doesnt work, the compiler is confused by return types How can I do something like this?

also why do you need intermediary variables? what's wrong with y -> y * 2 + x / 2? — njzk2
– njzk2, Commented Apr 17, 2021 at 22:02
the exact example is a toy example. But really I just need to write some code that will we infeasible to write in a single oneliner — GarseB010
– GarseB010, Commented Apr 17, 2021 at 22:16
Have you tried specifying explicit types for the lambda params? — gidds
– gidds, Commented Apr 17, 2021 at 22:24

AlexT · Accepted Answer · 2021-04-18 00:38:35Z

2

The reason your code in particular does not work is because you are adding an extra { after y -> in the second example:

val fs = num.map<Int, (Int) -> Int> { x ->
    { y ->
        { // this is not needed
            val tmp = y * 2
            val tmp1 = x / 2
            tmp + tmp1
            
        }// this is not needed
    }
}

So you are creating a "block" of {} which in kotlin means that you are creating a lambda function. The reason block is in quotes is because if you truly want a block in kotlin the syntax is run {}, as opposed to the java {}. But again, you do not need it here.

All you have to do is remove the extra brackets and your code is fine:

val fs = num.map <Int, (Int) -> Int > { x  -> {
        y ->
    val tmp = y *2
    val tmp1 = x / 2
    tmp + tmp1
}}

answered Apr 18, 2021 at 0:38

AlexT

2,9911 gold badge15 silver badges29 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

IR42 · Accepted Answer · 2021-04-17 22:43:53Z

1

There is no difference between single-line and multi-line lambda

val fs = num.map { x ->
    { y: Int ->
        val tmp = y * 2
        val tmp1 = x / 2
        tmp + tmp1
    }
}

edited Apr 17, 2021 at 22:43

answered Apr 17, 2021 at 22:28

IR42

9,8632 gold badges28 silver badges41 bronze badges

Collectives™ on Stack Overflow

using val in kotlin lambda

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related