Appending to an ArrayBuffer in Scala in Scala

A Scala List is immutable. When you need to append elements in place, use scala.collection.mutable.ArrayBuffer.

val arr = scala.collection.mutable.ArrayBuffer(1, 2, 3)
arr += 4

// arr is now ArrayBuffer(1, 2, 3, 4)

Assignment
Follow the Coding Tutorial and append an item.

Hint
Use += with an ArrayBuffer.