Scala Cookbook

10.18. Extracting a Sequence of Elements from a Collection

Problem

You want to extract a sequence of contiguous elements from a collection, either by specifying a starting position and length, or a function.

Solution

There are quite a few collection methods you can use to extract a contiguous list of elements from a sequence, including drop, dropWhile, head, headOption, init, last, lastOption, slice, tail, take, takeWhile.

Given the following Array:

scala> val x = (1 to 10).toArray
x: Array[Int] = Array(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)

The drop method drops the number of elements you specify from the beginning of the sequence:

scala> val y = x.drop(3)
y: Array[Int] = Array(4, 5, 6, 7, 8, 9, 10)

The dropWhile method drops elements as long as the predicate you supply returns true:

scala> val y = x.dropWhile(_ < 6)
y: List[Int] = List(6, 7, 8, 9, 10)

The dropRight method works like drop, but starts at the end of the collection and works forward, dropping elements from the end of the sequence:

scala> val y = x.dropRight(4)
y: Array[Int] = Array(1, 2, 3, 4, 5, 6)

take extracts the first N elements from the sequence:

scala> val y = x.take(3)
y: Array[Int] = Array(1, 2, 3)

takeWhile returns elements as long as the predicate you supply returns true:

scala> val y = x.takeWhile(_ < 5)
y: Array[Int] = Array(1, 2, 3, 4)

takeRight works the same way take works, but starts at the end of the sequence and moves forward, taking the specified number of elements from the end of the sequence:

scala> val y = x.takeRight(3) ...