toMap when keys are repeated with different values

Question

I have a list

val data = List(2, 4, 3, 2, 1, 1, 1,7)

with which I want to create a map such that values in above list are keys to new one with indeces as new values I tried

scala> data.zipWithIndex.toMap
res5: scala.collection.immutable.Map[Int,Int] = Map(1 -> 6, 2 -> 3, 7 -> 7, 3 -> 2, 4 -> 1)

but strangely it gives res5(1) as 6 but I want it to be 4.

I could solve it by

data.zipWithIndex groupBy (_._1) mapValues (w=>w.map(tuple=>tuple._2) min)

but is there any way I can pass a function f to toMap so that it creates map in desired way.

Answer 1

toMap is going to add each pair to the map in the order of the zipped list, and when you add a mapping k -> v to a map that already contains a k, the old value is simply replaced.

An easy fix is just to reverse the list after zipping the indices and before converting to a map:

data.zipWithIndex.reverse.toMap

Now the mappings 1 -> 6 and 1 -> 5 will be added before 1 -> 4, which means 1 -> 4 is the one you'll see in the result.