How to make immutable 2D Array in Scala

Question

I have the following class:

class Matrix(val matrix: Array[Array[Int]]) {
// some other methods
override def toString: String = {
    return matrix.map(_.mkString(" ")).mkString("\n")
  }   
}

I have declared class variable as val to prevent further modification in matrix.

object Main {
  def main(args: Array[String]) {

    val > = Array
    val x: Array[Array[Int]] = >(
      >(1, 2, 3),
      >(4, 5, 6),
      >(7, 8, 9))

    val m1 = new Matrix(x)
    println("m1 -->\n" + m1)
    x(1)(1) = 101 // Need to prevent this type of modification.
    println("m1 -->\n" + m1)
  }
}

After doing x(1)(1) = 101 the output of the program is

But I want to prevent this modification and get the original matrix as

val only makes the variable cannot be reassigned by other collection but you still can reassign the field in the collection — Tony Ng Wei Shyang
– Tony Ng Wei Shyang, Commented Jul 3, 2017 at 7:08
Use an immutable data type instead of a mutable one. Array is mutable (even if the reference to it is not). You can simply use IndexedSeq which serves as a generic trait for sequences with constant random access. The official documentation has a very interesting chapter on collections and their characteristics: docs.scala-lang.org/overviews/collections/overview.html — stefanobaghino
– stefanobaghino, Commented Jul 3, 2017 at 7:52

Tony Ng Wei Shyang · Accepted Answer · 2017-07-03 07:34:31Z

1

Instead of using Array, maybe you could use List instead, and it is immutable :

scala> val num:List[Int] = List(1,2,3)
num: List[Int] = List(1, 2, 3)

scala> num(1) = 3
<console>:13: error: value update is not a member of List[Int]
   num(1) = 3
   ^

answered Jul 3, 2017 at 7:34

Tony Ng Wei Shyang

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3 Comments

Abir Chokraborty Over a year ago

I am using ListBuffer. Could you tell me how can I cast scala.collection.mutable.ListBuffer[scala.collection.mutable.ListBuffer[Int]] to List[List[Int]]?

jwvh Over a year ago

buff.map(_.toList).toList

Mark Lewis Over a year ago

You really don't want Lists for matrices. They are likely going to be random access so this will have a lot of overhead.

jwvh · Accepted Answer · 2017-07-03 07:24:59Z

0

There's a difference between a mutable/immutable variable (e.g. var x and val x) and a mutable/immutable collection. Declaring one (the variable) doesn't effect the other (the collection).

The Scala Array is inherited from Java and, as such, is mutable. There are many fine immutable collections. Array isn't one of them.

answered Jul 3, 2017 at 7:24

jwvh

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1 Comment

Mark Lewis Over a year ago

The fact that this is a matrix leads me to believe that there is likely to be a lot of random access. As such, the best option from the built-in immutable collections would be Vector. However, looking at the code for Vector it doesn't appear that it is specialized as of Scala 2.12.2. That means that all the Ints will be wrapped values, not the 4-byte primitives. I would suggest writing a bit more code so you create an immutable wrapper around Array[Array[Int]] with an appropriate apply method. If you would like a full answer on this approach, let me know.

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