Scala Cookbook

10.10. Looping over a Collection with a for Loop

Problem

You want to loop over the elements in a collection using a for loop, possibly creating a new collection from the existing collection using the for/yield combination.

Solution

You can loop over any Traversable type (basically any sequence) using a for loop:

scala> val fruits = Traversable("apple", "banana", "orange")
fruits: Traversable[String] = List(apple, banana, orange)

scala> for (f <- fruits) println(f)
apple
banana
orange

scala> for (f <- fruits) println(f.toUpperCase)
APPLE
BANANA
ORANGE

If your algorithm is long, perform the work in a block following a for loop:

scala> val fruits = Array("apple", "banana", "orange")
fruits: Array[String] = Array(apple, banana, orange)

scala> for (f <- fruits) {
     |   // imagine this required multiple lines
     |   val s = f.toUpperCase
     |   println(s)
     | }
APPLE
BANANA
ORANGE

This example shows one approach to using a counter inside a for loop:

scala> for (i <- 0 until fruits.size) println(s"element $i is ${fruits(i)}")
element 0 is apple
element 1 is banana
element 2 is orange

You can also use the zipWithIndex method when you need a loop counter:

scala> for ((elem, count) <- fruits.zipWithIndex) {
     |   println(s"element $count is $elem")
     | }
element 0 is apple
element 1 is banana
element 2 is orange

Note

When using zipWithIndex, consider calling view before zipWithIndex:

// added a call to 'view'
for ((elem, count) <- fruits.view.zipWithIndex) {
  println(s"element $count is $elem")
}

See the next recipe for details. ...