Scala local variable inside primary constructor

Question

How in Scala I can define local variable in primary constructor?

I need to solve this exercise from Scala for the impatient book:

Write a class Person with a primary constructor that accepts a string containing a ﬁrst name, a space, and a last name, such as new Person("Fred Smith"). Supply read-only properties firstName and lastName. Should the primary constructor parameter be a var, a val, or a plain parameter? Why?

And for now my solution looks like this:

class Person(firstLast: String) {
  private[this] val firstLastAsArr = firstLast.trim.split(" ")

  val firstName = firstLastAsArr (0)
  val lastName = firstLastAsArr (1)
}

How I can restrict firstLastAsArr variable visibility to primary constructor scope (now it have class scope)?

possible duplicate of [How do you define a local var/val in the primary constructor in Scala?](http://stackoverflow.com/questions/1118669/how-do-you-define-a-local-var-val-in-the-primary-constructor-in-scala) — Jukka Suomela, Jun 10 '14 at 13:46

score 5 · Accepted Answer · answered Jun 20 '13 at 10:22

5

One solution is to initialize firstName and lastName at once, thereby allowing to turn firstLastAsArr into a local temporary value inside your initialization block:

class Person(firstLast: String) {
  val (firstName, lastName) = {
    val firstLastAsArr = firstLast.trim.split(" ")
    (firstLastAsArr(0), firstLastAsArr(1))
  }
}

answered Jun 20 '13 at 10:22

Régis Jean-Gilles

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`firstLastAsArr(1)` will actually contain the separator, i.e., `" "` – Malte Schwerhoff Jun 20 '13 at 10:25
3

@MalteSchwerhoff whoa? `"John Doe".trim.split(" ")` results to `Array(John, Doe)` – om-nom-nom Jun 20 '13 at 10:26
@om-nom-nom Interesting. I tried it locally as a Scala 2.10.1 script (https://gist.github.com/anonymous/5821751), but http://ideone.com/4b27Dr shows the behaviour as well with Scala 2.10.0. – Malte Schwerhoff Jun 20 '13 at 10:36
@MalteSchwerhoff that is because you have **two** spaces in `"John Doe"` and `(,1)` is the void between them – om-nom-nom Jun 20 '13 at 10:40
@om-nom-nom Oh boy, missing that is fairly embarrassing. Thanks! – Malte Schwerhoff Jun 20 '13 at 13:20

score 4 · Answer 2 · answered Jun 20 '13 at 10:25

4

It is not a general answer, but in this particular way you may write:

  val Array(firstName, lastName) = firstLast.trim.split(" ")

answered Jun 20 '13 at 10:25

om-nom-nom

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Malte Schwerhoff · Answer 3 · 2013-06-20T13:17:22.167

You don't strictly need the intermediate variable:

class Person(firstLast: String) {
  val (firstName, lastName) =
    firstLast.trim.split(" ") match {case Array(first, last) => (first, last)}
}

However, if your transformation from firstLast to firstName and lastName grows a big longer, for example, because you check that there is exactly one first and one last name, then I would encapsulate the whole splitting-business in a dedicated method:

class Person(firstLast: String) {
  val (firstName, lastName) = split(firstLast)

  private def split(firstLast: String): (String, String) = {
    val firstLastAsArr = firstLast.trim.split(" ")
    ...
    (first, last)
  }
}

senia · Answer 4 · 2013-06-20T11:10:14.203

1

Pattern matching in constructor works just fine, but you should consider moving such logic from constructor to factory method:

case class Person(firstName: String, lastName: String)
object Person{
  def apply(firstLast: String) = {
    val firstLastAsArr = firstLast.trim.split(" ")
    new Person(firstLastAsArr(0), firstLastAsArr(1))
  }
}

val p = Person("My Title")

edited Jun 20 '13 at 11:10

answered Jun 20 '13 at 10:46

senia

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@pagoda_5b: this is really not a correct answer in this case, but I can't add formated code sample to comment. I think the one who need to initialize 2 fields from 1 constructor parameter should use a factory method. SO is not about homework. – senia Jun 20 '13 at 12:58
+1 for the factory method solution -- I'd also go for this pattern, as it avoids creating temporary tuples. – axel22 Jun 20 '13 at 13:08

score 0 · Answer 5 · answered May 21 '14 at 08:15

Pattern maching in primary constructor works well

  class Person(_fullName:String) {
    val (firstName, lastName) =  _fullName.split(" ") match {
      case Array(x:String, y:String, _*) => (x,y)
      case _ => (null,null)
    }
  }

See my github for full answer https://github.com/BasileDuPlessis/scala-for-the-impatient/blob/master/src/main/scala/com/basile/scala/ch05/Ex07.scala

Scala local variable inside primary constructor

5 Answers5

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