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

Question

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

You are right. I just noticed it. But, which is the equivalent of null by for AnyVal? — Malvrok
– Malvrok, Commented Dec 28, 2016 at 18:55
If you're talking about the val aux then perhaps you should declare your T. It feels like you are missing too much code here for us to be able to help you. — 2rs2ts
– 2rs2ts, Commented Dec 28, 2016 at 19:16

Community · Accepted Answer · 2020-06-20 09:12:55Z

3

`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.

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Dec 28, 2016 at 19:13

evan.oman

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5 Comments

Malvrok Over a year ago

The thing is that I algo want to use the matrix to contain other data type, not only Numeric values. That's why I want to initialize an array with a default value, and asign it to a variable (or value in this case)

evan.oman Over a year ago

Fair enough, but the general idea still holds that your default value will need to be of type T (so if you can restrict T to some tight supertype of your data and then use a value of that supertype then that would be ideal)

Malvrok Over a year ago

For example, in Java, all classes have Object like last parent. So, the default value of Object when creating an array is null. In Scala, which is the default value? There should be something similar. I know that Scala "doesn't like" nulls, and thats why they use Options, but I think there should be a default value like null but for Scala.

evan.oman Over a year ago

You need to say that it can be null explicitly using a T >: Null <: AnyRef, see my updated answer

evan.oman Over a year ago

@Malvrok does this answer your question?

Alexey Romanov · Accepted Answer · 2016-12-28 22:30:52Z

1

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.

answered Dec 28, 2016 at 22:30

Alexey Romanov

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