Kotlin - Adding value to an empty array

Question

I am learning kotlin concepts and had the following question when I was going through the topic Arrays.

I created an empty

val empty = arrayOf<Int>()
empty[0] = 2

So the above code will fail and cause ArrayIndexOutOfBoundsException which makes sense because the size of an array cannot be changed and since the array was initialized with no size it is treated as an empty array.

Here is where i thought this gets interesting

var emptyArray = arrayOf<Int>()
emptyArray += 1
emptyArray += 2

The above code doesn't fail and when i print the items I am getting the results back. I am assuming there is something going on when trying to add an element to an index versus adding it directly but I couldn't find any documentation that explains what is going on under the hood. Can someone please explain

kotlin.collections has a plus operator for Arrays that return an array containing the original array plus the new element. And since you made emptyArray a var it's possible to reassign this new array to emptyArray. — Michael
– Michael, Commented Mar 14, 2022 at 14:27
By the way, the ultimate answer to most questions about arrays in Kotlin is: “Don't use an array, use a list!” Arrays are mostly a legacy structure; needed for varargs, for interacting with old APIs, and for implementing higher-level structures — for just about everything else, lists are simpler and better supported. — gidds
– gidds, Commented Mar 14, 2022 at 17:09

Sweeper · Accepted Answer · 2022-03-14 14:28:39Z

This is because of how Kotlin's augmented assignment operators are overloaded.

For the assignment operations, for example a += b, the compiler performs the following steps:

If the function from the right column is available:

[...]

Otherwise, try to generate code for a = a + b (this includes a type check: the type of a + b must be a subtype of a).

"the function from the right column" refers to the right column of the table which I did not include in the quote, which are basically the operator functions plusAssign, minusAssign, timesAssign etc. Of course, these functions do not exist for arrays, so the second bullet point applies here.

To generate code for a = a + b, there needs to be a plus operator function for arrays, taking an element of that array, and returning an array, so that it is translated to a = a.plus(b). There is indeed such an operator function for Array<T>.

operator fun <T> Array<T>.plus(element: T): Array<T>

So to summarise:

emptyArray += 1

is translated into

emptyArray = emptyArray + 1

which is translated into:

emptyArray = emptyArray.plus(1)

RobCo · Accepted Answer · 2022-03-14 14:28:48Z

In the second example, the + operator calls a function called plus, and then assigns the result back to the variable.

A look at the definition gives the following function:

/**
 * Returns an array containing all elements of the original array and then the given [element].
 */
public actual operator fun <T> Array<T>.plus(element: T): Array<T> {
    val index = size
    val result = java.util.Arrays.copyOf(this, index + 1)
    result[index] = element
    return result
}

As you see, a copy of the original is made but one larger. The new element is then assigned to the new, empty slot.

In the example a total of three arrays are allocated. First an empty, then a 1-size and finally a 2-size array, which is the final result. Each array still cannot change its size.

Note that you have to use var, and val will lead to a 'val cannot be reassigned' error.

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