Two-argument indexOf does not work for Iterator

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Iterator(5).indexOf(5,0) does not work. In fact, every call to the two-arg version of indexOf seems to return -1. I presume the iterator is being consumed completely instead of partially.

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Imported From: https://issues.scala-lang.org/browse/SI-8552?orig=1
Reporter: @Ichoran
Affected Versions: 2.11.0

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@ruippeixotog said (edited on May 17, 2014 10:18:25 PM UTC):
It seems Iterator doesn't have a two-arg version of indexOf. The two arg version present in other collections comes from GenSeqLike, which Iterator does not implement:

def indexOf[B >: A](elem: B, from: Int): Int = indexWhere(elem == _, from)

When indexOf is called on an Iterator with two arguments, the compiler is converting it to a call to the one-arg version with a tuple argument:

scala> Iterator(5 -> 0).indexOf(5,0)
res0: Int = 0

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@Ichoran said:
Ah, good catch. It's still a bug, I think, to not have it (especially since it's trivially implementable), but since it's not there I can't fix it in 2.11 (binary & source compatibility issues).

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@ruippeixotog said:
@Ichoran I'll add the new method to Iterator and create a pull request for 2.12.x. Is that ok with you?

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@ruippeixotog said:
Opened up a pull request for this at scala/scala#4429.

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Kevin Meredith (kevinmeredith) said (edited on Apr 16, 2015 4:49:08 PM UTC):
Note - I have not contributed to scala or the collections library at all.

I've noticed that other Iterator.scala code, as well as your PR, @rui, included var's.

Why not implement indexWhere in a functional way? Something like:

  override def indexWhere[B >: A](p: A => Boolean, from: Int): Int = 
   self.zipWithIndex.drop(from).find(p(_._1)) match {
      case None             => -1
      case Some((_, index)) => index
    } 

Note - I don't know if self is correct here. But this REPL examples shows a few simple examples working:

scala> Iterator(1,2,3).zipWithIndex.drop(0).find(_._1 == 3)
res2: Option[(Int, Int)] = Some((3,2))

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@Ichoran said:
[~kevinmeredith] You wouldn't do it that way because that implementation is much slower, and core library functions ought not assume that the user doesn't care how fast they are. You could write a tail-recursive function, but a simple loop with a mutable var isn't that hard to understand. The var is completely contained and the logic is straightforward.

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@Ichoran said:
scala/scala#4429