CODEWITHSUNDEEP
arraysscalagenerics
Malvrok
Malvrok

Reputation: 389

How to initialize a generic array with a default value in Scala

Im having troubles when trying to initialize an array with a generic type and storing it in a value (val)

case class Matrix[T <: AnyVal](val structure : Array[Array[Array[T]]])

//A method in another class
def foo() : Unit = {
    val aux: Array[Array[Array[T]]] = Array.fill(this.structure.length, this.structure.head.length, this.structure.head.head.length)(null)
    ...
    ...
}

The problem is that, when trying to create an array of type T, I want to initialize it with a default value of null, but it says that:

Expression of type Array[Array[Array[T]]] doesn't conform to expected type Array[Array[Array[T]]]

How can I solve this problem? Thank you

Upvotes: 1

Views: 3197

Answers (2)

evan.oman
evan.oman

Reputation: 5572

T:Numeric

If I were you I would restrict T to Numeric types and then use the zero value as the default value for an empty matrix:

scala> :pa
// Entering paste mode (ctrl-D to finish)

import scala.reflect.ClassTag

class Matrix[T:Numeric](val structure: Array[Array[T]])
{
    def *(that: Matrix[T]): Matrix[T] = ???
    def +(that: Matrix[T]): Matrix[T] = ???
    override def toString: String = structure.map(_.mkString(",")).mkString("\n")
}

object Matrix
{
    def apply[T:Numeric:ClassTag](rows: Int, cols: Int): Matrix[T] =
    {
        val zero: T = implicitly[Numeric[T]].zero
        new Matrix[T](Array.fill[T](rows, cols)(zero))
    }

    def apply[T:Numeric](data: Array[Array[T]]): Matrix[T] = new Matrix(data)
}

// Exiting paste mode, now interpreting.

import scala.reflect.ClassTag
defined class Matrix
defined object Matrix

scala> val empty = Matrix[Int](3,3)
empty: Matrix[Int] =
0,0,0
0,0,0
0,0,0

scala> val ex = Matrix[Int](Array(Array(1,2), Array(3,4)))
ex: Matrix[Int] =
1,2
3,4

T <: AnyRef

Otherwise if you want to use non-numeric values with null as default you can do something like this:

scala> :pa
// Entering paste mode (ctrl-D to finish)

import scala.reflect.ClassTag

class Matrix[T >: Null <: AnyRef](val structure : Array[Array[T]])
{
    override def toString: String = structure.map(_.mkString(",")).mkString("\n")
}

object Matrix
{
    def apply[T >: Null <: AnyRef : ClassTag](rows: Int, cols: Int): Matrix[T] =
    {
        new Matrix[T](Array.fill[T](rows, cols)(null))
    }

    def apply[T >: Null <: AnyRef](data: Array[Array[T]]): Matrix[T] = new Matrix(data)
}

// Exiting paste mode, now interpreting.

import scala.reflect.ClassTag
defined class Matrix
defined object Matrix

scala> case class Person(name: String)
defined class Person

scala> val empty = Matrix[Person](3,3)
empty: Matrix[Person] =
null,null,null
null,null,null
null,null,null

scala> val data = Array(Array(Person("a"), Person("b")), Array(Person("c"), Person("d")))
data: Array[Array[Person]] = Array(Array(Person(a), Person(b)), Array(Person(c), Person(d)))

scala> val ex = Matrix[Person](data)
ex: Matrix[Person] =
Person(a),Person(b)
Person(c),Person(d)

Note that you must manually specify that T can be a supertype of Null by adding the bound T >: Null.

Upvotes: 3

Alexey Romanov
Alexey Romanov

Reputation: 170899

If you want the default value, just use Array.ofDim(this.structure.length, this.structure.head.length, this.structure.head.head.length) instead of Array.fill. The values will be, like in Java, null for objects, 0 for Int, 0.0 for Double, etc. It's cheaper as well.

You'll need a ClassTag[T] in scope, but you need it for fill as well.

Upvotes: 1

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