How can I get a random number in Kotlin?

A generic method that can return a random integer between 2 parameters like ruby does with `rand(0..n)`.

Any suggestion?

First, you need a RNG. In Kotlin you currently need to use the platform specific ones (there isn't a Kotlin built in one). For the JVM it's [`java.util.Random`][1]. You'll need to create an instance of it and then call `random.nextInt(n)`.


[1]:

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Generate a random integer between `from`(inclusive) and `to`(exclusive)

import java.util.Random

val random = Random()

fun rand(from: Int, to: Int) : Int {
return random.nextInt(to - from) + from
}

There is no standard method that does this but you can easily create your own using either `Math.random()` or the class `java.util.Random`. Here is an example using the `Math.random()` method:

fun random(n: Int) = (Math.random() * n).toInt()
fun random(from: Int, to: Int) = (Math.random() * (to - from) + from).toInt()
fun random(pair: Pair<Int, Int>) = random(pair.first, pair.second)

fun main(args: Array<String>) {
val n = 10

val rand1 = random(n)
val rand2 = random(5, n)
val rand3 = random(5 to n)

println(List(10) { random(n) })
println(List(10) { random(5 to n) })
}
This is a sample output:

[9, 8, 1, 7, 5, 6, 9, 8, 1, 9]
[5, 8, 9, 7, 6, 6, 8, 6, 7, 9]

Building off of [@s1m0nw1][1] excellent answer I made the following changes.

1. (0..n) implies inclusive in Kotlin
2. (0 until n) implies exclusive in Kotlin
3. Use a singleton object for the Random instance (optional)

Code:

private object RandomRangeSingleton : Random()

fun ClosedRange<Int>.random() = RandomRangeSingleton.nextInt((endInclusive + 1) - start) + start


Test Case:

fun testRandom() {
Assert.assertTrue(
(0..1000).all {
(0..5).contains((0..5).random())
}
)
Assert.assertTrue(
(0..1000).all {
(0..4).contains((0 until 5).random())
}
)
Assert.assertFalse(
(0..1000).all {
(0..4).contains((0..5).random())
}
)
}


[1]:

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Kotlin standard lib doesn't provide Random Number Generator API. If you aren't in a multiplatform project, it's better to use the platform api *(all the others answers of the question talk about this solution)*.

**But if you are in a multiplatform context, the best solution is to implement random by yourself in pure kotlin for share the same random number generator between platforms.** It's more simple for dev and testing.

To answer to this problem in my personal project, i implement a pure Kotlin [Linear Congruential Generator][1]. LCG is the algorithm used by `java.util.Random`. Follow this link if you want to use it :

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My implementation purpose `nextInt(range: IntRange)` for you ;).

Take care about my purpose, LCG is good for most of the use cases *(simulation, games, etc...)* but is not suitable for cryptographically usage because of the predictability of this method.


[1]:

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As of kotlin 1.2, you could write:

`(1..3).shuffled().last()`

Just be aware it's big O(n), but for a small list (especially of unique values) it's alright :D

Examples random in the range [1, 10]

val random1 = (0..10).shuffled().last()

or utilizing Java Random

val random2 = Random().nextInt(10) + 1

to be super duper ))

fun rnd_int(min: Int, max: Int): Int {
var max = max
max -= min
return (Math.random() * ++max).toInt() + min
}

If the numbers you want to choose from are not consecutive, you can use [`random()`][1].

Usage:

val list = listOf(3, 1, 4, 5)
val number = list.random()

Returns one of the numbers which are in the list.


[1]:

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