How to iterate over Scala Lists with ‘foreach’ and ‘for’

Scala List/sequence FAQ: How do I iterate over a Scala List (or more generally, a sequence) using the foreach method or for loop?

There are a number of ways to iterate over a Scala List using the foreach method (which is available to Scala sequences like List, Array, ArrayBuffer, Vector, Seq, etc.) and for comprehension, and I'll show a few of those approaches here.

Back to top

Iterating over lists with ‘foreach’

A common way to iterate over a Scala List is with the foreach method. Here's a quote about foreach 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 foreach 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 familiar to you.

Note that this is a relatively common way to use the foreach method. Because foreach takes a procedure that doesn’t return anything, and because the result of foreach is also Unit, the foreach method is typically used for its side effects — something like this example where output is printed for a user to see.

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

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

Note that this second example is not a common or preferred way to use foreach; I’m just trying to show some different possibilities. (When I first wrote this example it wasn’t the worst thing in the world to use a var field, but with more and more developers preferrring functional programming, the use of var fields is discouraged.)

Back to top

Scala Lists and the “for comprehension”

The Scala for comprehension is not specific to lists, but is an extremely powerful way to operate on a List and other sequences. Here's a simple example of how to iterate over a sequence using the for comprehension (also known as a “for loop”):

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

Back to top

More detailed examples

I apologize that these examples are not as detailed as I prefer. If I had more free time I’d expand on them here, but sadly I don’t have that free time right now. So I’ll just have to say, “Please see the Scala Cookbook, where I cover the for loop and foreach method in great detail”:

Back to top

Summary: Iterating Scala lists with foreach and for

I hope this short tutorial on how to iterate over a Scala List (and other sequences) 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.

Back to top

Share it!

There’s just one person behind this website; if this article was helpful (or interesting), I’d appreciate it if you’d share it. Thanks, Al.

Add new comment

The content of this field is kept private and will not be shown publicly.

Anonymous format

  • Allowed HTML tags: <em> <strong> <cite> <code> <ul type> <ol start type> <li> <pre>
  • Lines and paragraphs break automatically.
By submitting this form, you accept the Mollom privacy policy.