How to implement 'takeUntil' of a list? - 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)

Answer 5

You could use following function,
def takeUntil(list: List[Int]): List[Int] = list match {
case x :: xs if (x != 7) => x :: takeUntil(xs)
case x :: xs if (x == 7) => List(x)
case Nil => Nil
}
val list = List(1,4,5,2,3,5,5,7,8,9,2,7,4)
takeUntil(list) //List(1,4,5,2,3,5,5,7)
Tail Recursive version
def takeUntilRec(list: List[Int]): List[Int] = {
#annotation.tailrec
def trf(head: Int, tail: List[Int], res: List[Int]): List[Int] = head match {
case x if (x != 7 && tail != Nil) => trf(tail.head, tail.tail, x :: res)
case x => x :: res
}
trf(list.head, list.tail, Nil).reverse
}

Answer 6

Some ways by use built-in functions:
val list = List(1, 4, 5, 2, 3, 5, 5, 7, 8, 9, 2, 7, 4)
//> list : List[Int] = List(1, 4, 5, 2, 3, 5, 5, 7, 8, 9, 2, 7, 4)
//Using takeWhile with dropWhile
list.takeWhile(_ != 7) ++ list.dropWhile(_ != 7).take(1)
//> res0: List[Int] = List(1, 4, 5, 2, 3, 5, 5, 7)
//Using take with segmentLength
list.take(list.segmentLength(_ != 7, 0) + 1)
//> res1: List[Int] = List(1, 4, 5, 2, 3, 5, 5, 7)
//Using take with indexOf
list.take(list.indexOf(7) + 1)
//> res2: List[Int] = List(1, 4, 5, 2, 3, 5, 5, 7)

Answer 7

Many of the solutions here are not very efficient, because they explore the whole list, rather than stopping early. Here is a short solution using the in-built functions:
def takeUntil[T](c: Iterable[T], f: T => Boolean): Iterable[T] = {
val index = c.indexWhere(f)
if (index == -1) c else c.take(index + 1)
}