Kotlin: Can I access a nested array from inside another array?

Question

This may be a dumb question, but I have an array:

var testarray = arrayOf(0,0,arrayOf(7,5,2))

So I tried to access the second array by typing testarray[2][1]. However, this throws an error:

error: unresolved reference. None of the following candidates is applicable because of receiver type mismatch: 
@InlineOnly public inline operator fun <@OnlyInputTypes K, V> Map<out Int, ???>.get(key: Int): ??? defined in kotlin.collections
testarray[2][1]
^
error: no get method providing array access
testarray[2][1]

Is there a way to access the nested array?

msrd0 · Accepted Answer · 2018-11-03 11:27:00Z

3

Your outer array contains Ints, and an array. So its actual declared type (that you could specify, or could let the IDE specify for you), is Array<Serializable>.

A Serializable could be anything. The compiler doesn't know that the element at index 2 happens to be an array. So it can't let you use the [] operator on it. You need to cast the element to an array to be able to do that:

println((testarray[2] as Array<Int>)[1])

That would fix the compilation issue, but not the design issue. Using arrays in general is already a smell, but using arrays containing mixed types is an even bigger smell. You should reconsider your design.

edited Nov 3, 2018 at 11:27

msrd0

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answered Nov 2, 2018 at 23:50

JB Nizet

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3 Comments

jfa Over a year ago

So use MutableLists instead in this case? Or would there be a better datatype in this case?

JB Nizet Over a year ago

I don't know enough about your use-case to recommend anything concrete. In general you should favor collections over arrays, and immutable collections over mutable ones.

jfa Over a year ago

I need to be able to modify the values inside the object, but I do not need to change the length.

Willi Mentzel · Accepted Answer · 2018-11-03 02:53:55Z

1

I also think that you should rethink your design as JB Nizet already said.

You could do that by generalizing your array to an array of arrays like this:

val testArray = arrayOf(arrayOf(0), arrayOf(0), arrayOf(7, 5, 2))

which would ensure that you can get and set elements like this:

testArray[2][1]
testArray[2][0] = 4

As long as you just want to access the elements in the fashion you described keeping your design you could create a (local) operator extension function get on Serializable like this:

operator fun Serializable.get(i: Int) = (this as? Array<*>)?.get(i) ?: this

val a = arrayOf(0, 0, arrayOf(7,5,2))

println(a[1][0]) // 0
println(a[2][1]) // 5

answered Nov 3, 2018 at 2:53

Willi Mentzel

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2 Comments

jfa Over a year ago

That's definitely feasible. Actually I like this idea the more I think about it. If it's a 2d array, I should have to pass the array element anyway.

jfa Over a year ago

I'm getting 'error: no set method providing array access' whenever I try to change the values in the array

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