Trying to create a list of unique numbers. Why is ...

# getting-started

[alex cole]

Trying to create a list of unique numbers. Why is this statement not working?

mineLocation = Set<Int>(mineCount) { Random.nextInt(0, length * width)}

[Platon Malyugin]

Set is interface or you have own implementation for this class?

[alex cole]

Well I was trying to create a unique list of random elements
Trying to avoid:

var mineLocations = arrayListOf<Int>()
while(mineCount != minesSet) {
    val i = Random.nextInt(0, length * width)
    if (!mineLocations.contains(i)) {
        mineLocations.add(i)
        minesSet++
    }
}

[Platon Malyugin]

Anyway, I think Kotlin doesn’t have this functional, need to do own solution, e.g.

val rands = mutableSetOf<Int>()
val mineCount = 10
        
while (rands.size != mineCount) {
  rands.add(Random.nextInt())
}
// return immutable set
rands.toSet()

[Platon Malyugin]

In theory about data structures, Set must contains only unique elements, so calling contains method is unnecessary

[alex cole]

This is for a minesweeper game. I think ideally a map should be used for each mine but I think I could get away with a set or list. What would be the better recommended approach?

[Platon Malyugin]

Each point of mine has two states: mined or not?

[mickeelm]

Are you trying tro create a function that takes in the parameter mineCount? That's not obvious to me since that parameter is not used...and where is length and width stated?

Also, Set is as pointed out before a data structure that only contains uniqiue elements. Moreover, it is by default unordered so there's no need of randomizing here

[mickeelm]

Also, I don't really understand why they have to be random here at all but I don't have the exact use case. Anyway, the code that you pasted above (with the array list etc) can be rewritten as

val width = 10
val height = 10
val size = width * height //I mean, shouldn't this be a grid (2d-array or so), rather than the product of two numbers?

//This is really all you need. The randomization seems completely unnecessary to me, but maybe I don't understand the use case?
(0..size).toSet()

[Matteo Mirk]

Alex, your initial code would be cool but sadly only List has a generator function (written to look like a constructor). You could define your own function anyway that encapsulates the creation code. Another take using sequences could be:
Alex, your initial code would be cool but sadly only List has a generator function (written to look like a constructor). You could define your own function anyway that encapsulates the creation code. Another take using sequences could be:

val mineLocation = generateSequence { Random.nextInt(0, length * width) }
        .distinct()
        .take(mineCount)
        .toSet()

[mickeelm]

Okay, NOW I understand...too early in the morning for me - didn't connect that we were trying to place out mines...sorry Alex... 😂

[Matteo Mirk]

@mickeelm you didn’t get the use case: he’s trying to generate a random subset of locations on a grid. Your code generates a set of all the locations. 😛

[mickeelm]

Yeah, I'm sorry about that 😂

[Matteo Mirk]

we wrote at the same time, sorry

[Matteo Mirk]

so you can encapsulate the above code inside this function to obtain your desired idiom:

inline fun <T> Set(size: Int, init: () -> T): Set<T> = ...

[alex cole]

I have not done inline functions like that yet, only lambda functions.

[Matteo Mirk]

nothing fancy here, just copying the initial List(Int, (Int) -> T)) definition from standard lib: this small optimization will let the compiler inline the init lambda when creating the Set and the Set function itself at the call site, to avoid creating a function object

[Matteo Mirk]

if you’re not familiar with this feature, just ignore the inline modifier, don’t worry