Generate a list with lambdas in kotlin












2















I'm new to Kotlin and lambdas and I'm trying to understand it. I'm trying to generate a list of 100 random numbers.
This works:



private val maxRandomValues = (1..100).toList()


But I want to do something like that:



private val maxRandomValues = (1..100).forEach { RandomGenerator().nextDouble() }.toList()


But this is not working. I'm trying to figure out how to use the values generated into forEach are used in the toList()






lambda kotlin







share|improve this question























  • The problem with your sample is that forEach does not return a value. In @Sergey Rybalkin's answer, they use map.

    – ordonezalex
    Nov 21 '18 at 17:14
















2















I'm new to Kotlin and lambdas and I'm trying to understand it. I'm trying to generate a list of 100 random numbers.
This works:



private val maxRandomValues = (1..100).toList()


But I want to do something like that:



private val maxRandomValues = (1..100).forEach { RandomGenerator().nextDouble() }.toList()


But this is not working. I'm trying to figure out how to use the values generated into forEach are used in the toList()






lambda kotlin







share|improve this question























  • The problem with your sample is that forEach does not return a value. In @Sergey Rybalkin's answer, they use map.

    – ordonezalex
    Nov 21 '18 at 17:14














2












2








2








I'm new to Kotlin and lambdas and I'm trying to understand it. I'm trying to generate a list of 100 random numbers.
This works:



private val maxRandomValues = (1..100).toList()


But I want to do something like that:



private val maxRandomValues = (1..100).forEach { RandomGenerator().nextDouble() }.toList()


But this is not working. I'm trying to figure out how to use the values generated into forEach are used in the toList()






lambda kotlin







share|improve this question














I'm new to Kotlin and lambdas and I'm trying to understand it. I'm trying to generate a list of 100 random numbers.
This works:



private val maxRandomValues = (1..100).toList()


But I want to do something like that:



private val maxRandomValues = (1..100).forEach { RandomGenerator().nextDouble() }.toList()


But this is not working. I'm trying to figure out how to use the values generated into forEach are used in the toList()






lambda kotlin




lambda kotlin






share|improve this question













share|improve this question











share|improve this question




share|improve this question










asked Nov 21 '18 at 16:29









KillrazorKillrazor

2,39083563




2,39083563













  • The problem with your sample is that forEach does not return a value. In @Sergey Rybalkin's answer, they use map.

    – ordonezalex
    Nov 21 '18 at 17:14



















  • The problem with your sample is that forEach does not return a value. In @Sergey Rybalkin's answer, they use map.

    – ordonezalex
    Nov 21 '18 at 17:14

















The problem with your sample is that forEach does not return a value. In @Sergey Rybalkin's answer, they use map.

– ordonezalex
Nov 21 '18 at 17:14





The problem with your sample is that forEach does not return a value. In @Sergey Rybalkin's answer, they use map.

– ordonezalex
Nov 21 '18 at 17:14












1 Answer
1






active

oldest

votes


















6














It's way better to use kotlin.collections function to do this:



List(100) {

    Random.nextInt()

}


According to Collections.kt



inline fun <T> List(size: Int, init: (index: Int) -> T): List<T> = MutableList(size, init)


It's also possible to generate using range like in your case:



(1..100).map { Random.nextInt() }


The reason you can't use forEach is that it return Unit (which is sort of like void in Java, C#, etc.). map operates Iterable (in this case the range of numbers from 1 to 100) to map them to a different value. It then returns a list with all those values. In this case, it makes more sense to use the List constructor above because you're not really "mapping" the values, but creating a list






share|improve this answer


























  • I used that List(100){Random.nextInt()} statement. What I don't understand is what is the origin of that kind of structure (Collection constructor and a body). I'm jumping from the verbosity of java and I'm completely lost.

    – Killrazor
    Nov 21 '18 at 17:21






  • 1





    @Killrazor lets check step by step: 1. List(10,(index: Int) -> { index*index })

    – Sergey Rybalkin
    Nov 21 '18 at 17:28








  • 1





    @Killrazor kotlin allows to omit 2. List(10, { index*index })

    – Sergey Rybalkin
    Nov 21 '18 at 17:30






  • 1





    @Killrazor it's possible to move last parameter out of () braces if it is lambda 3: List(10) { index*index }

    – Sergey Rybalkin
    Nov 21 '18 at 17:31











  • Nice explanation! thanks.

