Lists in Scala

A tide log held in a List, extended at both ends and read back safely.

// a List literal holds its elements in order and never changes
val tideHeights: List[Int] = List(4, 7, 11, 9, 3)

// :: adds to the front and :+ appends to the back, both returning
// a new list rather than editing the old one
val withDawn: List[Int] = 2 :: tideHeights
val withDusk: List[Int] = tideHeights :+ 5

// the original still has its five readings after both operations
val stillFive: Boolean = tideHeights.length == 5

// head is the first element and tail is everything after it
val firstReading: Int = withDawn.head
val laterReadings: List[Int] = withDawn.tail

// head throws on an empty list, so headOption answers with an
// Option instead: Some for a reading, None for no readings at all
val maybeFirst: Option[Int] = List.empty[Int].headOption
val realFirst: Option[Int] = withDusk.headOption

How it works

1. val tideHeights: List[Int] = List(4, 7, 11, 9, 3) is a list literal holding five readings in the order written.
2. 2 :: tideHeights adds to the front and tideHeights :+ 5 appends to the back, each returning a new list, and stillFive confirms the original still has its five readings.
3. withDawn.head is the first reading and withDawn.tail is the rest, while List.empty[Int].headOption hands back an Option[Int] where head would throw.

Keywords and builtins used here

• Boolean
• Int
• List
• Option
• val

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Step 1 of 3 in Collections, step 10 of 27 in Language basics.

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Lists in other languages

• C#
• Java
• R