Iterating over Scala Lists with foreach and for

Scala List/foreach FAQ: How do I iterate over a Scala List using the foreach method and for loop?

There are a number of ways to iterate over a Scala List using the foreach operator and for comprehension, and I'll show a few of those approaches here.

Iterating lists with foreach

A common way to iterate over a Scala List is with the foreach method. Here's a quote about the foreach method from the book Programming in Scala:

foreach takes a procedure (a function with a result type Unit) as the right operand. It simply applies the procedure to each List element. The result of the operation is again Unit; no list of results is assembled.

Here's a simple example showing how to use the foreach function to print every item in a List:

scala> val x = List(1,2,3)
x: List[Int] = List(1, 2, 3)

scala> x.foreach { println }
1
2
3

If you've used a programming language like Ruby, this syntax will look very familiar to you.

This next example shows a way to sum all the elements in a list using the foreach method:

scala> var sum = 0
sum: Int = 0

scala> val x = List(1,2,3)
x: List[Int] = List(1, 2, 3)

scala> x.foreach(sum += _)

scala> println(sum)
6

Scala Lists and the for comprehension

The Scala for comprehension is not specific to lists, but is an extremely powerful way to operate on lists. Here's a simple example of how to iterate over a list using the for comprehension:

scala> val names = List("Bob", "Fred", "Joe", "Julia", "Kim")
names: List[java.lang.String] = List(Bob, Fred, Joe, Julia, Kim)

scala> for (name <- names) println(name)
Bob
Fred
Joe
Julia
Kim

So far, so good. Now let's add a simple "if" clause to the for comprehension to print only the elements we want to print:

scala> val names = List("Bob", "Fred", "Joe", "Julia", "Kim")
names: List[java.lang.String] = List(Bob, Fred, Joe, Julia, Kim)

scala> for (name <- names if name.startsWith("J"))
     | println(name)
Joe
Julia

If you already know about the for comprehension, you know that you can add multiple if clauses, and much more functionality. I could easily write an entire tutorial on the Scala for comprehension, so to keep this tutorial short, I'll stop here for now.

Before leaving, I will add these notes however, from the book Programming in Scala:

Scala provides the for comprehension, which provides syntactically pleasing nesting of map, flatMap, and filter ... The for comprehension is not a looping construct, but is a syntactic construct the compiler reduces to map, flatMap, and filter.

Iterating Scala lists with foreach and for

I hope this short tutorial on how to iterate over a Scala List using the foreach method and for comprehension have been helpful. As you can tell from these examples, there's much more power available to you with both approaches, which is one of the great things about the Scala programming language.