Scala Cookbook

10.9. Looping over a Collection with foreach

Problem

You want to iterate over the elements in a collection with the foreach method.

Solution

The foreach method takes a function as an argument. The function you define should take an element as an input parameter, and should not return anything. The input parameter type should match the type stored in the collection. As foreach executes, it passes one element at a time from the collection to your function until it reaches the last element in the collection.

The foreach method applies your function to each element of the collection, but it doesn’t return a value. Because it doesn’t return anything, it’s said that it’s used for its “side effect.”

As an example, a common use of foreach is to output information:

scala> val x = Vector(1, 2, 3)
x: scala.collection.immutable.Vector[Int] = Vector(1, 2, 3)

scala> x.foreach((i: Int) => println(i))
1
2
3

That’s the longhand way of writing that code. For most expressions, Scala can infer the type, so specifying i: Int isn’t necessary:

args.foreach(i => println(i))

You can further shorten this expression by using the ubiquitous underscore wildcard character instead of using a temporary variable:

args.foreach(println(_))

In a situation like this, where a function literal consists of one statement that takes a single argument, it can be condensed to this form:

args.foreach(println)

For a simple case like this, the syntax in the last example is typically used.

Discussion

As long as your function (or method) takes ...