Scala Cookbook

11.4. Deleting Elements from a List (or ListBuffer)

Problem

You want to delete elements from a List or ListBuffer.

Solution

A List is immutable, so you can’t delete elements from it, but you can filter out the elements you don’t want while you assign the result to a new variable:

scala> val originalList = List(5, 1, 4, 3, 2)
originalList: List[Int] = List(5, 1, 4, 3, 2)

scala> val newList = originalList.filter(_ > 2)
newList: List[Int] = List(5, 4, 3)

Rather than continually assigning the result of operations like this to a new variable, you can declare your variable as a var and reassign the result of the operation back to itself:

scala> var x = List(5, 1, 4, 3, 2)
x: List[Int] = List(5, 1, 4, 3, 2)

scala> x = x.filter(_ > 2)
x: List[Int] = List(5, 4, 3)

See Chapter 10 for other ways to get subsets of a collection using methods like filter, partition, splitAt, take, and more.

ListBuffer

If you’re going to be modifying a list frequently, it may be better to use a ListBuffer instead of a List. A ListBuffer is mutable, so you can remove items from it using all the methods for mutable sequences shown in Chapter 10. For example, assuming you’ve created a ListBuffer like this:

import scala.collection.mutable.ListBuffer
val x = ListBuffer(1, 2, 3, 4, 5, 6, 7, 8, 9)

You can delete one element at a time, by value:

scala> x -= 5
res0: x.type = ListBuffer(1, 2, 3, 4, 6, 7, 8, 9)

You can delete two or more elements at once:

scala> x -= (2, 3)
res1: x.type = ListBuffer(1, 4, 6, 7, 8, 9)

(That method looks ...