Scala ArrayBuffer class: methods, syntax, and examples

By Alvin Alexander. Last updated: February 21, 2020

Summary: This page contains a large collection of examples of how to use the Scala ArrayBuffer class.

ArrayBuffer introduction

Use the ArrayBuffer class when you want to use a general-purpose, mutable indexed sequence in your Scala applications:

Mutable: you can change the elements in the collection, and resize the collection
Indexed: random access of elements is very fast; this means you can access elements quickly by their element number, such as people(12345432) (a linked list would be very slow for this purpose)

Just as the Vector class is the recommended “go to” class for immutable, indexed sequential collections, the ArrayBuffer class is recommended as the general-purpose collections class for mutable, indexed sequential collections.

ArrayBuffer is an indexed sequential collection. Use ListBuffer if you prefer a linear sequential collection that is mutable.

Import it

To use an ArrayBuffer you need to import it into the scope of your application:

import scala.collection.mutable.ArrayBuffer

How to create an initial, empty ArrayBuffer

You create a new, empty ArrayBuffer without the new keyword:

var fruits = ArrayBuffer[String]()
var ints = ArrayBuffer[Int]()
var people = ArrayBuffer[Person]()

If you know your ArrayBuffer will need to hold a lot of elements you can create it with an initial size:

// ready to hold 100,000 ints
val buf = new ArrayBuffer[Int](100000)

How to create a new ArrayBuffer with initial elements

To create a new ArrayBuffer with initial elements, just specify the elements to add, like this:

val nums = ArrayBuffer(1, 2, 3)

val people = ArrayBuffer(
    Person("Al"),
    Person("Kim"),
    Person("Jim")
)

Create a new ArrayBuffer by populating it

You can also create a new ArrayBuffer that’s populated with initial elements using a Range:

# to, until
(1 to 5).toBuffer                   # ArrayBuffer(1, 2, 3, 4, 5)
(1 until 5).toBuffer                # ArrayBuffer(1, 2, 3, 4)

(1 to 10 by 2).toBuffer             # ArrayBuffer(1, 3, 5, 7, 9)
(1 until 10 by 2).toBuffer          # ArrayBuffer(1, 3, 5, 7, 9)
(1 to 10).by(2).toBuffer            # ArrayBuffer(1, 3, 5, 7, 9)

('d' to 'h').toBuffer               # ArrayBuffer(d, e, f, g, h)
('d' until 'h').toBuffer            # ArrayBuffer(d, e, f, g)

('a' to 'f').by(2).toBuffer         # ArrayBuffer(a, c, e)

# range method
ArrayBuffer.range(1, 3)             # ArrayBuffer(1, 2)
ArrayBuffer.range(1, 6, 2)          # ArrayBuffer(1, 3, 5)

You can also use the fill and tabulate methods:

ArrayBuffer.fill(3)("foo")               # ArrayBuffer(foo, foo, foo)
ArrayBuffer.tabulate(3)(n => n * n)      # ArrayBuffer(0, 1, 4)
ArrayBuffer.tabulate(4)(n => n * n)      # ArrayBuffer(0, 1, 4, 9)

fill is a nice way to populate an ArrayBuffer with random data:

val r = scala.util.Random
ArrayBuffer.fill(5)(r.nextInt(10))         # ArrayBuffer(4, 4, 3, 9, 1)
ArrayBuffer.fill(5)(r.nextPrintableChar)   # ArrayBuffer(|, ], U, X, l)

If you have a populated ArrayBuffer you can also modify the values of each element in place with transform:

val a = ArrayBuffer.range(1,5)        # ArrayBuffer(1, 2, 3, 4)
a.transform(_ * 2)                    # a = ArrayBuffer(2, 4, 6, 8)

Adding elements to an ArrayBuffer

In general, to add new elements to an ArrayBuffer use the += and ++= methods:

Method	Description
`+=`	add a single element to an ArrayBuffer
`++=`	add a sequence to an ArrayBuffer

You can also use these methods:

append, appendAll
insert, insertAll
prepend, prependAll

Examples

This is how to use += and ++=:

var nums = ArrayBuffer(1, 2, 3)   # ArrayBuffer(1, 2, 3)
nums += 4                         # ArrayBuffer(1, 2, 3, 4)
nums += (5, 6)                    # ArrayBuffer(1, 2, 3, 4, 5, 6)
nums ++= List(7, 8)               # ArrayBuffer(1, 2, 3, 4, 5, 6, 7, 8)

