Finding the index of an element in a list scala

Question

How do I find the index of an element in a Scala list.

val ls = List("Mary", "had", "a", "little", "lamb")

I need to get 3 if I ask for the index of "little"

score 96 · Accepted Answer · answered Jul 26 '13 at 05:38

96

scala> List("Mary", "had", "a", "little", "lamb").indexOf("little")
res0: Int = 3

You might try reading the scaladoc for List next time. ;)

answered Jul 26 '13 at 05:38

DaoWen

List("Mary", "had", "a", "little", "little").indexOf("little") will always return 3 – m-bhole May 07 '19 at 14:10

Rok Kralj · Answer 2 · 2020-07-29T10:29:50.907

52

If you want to search by a predicate, use .indexWhere(f):

val ls = List("Mary", "had", "a", "little", "lamb","a")
ls.indexWhere(_.startsWith("l"))

This returns 3, since "little" is the first word beginning with letter l.

edited Jul 29 '20 at 10:29

answered Feb 08 '15 at 08:44

Rok Kralj

score 40 · Answer 3 · edited Aug 15 '14 at 13:50

40

If you want list of all indices containing "a", then:

val ls = List("Mary", "had", "a", "little", "lamb","a")
scala> ls.zipWithIndex.filter(_._1 == "a").map(_._2)
res13: List[Int] = List(2, 5)

edited Aug 15 '14 at 13:50

prosseek

answered Jul 26 '13 at 05:42

Jatin

23

Interesting! I think this is what _collect_ is for though: `ls.zipWithIndex.collect { case ("a",i) => i }` – DaoWen Jul 26 '13 at 17:04
1

Is there a way to use `.collect` with a variable ? Like `ls.zipWithIndex.collect { case (someVariableIWantToMatch,i) => i }` ? – Arthur Attout Mar 07 '19 at 09:30
2

Nevermind, that's about [Stable identifiers](https://stackoverflow.com/questions/7078022/why-does-pattern-matching-in-scala-not-work-with-variables) – Arthur Attout Mar 07 '19 at 10:04

3 Answers3