Transform a list of object to lists of its field

Question

I have a List[MyObject], with MyObject containing the fields field1, field2 and field3.

I'm looking for an efficient way of doing :

Tuple3(_.map(_.field1), _.map(_.field2), _.map(_.field3))

In java I would do something like :

Field1Type f1 = new ArrayList<Field1Type>();
Field2Type f2 = new ArrayList<Field2Type>();
Field3Type f3 = new ArrayList<Field3Type>();


for(MyObject mo : myObjects) {
    f1.add(mo.getField1());
    f2.add(mo.getField2());
    f3.add(mo.getField3());
}

I would like something more functional since I'm in scala but I can't put my finger on it.

gatear · Accepted Answer · 2021-11-23 13:57:53Z

3

Get 2\3 sub-groups with `unzip\unzip3`

Assuming the starting point:

val objects: Seq[MyObject] = ???

You can unzip to get all 3 sub-groups:

val (firsts, seconds, thirds) =  
  objects
    .unzip3((o: MyObject) => (o.f1, o.f2, o.f3))

What if I have more than 3 relevant sub-groups ?

If you really need more sub-groups you need to implement your own unzipN however instead of working with Tuple22 I would personally use an adapter:


case class MyObjectsProjection(private val objs: Seq[MyObject]) {
  
  lazy val f1s: Seq[String] =
    objs.map(_.f1)

  lazy val f2s: Seq[String] =
    objs.map(_.f2)

    ... 
  
  lazy val f22s: Seq[String] =
    objs.map(_.f3)
}

val objects: Seq[MyClass] = ???
val objsProjection = MyObjectsProjection(objects)

objs.f1s
objs.f2s
...
objs.f22s

Notes:

Change MyObjectsProjection according to your needs.
This is from a Scala 2.12\2.11 vanilla perspective.

edited Nov 23, 2021 at 13:57

answered Nov 23, 2021 at 11:17

gatear

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4 Comments

gianluca aguzzi Over a year ago

For scala 3 users: you can leverage generic tuple: elements.map(Tuple.fromProductTyped(_)).unzip3

Omegaspard Over a year ago

What if I have more than 3 fields later ?

gatear Over a year ago

Even if MyClass has more fields you can select just 2\3 relevant fields with unzip\unzip3. You need to add your own implementations for bigger tuples or just rethink your algorithm. TBH instead of working with Tuple20 i would make a case class adapter.

gatear Over a year ago

I updated the post with a generic adapter as well

Jarrod Baker · Accepted Answer · 2021-11-23 11:10:36Z

2

The following will decompose your objects into three lists:

case class MyObject[T,S,R](f1: T, f2: S, f3: R)

val myObjects: Seq[MyObject[Int, Double, String]] = ???

val (l1, l2, l3) = myObjects.foldLeft((List.empty[Int], List.empty[Double], List.empty[String]))((acc, nxt) => {
  (nxt.f1 :: acc._1, nxt.f2 :: acc._2, nxt.f3 :: acc._3)
})

answered Nov 23, 2021 at 11:10

Jarrod Baker

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Transform a list of object to lists of its field

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Get 2\3 sub-groups with `unzip\unzip3`

What if I have more than 3 relevant sub-groups ?

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2 Answers 2

Get 2\3 sub-groups with unzip\unzip3

What if I have more than 3 relevant sub-groups ?

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Get 2\3 sub-groups with `unzip\unzip3`