Left join two lists

user_7309473 0
Viewed 383

I would like to left join two list, without lost of the data. Example below:

// two lists
val b = List("John","Alice","Gregor","Mike")                 // base list
val l = List((List(1,2,3),"Mike"), (List(3,1,2), "Alice"))  // list to left join

// sorted
val bs = b.sorted        // List("Alice","Gregor","John","Mike")
val ls = l.sortBy(_._2)  // List((List(3,1,2), "Alice"),(List(1,2,3),"Mike"))


// left join - expected result:
// Alice and Mike found in both lists, Gregor and John appear only in the base list
// List(("Alice",(List(3,1,2),"Alice"), ("Gregor",Nil), ("John",Nil), ("Mike",List("Alice","Gregor","John","Mike"))))

I found some methods, but they seem to be not proper for me:
- zip - simple 'copy/paste' two lists,
- flatMap - remove elements, which doesn't appear in both lists.

Thank you in advance.

list scala
3 Answers

Try 

b
  .map(key => key -> l.find(_._2 == key))
  .map {case (key, value) => key -> value.getOrElse(Nil) }

which outputs

List((John,List()), (Alice,(List(3, 1, 2),Alice)), (Gregor,List()), (Mike,(List(1, 2, 3),Mike))
user_52050220

Using this

bs.map{
  str =>
    (str, ls.find(_._2 == str).getOrElse(Nil))
}

you will get an output as 

List((Alice,(List(3, 1, 2),Alice)), (Gregor,List()), (John,List()), (Mike,(List(1, 2, 3),Mike)))
user_78037970

You can do it, using a Map:

val m = l.map(_.swap).toMap

val res = bs.map(elem => elem -> m.get(elem).map(_ -> elem).getOrElse(Nil))
println(res)
// Displays
// List((Alice,(List(3, 1, 2),Alice)), (Gregor,List()), (John,List()), (Mike,(List(1, 2, 3),Mike)))
user_58263490
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