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.