    – Killrazor
    Nov 22 '18 at 7:32











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1 Answer
1






active

oldest

votes








1 Answer
1






active

oldest

votes









active

oldest

votes






active

oldest

votes









6














It's way better to use kotlin.collections function to do this:



List(100) {

    Random.nextInt()

}


According to Collections.kt



inline fun <T> List(size: Int, init: (index: Int) -> T): List<T> = MutableList(size, init)


It's also possible to generate using range like in your case:



(1..100).map { Random.nextInt() }


The reason you can't use forEach is that it return Unit (which is sort of like void in Java, C#, etc.). map operates Iterable (in this case the range of numbers from 1 to 100) to map them to a different value. It then returns a list with all those values. In this case, it makes more sense to use the List constructor above because you're not really "mapping" the values, but creating a list






share|improve this answer


























  • I used that List(100){Random.nextInt()} statement. What I don't understand is what is the origin of that kind of structure (Collection constructor and a body). I'm jumping from the verbosity of java and I'm completely lost.

    – Killrazor
    Nov 21 '18 at 17:21






  • 1





    @Killrazor lets check step by step: 1. List(10,(index: Int) -> { index*index })

    – Sergey Rybalkin
    Nov 21 '18 at 17:28








  • 1





    @Killrazor kotlin allows to omit 2. List(10, { index*index })

    – Sergey Rybalkin
    Nov 21 '18 at 17:30






  • 1





    @Killrazor it's possible to move last parameter out of () braces if it is lambda 3: List(10) { index*index }

    – Sergey Rybalkin
    Nov 21 '18 at 17:31











  • Nice explanation! thanks.

    – Killrazor
    Nov 22 '18 at 7:32
















6














It's way better to use kotlin.collections function to do this:



List(100) {

    Random.nextInt()

}


According to Collections.kt



inline fun <T> List(size: Int, init: (index: Int) -> T): List<T> = MutableList(size, init)


It's also possible to generate using range like in your case:



(1..100).map { Random.nextInt() }


The reason you can't use forEach is that it return Unit (which is sort of like void in Java, C#, etc.). map operates Iterable (in this case the range of numbers from 1 to 100) to map them to a different value. It then returns a list with all those values. In this case, it makes more sense to use the List constructor above because you're not really "mapping" the values, but creating a list






share|improve this answer


























  • I used that List(100){Random.nextInt()} statement. What I don't understand is what is the origin of that kind of structure (Collection constructor and a body). I'm jumping from the verbosity of java and I'm completely lost.

    – Killrazor
    Nov 21 '18 at 17:21






  • 1





    @Killrazor lets check step by step: 1. List(10,(index: Int) -> { index*index })

    – Sergey Rybalkin
    Nov 21 '18 at 17:28








  • 1





    @Killrazor kotlin allows to omit 2. List(10, { index*index })

    – Sergey Rybalkin
    Nov 21 '18 at 17:30






  • 1





    @Killrazor it's possible to move last parameter out of () braces if it is lambda 3: List(10) { index*index }

    – Sergey Rybalkin
    Nov 21 '18 at 17:31











  • Nice explanation! thanks.

    – Killrazor
    Nov 22 '18 at 7:32














6












6








6







It's way better to use kotlin.collections function to do this:



List(100) {

    Random.nextInt()

}


According to Collections.kt



inline fun <T> List(size: Int, init: (index: Int) -> T): List<T> = MutableList(size, init)


It's also possible to generate using range like in your case:



(1..100).map { Random.nextInt() }


The reason you can't use forEach is that it return Unit (which is sort of like void in Java, C#, etc.). map operates Iterable (in this case the range of numbers from 1 to 100) to map them to a different value. It then returns a list with all those values. In this case, it makes more sense to use the List constructor above because you're not really "mapping" the values, but creating a list






share|improve this answer















It's way better to use kotlin.collections function to do this:



