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? — Jukka Suomela
– Jukka Suomela, Commented Jun 10, 2014 at 13:46

Régis Jean-Gilles · Accepted Answer · 2013-06-20 10:22:15Z

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, 2013 at 10:22

Régis Jean-Gilles

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5 Comments

Malte Schwerhoff Over a year ago

firstLastAsArr(1) will actually contain the separator, i.e., " "

om-nom-nom Over a year ago

@MalteSchwerhoff whoa? "John Doe".trim.split(" ") results to Array(John, Doe)

Malte Schwerhoff Over a year ago

@om-nom-nom Interesting. I tried it locally as a Scala 2.10.1 script (gist.github.com/anonymous/5821751), but ideone.com/4b27Dr shows the behaviour as well with Scala 2.10.0.

om-nom-nom Over a year ago

@MalteSchwerhoff that is because you have two spaces in "John Doe" and (,1) is the void between them

Malte Schwerhoff Over a year ago

@om-nom-nom Oh boy, missing that is fairly embarrassing. Thanks!

om-nom-nom · Accepted Answer · 2013-06-20 10:25:28Z

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, 2013 at 10:25

om-nom-nom

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Comments

Malte Schwerhoff · Accepted Answer · 2013-06-20 13:17:22Z

2

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)
  }
}

edited Jun 20, 2013 at 13:17

answered Jun 20, 2013 at 10:24

Malte Schwerhoff

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Comments

senia · Accepted Answer · 2013-06-20 11:10:14Z

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, 2013 at 11:10

answered Jun 20, 2013 at 10:46

senia

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2 Comments

senia Over a year ago

@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.

axel22 Over a year ago

+1 for the factory method solution -- I'd also go for this pattern, as it avoids creating temporary tuples.

Basile du Plessis · Accepted Answer · 2014-05-21 08:15:11Z

0

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

answered May 21, 2014 at 8:15

Basile du Plessis

1

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