Finding the index of an element in a list scala

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.

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)

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

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

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