List(100) {

    Random.nextInt()

}


According to Collections.kt



inline fun <T> List(size: Int, init: (index: Int) -> T): List<T> = MutableList(size, init)


It's also possible to generate using range like in your case:



(1..100).map { Random.nextInt() }


The reason you can't use forEach is that it return Unit (which is sort of like void in Java, C#, etc.). map operates Iterable (in this case the range of numbers from 1 to 100) to map them to a different value. It then returns a list with all those values. In this case, it makes more sense to use the List constructor above because you're not really "mapping" the values, but creating a list







share|improve this answer














share|improve this answer



share|improve this answer








edited Nov 21 '18 at 16:56

























answered Nov 21 '18 at 16:32









Sergey RybalkinSergey Rybalkin

1,354316




1,354316













  • I used that List(100){Random.nextInt()} statement. What I don't understand is what is the origin of that kind of structure (Collection constructor and a body). I'm jumping from the verbosity of java and I'm completely lost.

    – Killrazor
    Nov 21 '18 at 17:21






  • 1





    @Killrazor lets check step by step: 1. List(10,(index: Int) -> { index*index })

    – Sergey Rybalkin
    Nov 21 '18 at 17:28








  • 1





    @Killrazor kotlin allows to omit 2. List(10, { index*index })

    – Sergey Rybalkin
    Nov 21 '18 at 17:30






  • 1





    @Killrazor it's possible to move last parameter out of () braces if it is lambda 3: List(10) { index*index }

    – Sergey Rybalkin
    Nov 21 '18 at 17:31











  • Nice explanation! thanks.

    – Killrazor
    Nov 22 '18 at 7:32



















  • I used that List(100){Random.nextInt()} statement. What I don't understand is what is the origin of that kind of structure (Collection constructor and a body). I'm jumping from the verbosity of java and I'm completely lost.

    – Killrazor
    Nov 21 '18 at 17:21






  • 1





    @Killrazor lets check step by step: 1. List(10,(index: Int) -> { index*index })

    – Sergey Rybalkin
    Nov 21 '18 at 17:28








  • 1





    @Killrazor kotlin allows to omit 2. List(10, { index*index })

    – Sergey Rybalkin
    Nov 21 '18 at 17:30






  • 1





    @Killrazor it's possible to move last parameter out of () braces if it is lambda 3: List(10) { index*index }

    – Sergey Rybalkin
    Nov 21 '18 at 17:31











  • Nice explanation! thanks.

    – Killrazor
    Nov 22 '18 at 7:32

















I used that List(100){Random.nextInt()} statement. What I don't understand is what is the origin of that kind of structure (Collection constructor and a body). I'm jumping from the verbosity of java and I'm completely lost.

– Killrazor
Nov 21 '18 at 17:21





I used that List(100){Random.nextInt()} statement. What I don't understand is what is the origin of that kind of structure (Collection constructor and a body). I'm jumping from the verbosity of java and I'm completely lost.

– Killrazor
Nov 21 '18 at 17:21




1




1





@Killrazor lets check step by step: 1. List(10,(index: Int) -> { index*index })

– Sergey Rybalkin
Nov 21 '18 at 17:28







@Killrazor lets check step by step: 1. List(10,(index: Int) -> { index*index })

– Sergey Rybalkin
Nov 21 '18 at 17:28






1




1





@Killrazor kotlin allows to omit 2. List(10, { index*index })

– Sergey Rybalkin
Nov 21 '18 at 17:30





@Killrazor kotlin allows to omit 2. List(10, { index*index })

– Sergey Rybalkin
Nov 21 '18 at 17:30




1




1





@Killrazor it's possible to move last parameter out of () braces if it is lambda 3: List(10) { index*index }

– Sergey Rybalkin
Nov 21 '18 at 17:31





@Killrazor it's possible to move last parameter out of () braces if it is lambda 3: List(10) { index*index }

– Sergey Rybalkin
Nov 21 '18 at 17:31













Nice explanation! thanks.

– Killrazor
Nov 22 '18 at 7:32





Nice explanation! thanks.

– Killrazor
Nov 22 '18 at 7:32


















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