Scala Collections: map and filter

Before you start

Scala basics.

Transform and filter an immutable List without loops.
Predict the sample output and explain the state changes.
Complete the practice task and check the boundary cases.

The concept

Scala's standard collections are immutable by default. map builds a new list by applying a function to every element, and filter keeps only the elements for which a condition is true. Neither changes the original list.

Chaining them is the usual way to express a pipeline. Here the scores are curved by 5, giving 87, 96, and 81, and then only those of at least 90 are kept and summed.

Code example

Scala

object Main {
  def main(args: Array[String]): Unit = {
    val scores = List(82, 91, 76)
    val curved = scores.map(s => s + 5)
    println(curved)
    println(curved.filter(s => s >= 90).sum)
  }
}

Output

List(87, 96, 81)
96

Example details

Follow the code

Open the walkthrough to inspect the example one step at a time.

Show guided walkthrough

Guided walkthroughReady

1object Main {

2 def main(args: Array[String]): Unit = {

3 val scores = List(82, 91, 76)

4 val curved = scores.map(s => s + 5)

5 println(curved)

6 println(curved.filter(s => s >= 90).sum)

7 }

8}

Follow the example

Start to inspect the source, values, and output. These prepared steps explain the published sample; grouped loop steps describe multiple iterations.

View the complete dry-run table

Prepared trace of the published program. Some rows group repeated iterations.
What happens	State after step	Output so far
An immutable list of three ints.	`scores = List(82, 91, 76)`	—
A new list with 5 added to each element.	`curved = List(87, 96, 81)`	—
A List prints in its constructor form.	`curved = List(87, 96, 81)`	List(87, 96, 81)
Only 96 remains, and its sum is 96.	`curved = List(87, 96, 81)`	List(87, 96, 81) 96

Behind the scenes

How Scala processes this code

Scala compiles for the JVM, which invokes the main entry point.

Cost and common mistakes

map and filter are O(n) time; each builds a new list of up to n elements.

Collections are immutable, so scores.map(...) on its own discards the result unless you keep it.
sum of an empty List is 0, but max of an empty List throws an exception.

Try it yourself

Predict scores.filter(s => s > 80).

Reveal answer and reasoning

It keeps 82 and 91, giving List(82, 91).