CODEWITHSUNDEEP
scaladata-structuresscala-collections
jcm
jcm

Reputation: 5659

Moving an element to the front of a list in Scala

I was wondering if there is a way to find an element in a list and move it to the front of the list in Scala? Is there any easy way to do this other than iterating over the list, then removing that element and then pre-pending it to the front of the list?

Upvotes: 4

Views: 2063

Answers (2)

elm
elm

Reputation: 20415

Consider the use of partition where multiple occurrences of the item of interest are moved to the head, like this,

implicit class RichList[A] (val a: List[A]) extends AnyVal {
  def toHead(e: A) = {
    val (l,r) = a.partition(_ != e)
    r ++ l
  }
}

Hence for example for val l = List(1,2,3,3,4,5,3) we have that

l.toHead(3)
res: List(3, 3, 3, 1, 2, 4, 5)

l.toHead(7)
res: List(1, 2, 3, 3, 4, 5, 3)

Upvotes: 0

jonderry
jonderry

Reputation: 23633

How about using span?:

def moveToFront[A](y: A, xs: List[A]): List[A] = {
  xs.span(_ != y) match {
    case (as, h::bs) => h :: as ++ bs
    case _           => xs
  }
}

Upvotes: 10

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