Scala group By First Occurrence - scala

Question

I have the following data:
List(Map("weight"->"1000","type"->"abc","match"->"first"),Ma‌​p("weight"->"1000","‌​type"->"abc","match"‌​->"third"),Map("weig‌​ht"->"182","type"->"‌​bcd","match"->"secon‌​d"),Map("weight"->"1‌​50","type"->"bcd","m‌​atch"->"fourth"),Map‌​("weight"->"40","typ‌​e"->"aaa","match"->"‌​fifth"))
After grouping by "type" i would want results of first "abc" then "bcd" then "aaa"
When I apply group by "type" the resulting map gives first key as aaa whereas I want the first key to be "abc" and all corresponding values.
how can I achieve this?

Answer 1

As mentioned in the comments you need a sorted map, which is not created by a simple group by. What you could do is:
val a = List(Map("weight"->"1000", "type"->"abc","match"->"first"), Map("weight"->"1000","type"->"abc","match"->"third"),Map("weig‌​ht"->"182","type"->"‌​bcd","match"->"secon‌​d"), Map("weight"->"1‌​50","type"->"bcd","m‌​atch"->"fourth"), Map("weight"->"40","type"->"aaa","match"->"‌​fifth"))
val sortedGrouping = SortedMap.empty[String,String] ++ a.groupBy { x => x("type") }
println(sortedGrouping)
What you get (printed) is:
Map(aaa -> List(Map(weight -> 40, type -> aaa, match -> ‌​fifth)), abc -> List(Map(weight -> 1000, type -> abc, match -> first), Map(weight -> 1000, type -> abc, match -> third)), bcd -> List(Map(weight -> 1‌​50, type -> bcd, m‌​atch -> fourth)), ‌​bcd -> List(Map(weig‌​ht -> 182, type -> ‌​bcd, match -> secon‌​d)))

Answer 2

I don't think there is out of the box solution for what you are trying to achieve. Map is used in two ways: get a value by a key, and iterate over it. The first part is unaffected by a sort order, so groupBy works perfectly well. The second part can be achieved by creating a list of keys in required order and then using that list to get key-value pairs from Map in a specific order. For example:
val xs = List(Map("weight"->"1000","type"->"abc","match"->"first"),
Map("weight"->"1000","type"->"abc","match"->"third"),
Map("weight"->"182","type"->"bcd","match"->"second"),
Map("weight"->"150","type"->"bcd","match"->"fourth"),
Map("weight"->"40","type"->"aaa","match"->"‌fifth"))
import collection.mutable.{ListBuffer, Set}
// List of types in the order of appearance in your list
val sortedKeys = xs.map(_("type")).distinct
//> sortedKeys: List[String] = List(abc, bcd, aaa)
Now when you need to iterate you simply do:
val grouped = xs.groupBy(_("type"))
sortedKeys.map(k => (k, grouped(k)))
// List[(String, List[scala.collection.immutable.Map[String,String]])] =
// List(
// (abc,List(Map(weight -> 1000, type -> abc, match -> first),
// Map(weight -> 1000, type -> abc, match -> third))),
// (bcd,List(Map(weight -> 182, type -> bcd, match -> second),
// Map(weight -> 150, type -> bcd, match -> fourth))),
// (aaa,List(Map(weight -> 40, type -> aaa, match -> ‌fifth)))
// )