Setting instance vars with apply

Question

With Array it's possible to get and set elements with val i = xs(0) and xs(0) = i syntax. How to implement this functionality in my own class? So far I was only able to implement getting the values.

class Matrix(val m : Int, val n : Int) {
  val matrix = Array.ofDim[Double](m, n)

  def apply(i:Int)(j:Int) = matrix(i)(j)
}

UPDATE: Thanks to Mauricio for answer about update method. That's the final version

class Matrix(val m:Int, val n:Int) {
  private val matrix = Array.ofDim[Double](m, n)
  def apply(i:Int) = new {
    def apply(j:Int) = matrix(i)(j)

    def update(j:Int, v:Double) = { matrix(i)(j) = v }
  }
}

it("matrix") {
  val m = new Matrix(3, 3)
  m(0)(1) = 10.0
  val x = m(0)(1)

  x should equal(10.0)
  x.isNegInfinity should be (false) // Implicits for Double work!
}

Maurício Linhares · Accepted Answer · 2013-05-02 02:59:18Z

2

You need to declare an update method:

class Matrix(val m : Int, val n : Int) {
  private val matrix = Array.ofDim[Double](m, n)

  def apply(i:Int)(j:Int) = matrix(i)(j)

  def update( i : Int, j : Int, value : Double) {
    matrix(i)(j) = value
  }

}

val m = new Matrix( 10, 10 )
m(9, 9) = 50

answered May 2, 2013 at 2:59

Maurício Linhares

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3 Comments

synapse Over a year ago

Thanks. So if I want to use exactly the same syntax as multidim array, I need to implement MatrixRow class with update and apply methods and return it from Matrix's apply. Is there a way to create some anonymous class instead instead of explicit class definition?

Maurício Linhares Over a year ago

A caller won't know what methods an anonymous class implements unless you make it a trait and return this trait implemented by the returned anonymous object.

synapse Over a year ago

It looks like it typechecks even without a trait. I've updated the question with the code that works.

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