Scala Cookbook

10.28. Sorting a Collection

Problem

You want to sort a sequential collection. Or, you want to implement the Ordered trait in a custom class so you can use the sorted method, or operators like <, <=, >, and >= to compare instances of your class.

Solution

See Recipe 11.10, for information on how to sort an Array. Otherwise, use the sorted or sortWith methods to sort a collection.

The sorted method can sort collections with type Double, Float, Int, and any other type that has an implicit scala.math.Ordering:

scala> val a = List(10, 5, 8, 1, 7).sorted
a: List[Int] = List(1, 5, 7, 8, 10)

scala> val b = List("banana", "pear", "apple", "orange").sorted
b: List[String] = List(apple, banana, orange, pear)

The “rich” versions of the numeric classes (like RichInt) and the StringOps class all extend the Ordered trait, so they can be used with the sorted method. (More on the Ordered trait in the Discussion.)

The sortWith method lets you provide your own sorting function. The following examples demonstrate how to sort a collection of Int or String in both directions:

scala> List(10, 5, 8, 1, 7).sortWith(_ < _)
res1: List[Int] = List(1, 5, 7, 8, 10)

scala> List(10, 5, 8, 1, 7).sortWith(_ > _)
res2: List[Int] = List(10, 8, 7, 5, 1)

scala> List("banana", "pear", "apple", "orange").sortWith(_ < _)
res3: List[java.lang.String] = List(apple, banana, orange, pear)

scala> List("banana", "pear", "apple", "orange").sortWith(_ > _)
res4: List[java.lang.String] = List(pear, orange, banana, apple)

Your sorting function ...