How to implement 'takeUntil' of a list?

Question

I want to find all items before and equal the first 7:

val list = List(1,4,5,2,3,5,5,7,8,9,2,7,4)

My solution is:

list.takeWhile(_ != 7) ::: List(7)

The result is:

List(1, 4, 5, 2, 3, 5, 5, 7)

Is there any other solution?

Answer 1

One-liner for impatient:

List(1, 2, 3, 7, 8, 9, 2, 7, 4).span(_ != 7) match {case (h, t) => h ::: t.take(1)}

More generic version:

It takes any predicate as argument. Uses span to do the main job:

  implicit class TakeUntilListWrapper[T](list: List[T]) {
    def takeUntil(predicate: T => Boolean):List[T] = {
      list.span(predicate) match {
        case (head, tail) => head ::: tail.take(1)
      }
    }
  }

  println(List(1,2,3,4,5,6,7,8,9).takeUntil(_ != 7))
  //List(1, 2, 3, 4, 5, 6, 7)

  println(List(1,2,3,4,5,6,7,8,7,9).takeUntil(_ != 7))
  //List(1, 2, 3, 4, 5, 6, 7)

  println(List(1,2,3,4,5,6,7,7,7,8,9).takeUntil(_ != 7))
  //List(1, 2, 3, 4, 5, 6, 7)

  println(List(1,2,3,4,5,6,8,9).takeUntil(_ != 7))
  //List(1, 2, 3, 4, 5, 6, 8, 9)

Tail-recursive version.

Just to illustrate alternative approach, it's not any more efficient than previous solution.

implicit class TakeUntilListWrapper[T](list: List[T]) {
  def takeUntil(predicate: T => Boolean): List[T] = {
    def rec(tail:List[T], accum:List[T]):List[T] = tail match {
      case Nil => accum.reverse
      case h :: t => rec(if (predicate(h)) t else Nil, h :: accum)
    }
    rec(list, Nil)
  }
}

Answer 2

Borrowing the takeWhile implementation from scala.collection.List and changing it a bit:

def takeUntil[A](l: List[A], p: A => Boolean): List[A] = {
    val b = new scala.collection.mutable.ListBuffer[A]
    var these = l
    while (!these.isEmpty && p(these.head)) {
      b += these.head
      these = these.tail
    }
    if(!these.isEmpty && !p(these.head)) b += these.head

    b.toList
  }

Answer 3

Here's a way to get there with foldLeft, and a tail recursive version to short circuit long lists.

There's also the tests I used while playing around with this.

import scala.annotation.tailrec
import org.scalatest.WordSpec
import org.scalatest.Matchers

object TakeUntilInclusiveSpec {
  implicit class TakeUntilInclusiveFoldLeft[T](val list: List[T]) extends AnyVal {
    def takeUntilInclusive(p: T => Boolean): List[T] =
      list.foldLeft( (false, List[T]()) )({
        case ((false, acc), x)      => (p(x), x :: acc)
        case (res @ (true, acc), _) => res
      })._2.reverse
  }
  implicit class TakeUntilInclusiveTailRec[T](val list: List[T]) extends AnyVal {
    def takeUntilInclusive(p: T => Boolean): List[T] = {
      @tailrec
      def loop(acc: List[T], subList: List[T]): List[T] = subList match {
        case Nil => acc.reverse
        case x :: xs if p(x) => (x :: acc).reverse
        case x :: xs => loop(x :: acc, xs)
      }
      loop(List[T](), list)
    }
  }
}

class TakeUntilInclusiveSpec extends WordSpec with Matchers {
  //import TakeUntilInclusiveSpec.TakeUntilInclusiveFoldLeft
  import TakeUntilInclusiveSpec.TakeUntilInclusiveTailRec

  val `return` = afterWord("return")
  object lists {
    val one = List(1)
    val oneToTen = List(1, 2, 3, 4, 5, 7, 8, 9, 10)
    val boat = List("boat")
    val rowYourBoat = List("row", "your", "boat")
  }

  "TakeUntilInclusive" when afterWord("given") {
    "an empty list" should `return` {
      "an empty list" in {
        List[Int]().takeUntilInclusive(_ == 7) shouldBe Nil
        List[String]().takeUntilInclusive(_ == "") shouldBe Nil
      }
    }

    "a list without the matching element" should `return` {
      "an identical list" in {
        lists.one.takeUntilInclusive(_ == 20) shouldBe lists.one
        lists.oneToTen.takeUntilInclusive(_ == 20) shouldBe lists.oneToTen
        lists.boat.takeUntilInclusive(_.startsWith("a")) shouldBe lists.boat
        lists.rowYourBoat.takeUntilInclusive(_.startsWith("a")) shouldBe lists.rowYourBoat
      }
    }

    "a list containing one instance of the matching element in the last index" should `return`
    {
      "an identical list" in {
        lists.one.takeUntilInclusive(_ == 1) shouldBe lists.one
        lists.oneToTen.takeUntilInclusive(_ == 10) shouldBe lists.oneToTen
        lists.boat.takeUntilInclusive(_ == "boat") shouldBe lists.boat
        lists.rowYourBoat.takeUntilInclusive(_ == "boat") shouldBe lists.rowYourBoat
      }
    }

    "a list containing one instance of the matching element" should `return` {
      "the elements of the original list, up to and including the match" in {
        lists.one.takeUntilInclusive(_ == 1) shouldBe List(1)
        lists.oneToTen.takeUntilInclusive(_ == 5) shouldBe List(1,2,3,4,5)
        lists.boat.takeUntilInclusive(_ == "boat") shouldBe List("boat")
        lists.rowYourBoat.takeUntilInclusive(_ == "your") shouldBe List("row", "your")
      }
    }

    "a list containing multiple instances of the matching element" should `return` {
      "the elements of the original list, up to and including only the first match" in {
        lists.oneToTen.takeUntilInclusive(_ % 3 == 0) shouldBe List(1,2,3)
        lists.rowYourBoat.takeUntilInclusive(_.length == 4) shouldBe List("row", "your")
      }
    }
  }
}

Answer 4

Possible way of doing this:

def takeUntil[A](list:List[A])(predicate: A => Boolean):List[A] =
  if(list.isEmpty) Nil
  else if(predicate(list.head)) list.head::takeUntil(list.tail)(predicate)
  else List(list.head)