Scala Cookbook

10.17. Using filter to Filter a Collection

Problem

You want to filter the items in a collection to create a new collection that contains only the elements that match your filtering criteria.

Solution

As listed in Recipe 10.3, a variety of methods can be used to filter the elements of an input collection to produce a new output collection. This recipe demonstrates the filter method.

To use filter on your collection, give it a predicate to filter the collection elements as desired. Your predicate should accept a parameter of the same type that the collection holds, evaluate that element, and return true to keep the element in the new collection, or false to filter it out. Remember to assign the results of the filtering operation to a new variable.

For instance, the following example shows how to create a list of even numbers from an input list using a modulus algorithm:

scala> val x = List.range(1, 10)
x: List[Int] = List(1, 2, 3, 4, 5, 6, 7, 8, 9)

// create a list of all the even numbers in the list
scala> val evens = x.filter(_ % 2 == 0)
evens: List[Int] = List(2, 4, 6, 8)

As shown, filter returns all elements from a sequence that return true when your function/predicate is called. There’s also a filterNot method that returns all elements from a list for which your function returns false.

Discussion

The main methods you can use to filter a collection are listed in Recipe 10.3, and are repeated here for your convenience: collect, diff, distinct, drop, dropWhile, filter, filterNot, find, ...