Scala: what is the best way to append an element to an Array?

113
votes

Say I have an Array[Int] like

val array = Array( 1, 2, 3 )

Now I would like to append an element to the array, say the value 4, as in the following example: 

val array2 = array + 4     // will not compile

I can of course use System.arraycopy() and do this on my own, but there must be a Scala library function for this, which I simply could not find. Thanks for any pointers!

Notes:

  1. I am aware that I can append another Array of elements, like in the following line, but that seems too round-about:

    val array2b = array ++ Array( 4 )     // this works

  2. I am aware of the advantages and drawbacks of List vs Array and here I am for various reasons specifically interested in extending an Array.

Edit 1

Thanks for the answers pointing to the :+ operator method. This is what I was looking for. Unfortunately, it is rather slower than a custom append() method implementation using arraycopy -- about two to three times slower. Looking at the implementation in SeqLike[], a builder is created, then the array is added to it, then the append is done via the builder, then the builder is rendered. Not a good implementation for arrays. I did a quick benchmark comparing the two methods, looking at the fastest time out of ten cycles. Doing 10 million repetitions of a single-item append to an 8-element array instance of some class Foo takes 3.1 sec with :+ and 1.7 sec with a simple append() method that uses System.arraycopy(); doing 10 million single-item append repetitions on 8-element arrays of Long takes 2.1 sec with :+ and 0.78 sec with the simple append() method. Wonder if this couldn't be fixed in the library with a custom implementation for Array?

Edit 2

For what it's worth, I filed a ticket: https://issues.scala-lang.org/browse/SI-5017

3
arraysscalaappend
Why not use ArrayBuffer and its += method? That will give you amortized O(1) append. – Fred Foo
In scala, System.arraycopy(...) is replaced by Array.copy(...) – paradigmatic
You are aware of the advantages and drawbacks of List vs Array, but are surprised of benchmark results of 10 million appends? – user unknown
Can you run again your benchmark using an ArrayBuffer which is converted after the last append to an array (with toArray) ? – paradigmatic
@paradigmatic: The benchmark of course was not 10 million appends to the same array but 10 million repetitions of a single-item append to an 8-element array. I updated the question accordingly. – Gregor Scheidt

3 Answers

214
votes

You can use :+ to append element to array and +: to prepend it:

0 +: array :+ 4

should produce:

res3: Array[Int] = Array(0, 1, 2, 3, 4)

It's the same as with any other implementation of Seq.

61
votes
val array2 = array :+ 4
//Array(1, 2, 3, 4)

Works also "reversed":

val array2 = 4 +: array
Array(4, 1, 2, 3)

There is also an "in-place" version:

var array = Array( 1, 2, 3 )
array +:= 4
//Array(4, 1, 2, 3)
array :+= 0
//Array(4, 1, 2, 3, 0)
8
votes

The easiest might be:

Array(1, 2, 3) :+ 4

Actually, Array can be implcitly transformed in a WrappedArray