CODEWITHSUNDEEP
androidarraylistkotlin
Jeremy Theintz
Jeremy Theintz

Reputation: 21

Why do some elements of my array go missing ? (Kotlin)

I don't understand why some elements go missing. My supposition is that I somehow declared my variables wrong. But if not I don't see what is wrong in the conceptualization of the code.

I tried to create an array at the beginning of the function which is supposed to be the same at the end, but the elements are still missing.

(Example :

val savedArray = nameArray

[some code where none of the two arrays are modified]

nameArray = savedArray)

import android.support.v7.app.AppCompatActivity 

import android.os.Bundle 

import android.util.Log

var nameArray = arrayListOf("Julia", "Maxime", "Thomas", "Dean", "Samuel")

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        Log.i("Before", "$nameArray") // ["Julia", "Maxime", "Thomas", "Dean", "Samuel"]

        val randomArray = choosePlayer()

        Log.i("After", "$nameArray") // [Maxime, Dean, Samuel] Where are Thomas and Julia ?

    }


    fun choosePlayer(): ArrayList<String> {
        val endArray = arrayListOf<String>()
        var editArray = nameArray

        val name1 = editArray.random()

        endArray.add(name1)
        editArray.remove(name1)

        val name2 = editArray.random()

        endArray.add(name2)
        editArray.remove(name2)


        Log.i("return value", "$endArray") // [Thomas, Julia]
        return endArray
    }
}

I expect the nameArray to be full but instead two elements are always missing. I noticed that they went missing once they are removed from editArray.

Upvotes: 1

Views: 219

Answers (2)

Cililing
Cililing

Reputation: 4753

ArrayList is mutable type. As @Blackbelt mentioned you assigned endArray and editArray to the same value.

So, what I would like to add to that answer is to suggest more "kotlin-like" solution - use unmutable collections. It's easier to predict what will happen in your code.

val nameArray = listOf("s1", "s2", "s3") // You cannot modify content of this list.

fun choosePlayer(): List<String> {
    return nameArray.shuffled().subList(0, 2)
}

method .shuffled() creates a copy of collection and doesn't modify content of original collection. So, you can be always sure that nameArray created as listOf<T> (which returns List<T>) will always have the same content.

If you need modify collection there is a way to do following:

val list = listOf(...)

// Create a mutable copy of list. 
// Whatever you do with that list you will not modify original list.
val mutable = list.toMutableList()

Upvotes: 0

Blackbelt
Blackbelt

Reputation: 157437

I expect the nameArray to be full but instead two elements are always missing.

It is because endArray and editArray are the same object. Here 

val endArray = arrayListOf<String>()
var editArray = nameArray

editArray and endArray are the same reference. Just create a new one

val endArray = arrayListOf<String>()
var editArray = arrayListOf<String>()

Upvotes: 1

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