How to find unique elements from list of tuples based on some elements using scala?

Question

I have following list

val a = List(("name1","add1","city1",10),("name1","add1","city1",10),

("name2","add2","city2",10),("name2","add2","city2",20),("name3","add3","city3",20))

I want distinct element from above list based on first three values of tuple. Fourth value should not be consider while finding distinct elements from list.

I want following output:

val output = List(("name1","add1","city1",10),("name2","add2","city2",10),

("name3","add3","city3",20))

Is it possible to get above output?

As per my knowledge, distinct works if whole tuple/value is duplicated. I tried out with distinct like following:

val b = List(("name1","add1","city1",10),("name1","add1","city1",10),("name2","add2","city2",10),
("name2","add2","city2",20),("name3","add3","city3",20)).distinct

but it gives output as -

List(("name1","add1","city1",10),("name2","add2","city2",10),
("name2","add2","city2",20),("name3","add3","city3",20))

Any alternate approach will also appreciated.

Answer 1

Use groupBy like this

a.groupBy( v => (v._1,v._2,v._3)).keys.toList

This constructs a Map where each key is by definition a unique triplet as required in the lambda function above.

Should it include also the last element in the tuple, fetch the first element for each key, like this

a.groupBy( v => (v._1,v._2,v._3)).mapValues(_.head)

Answer 2

If the order of the output list isn't important (i.e. you are happy to get List(("name3","add3","city3",20),("name1","add1","city1",10),("name2","add2","city2",10))), the following works as specified:

a.groupBy(v => (v._1,v._2,v._3)).values.map(_.head).toList

(Due to Scala collections design, you'll see the order kept for output lists up to 4 elements, but above that size HashMap will be used.) If you do need to keep the order, you can do something like (generalizing a bit)

def distinctBy[A, B](xs: Seq[A], f: A => B) = {
  val seen = LinkedHashMap.empty[B, A]
  xs.foreach { x =>
    val key = f(x)
    if (!seen.contains(key)) { seen.update(key, x) }
  }
  seen.values.toList
}

distinctBy(a, v => (v._1, v._2, v._3))