Other ways to add elements to an ArrayBuffer:

val a = ArrayBuffer(1, 2, 3)      # ArrayBuffer(1, 2, 3)
a.append(4)                       # ArrayBuffer(1, 2, 3, 4)
a.append(5, 6)                    # ArrayBuffer(1, 2, 3, 4, 5, 6)
a.appendAll(Seq(7,8))             # ArrayBuffer(1, 2, 3, 4, 5, 6, 7, 8)

a.clear                           # ArrayBuffer()

val a = ArrayBuffer(9, 10)        # ArrayBuffer(9, 10)
a.insert(0, 8)                    # ArrayBuffer(8, 9, 10)
a.insert(0, 6, 7)                 # ArrayBuffer(6, 7, 8, 9, 10)
a.insertAll(0, Vector(4, 5))      # ArrayBuffer(4, 5, 6, 7, 8, 9, 10)
a.prepend(3)                      # ArrayBuffer(3, 4, 5, 6, 7, 8, 9, 10)
a.prepend(1, 2)                   # ArrayBuffer(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
a.prependAll(Seq(0))              # ArrayBuffer(0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10)

Note that when using insert, the first parameter is the position at which new elements should be inserted:

val a = ArrayBuffer('a', 'b', 'd')   # ArrayBuffer(a, b, d)
a.insert(2, 'c')                     # ArrayBuffer(a, b, c, d)

Removing elements from an ArrayBuffer

An ArrayBuffer is a mutable sequence, so:

Method	Description
`-=`	Remove a single element or tuple of elements
`--=`	remove multiple elements that are declared in another collection
`clear`	removes all the elements from an ArrayBuffer
`reduceToSize`	removes all elements after the index you specify
`remove`	Use `remove` to delete one element by its position in the ArrayBuffer, or a series of elements beginning at a starting position
`trimStart`	remove elements from the beginning
`trimEnd`	remove elements from the end

Examples

-= and --=:

val a = ArrayBuffer.range('a', 'h')   # ArrayBuffer(a, b, c, d, e, f, g)
x -= 'a'
x -= ('b', 'c')
x --= Seq('a', 'b')
x --= Set('d')

remove:

val a = ArrayBuffer.range('a', 'h')   # ArrayBuffer(a, b, c, d, e, f, g)
a.remove(0)                           # ArrayBuffer(b, c, d, e, f, g)
a.remove(2, 3)                        # ArrayBuffer(b, c, g)

trimStart and trimEnd:

val a = ArrayBuffer.range('a', 'h')   # ArrayBuffer(a, b, c, d, e, f, g)
a.trimStart(2)                        # ArrayBuffer(c, d, e, f, g)
a.trimEnd(2)                          # ArrayBuffer(c, d, e)

clear:

val a = ArrayBuffer.range('a', 'h')   # ArrayBuffer(a, b, c, d, e, f, g)
a.clear                               # ArrayBuffer[Char] = ArrayBuffer()

reduceToSize:

val a = ArrayBuffer.range('a', 'h')   # ArrayBuffer(a, b, c, d, e, f, g)
a.reduceToSize(4)                     # ArrayBuffer(a, b, c, d)

Updating ArrayBuffer elements

To update elements in an ArrayBuffer:

val a = ArrayBuffer.range(1,5)        # ArrayBuffer(1, 2, 3, 4)
a(2) = 50                             # ArrayBuffer(1, 2, 50, 4)
a.update(0, 10)                       # ArrayBuffer(10, 2, 50, 4)
a.update(3, 40)                       # ArrayBuffer(10, 2, 50, 40)

Using functional methods with `ArrayBuffer`

Even though ArrayBuffer is a mutable sequence, you can also use all of the usual functional methods to create new values from an ArrayBuffer:

val a = ArrayBuffer.range(0,5)        # ArrayBuffer(0, 1, 2, 3, 4)
val b = a.head                        # b = 0
val b = a.tail                        # b = ArrayBuffer(1, 2, 3, 4)
val b = a.drop(2)                     # b = ArrayBuffer(2, 3, 4)
val b = a.dropWhile(_ < 3)            # b = ArrayBuffer(3, 4)
val b = a.take(2)                     # b = ArrayBuffer(0, 1)
val b = a.takeWhile(_ < 3)            # b = ArrayBuffer(0, 1, 2)
val b = a.map(_ * 2)                  # b = ArrayBuffer(0, 2, 4, 6, 8)
val b = a.filter(_ > 2)               # b = ArrayBuffer(3, 4)

val a = ArrayBuffer.range('a', 'e')   # ArrayBuffer(a, b, c, d)
val b = a.splitAt(2)                  # b = (ArrayBuffer(a, b),ArrayBuffer(c, d))

You can also convert ArrayBuffer into other Scala sequences:

val a = ArrayBuffer.range(0,4)        # ArrayBuffer(0, 1, 2, 3)
val b = a.toList                      # b = List(0, 1, 2, 3)
val b = a.toVector                    # b = Vector(0, 1, 2, 3)
val b = a.toSet                       # b = Set(0, 1, 2, 3)

For many more examples of using functional methods on Scala sequences, see my Examples of Scala Vector methods tutorial.

Scala ArrayBuffer summary

I hope these ArrayBuffer examples are helpful. If I made a mistake, or you know another way to do something with an ArrayBuffer I haven’t shown, leave a note in the Comments section.

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